Ascidia class - Ascidiacea(from the Greek askidion - bag). Solitary and colonial sessile at the adult stage of the form; in the latter case - with a common tunic. Reproduction is both sexual and asexual - by external budding or the formation of gemmules (internal buds).
The class includes subclasses Alousobranchia, Phlebobranchia, Stolidobranchia, several orders, about 100 genera, about 2000 species. Widespread in all seas.
Adult ascidians adhere to the surface and do not move, but in their youth, as larvae, swim freely in the water. After several days of wandering, they find a suitable place, usually on a rock, and become settled. Some ascidians live alone, some in extensive colonies.
The length is from 0.1 mm to 30 cm. The body is covered with a thick layer (tunic) formed from secretions of the mantle surrounding the body of the sea squirt.
Internal structure
Pharynx with numerous gill slits (stigmas) opening into the atrial cavity, which communicates with the external environment by a siphon. The genital ducts and the hindgut also open into the atrial cavity.
The secondary cavity of the body is represented by the pericardium and epicardium (a pair of cell tubes growing from the pharyngeal wall).
Ascidians are chordates, that is, at certain stages of development they have similar organs, such as a rod-like support in the back (chord), from which the spine subsequently develops in vertebrates. The notochord is formed by highly vacuolated cells. However, signs of chordates in ascidians are observed only in the larval stage, in which they almost completely coincide with vertebrate larvae.
The body of ascidians is covered with a shell - a tunic, which has a complex structure. Outside, it is dressed in a thin but hard cuticle, under which lies a layer of cells containing tunicin. This is the only case of the formation of a fiber-like substance in the animal body. Under the tunic lies a skin-muscular sac, consisting of a single-layer epithelium and transverse and longitudinal muscle sacs fused with it.
Also, ascidians in the larval stage have a brain rudiment, which, however, completely disappears in an adult animal and only the so-called ganglion, a bunch of nerves, remains. It is associated with a neural gland (homologue of the vertebrate pituitary) that opens into the pharynx. Also, ascidian larvae have a notochord. Therefore, it is assumed that the first chordates could have appeared from the neotenic larvae of some ancient ascidians. The ganglion can, like the tail of a lizard, renew itself.
Ascidians are hermaphrodites, some of them practice same-sex reproduction.
Through a special hole, ascidians suck water into a special cavity, where food is filtered from the water. The filtered water is then released through another opening.
Features of metabolism
Some of the varieties of ascidians have a unique feature: they contain vanadium in their blood. Ascidians absorb it from the water.
In Japan, ascidians are bred on underwater plantations, harvested, burned and ash is obtained, in which vanadium is contained in a higher concentration than in the ore of many of its deposits.
Possible role in the origin of chordates
According to one version, the larval form of organisms similar to ascidians could give rise to a branch of chordates.
Image of sea squirts from the atlas of Ernst Haeckel Kunstformen der Natur 1904
From top to bottom, left to right:
Cynthia melocactus
Synoecum turgens
Botryllus schlosseri = Botryllus polycyclus
Botryllus marionis
Polyclinum constellatum
Polyclinum constellatum
Cynthia melocactus
Cynthia melocactus
Polycyclus cyaneus
Sidnyum elegans, = Fragarium elegans
Molgula tubulosa
Botryllus rubigo
Botryllus helleborus
Botrylloides purpureus
The opening of the oral siphon leads to the mouth, which is surrounded by tentacles. Next is a large saccular pharynx, the walls of which are pierced by numerous gill openings - stigmas that open into the atrial, or navcolosyabral cavity. The endostyle passes along the ventral side of the pharynx - the gutter is expelled by flashing epithelium and has glandular fields, the mucus secreted by them has thyroid-stimulating hormone. On the opposite side, a thin movable fold, the dorsal plate, enters the pharyngeal cavity. The movement of the cilia of the ciliated epithelium, bordering the edges of the stigmas, creates a flow of mucus secreting the endostyle in the direction of the back of the plate. Thus, a continuous veil of mucus is created, trapping food particles. due to the back of the plate, a mucous tourniquet is formed, which slides into the esophagus at the bottom of the pharynx, which expands into the saccular stomach. The latter passes into the intestine itself, which opens with an anus into the navcolosyabrov cavity. Together with water, small marine organisms enter the throat. Undigested food remains from the atrial cavity are brought out through the cloacal siphon.
excretory organs in most ascidian are numerous vesicles hanging down into the atrial cavity of the accumulation bud; in some species, one large vesicle develops. They are located along the walls of the mantle, filled with uric acid crystals, the removal of bubbles does not occur throughout the life of the individual. In some colonial ascidians (Botryllus), the products of nitrogen metabolism are excreted into the environment in the form of ammonia, while concretions of uric acid accumulate in the accumulation kidneys.
The nervous system consists of a ganglion, which is located between the oral and cloacal siphons, has no internal cavity - the neurocoel. From the ganglion along the dorsal side of the body, nerves depart to the mouth opening, as well as to the genitals.
There are no sense organs, except for the tentacles, which perform the function of touch.
All tunicates are hermaphrodites. Reproduction occurs by both polo and budding. During asexual reproduction on the maternal individual, on the ventral side, a protrusion (stolon) is formed, including internal organs. Kidneys are formed on stolons, in which all future organs are laid. Further, single buds are separated, and colonial ones remain and further multiply by budding. Sexual reproduction was studied by A. Kovalevsky. Sex glands in ascidians mature at different times; the same individual functions either as a male or as a female. Mature eggs are excreted by the genital ducts into the navcolozyabr cavity, where fertilization occurs with spermatozoa that penetrate with water. Fertilized eggs are carried out through the cloacal siphon.
Halocynthia papillosa off the coast of Croatia
From a fertilized egg, a fork-swimming larva is formed, which is not similar to adult forms. The larvae have fewer gill openings. On the dorsal side of the embryo from the ectoderm, the nervous system is laid, which appears in the form of a plate, the edges of which are wrapped, forming a gutter, and closes. Then into the neural tube, the cavity of which is a typical neurocoel. In the neural tube lies the chord, which is formed from the dorsal part of the endodermal rudiment of the intestine, and on the sides of it, the mesodermal stripes give, by the way, the beginning of the musculature. The mouth develops as an ectodermal cavity that grows towards the endodermal gut. From the wall of the latter, branchial protrusions develop, which break through holes into the ectodermal navkolozyabrova (atrial) cavity. As a result, a fork-swimming larva with all the main features of chordates is formed. In this state, it is several hours, or within one day. With the help of suckers, which are located at the front end of the body, it attaches to an underwater object, turns into a sessile ascidian, and the restructuring of its organization is in the nature of a regressive metamorphosis. The tunic begins to develop rapidly. The chord disappears, decreases in size, and then the neural tube disappears, a sensitive vesicle, a point. However, it remains nervous system the back of the vesicle, which forms the ganglion (ganglion). Further, the pharynx itself expands, the number of gill slits increases. The oral and anal siphons move upwards. The body gets bag-shaped, highlights the tunic. That is, in addition to numerous gill openings opening into the atrial cavity, almost no signs are preserved that would show that these animals belong to chordates. So, regressive changes are very pronounced in ascidians, and, obviously, this regression is associated with a transition to a sedentary lifestyle, in which both the muscles and sensory organs with a highly developed nervous system have lost their significance. The forked-floating ascidian larva has all the signs of chordate animals: notochord, spinal cord, complex sensory organs in the cerebral vesicles. Attaching to the bottom, it changes and loses these signs.
MATERIAL AND EQUIPMENT
SYSTEMATIC POSITION OF THE OBJECT
Topic 1. STRUCTURE OF SHELLS
Type Chordates, Chordata
Subtype tunicates, Tunicata
Ascidia class, Ascidiae
Representative - Ascidia, Ascidiae sp.
______________________________________________________________________________________________________________
Tables: The structure of the ascidia, the scheme of the structure of the larva of the ascidia, the successive stages of metamorphosis of the larva of the ascidia, the structure of the salpa and the cask, the structure of the appendicularia.
For one or two students you need:
1. Fixed sea squirt.
2. Fixed salps placed in Petri dishes in water (black paper must be placed under the Petri dish.
3. Wet preparations of ascidians, salps.
4. Bath.
5. Dissecting needles - 2.
6. Hand magnifier 4–6 X.
EXERCISE
Consider the appearance of a fixed single ascidian, a single salp and a colonial ascidian - pyrosome. Make the following drawings:
1. Scheme of the internal structure of the sea squirt.
2. Scheme of the structure of the ascidian larva.
3. Scheme of the structure of a single salp.
4. Scheme of the structure of the pyrosome.
In appearance, a single ascidian resembles a two-necked jar (Fig. 1), tightly attached to the substrate with its base and having two openings - oral and cloacal (atrial) siphons. The body is covered on the outside with a tunic that has a complex structure: it is dressed in a thin, usually hard cuticle, under which lies a dense fibrous network containing a fiber-like substance - tunicin (this is the only case in the animal world of the formation of a large amount of a substance close to plant fiber (cellulose) and acidic mucopolysaccharides.The tunic is secreted by the epithelium and is usually impregnated with inorganic salts, turning into an elastic and dense protective shell.Individual epithelial and mesenchymal cells, and often blood vessels, penetrate into it.In some species of ascidia, the tunic is thin, smooth, translucent, sometimes gelatinous or jelly-like, in in others, it is thick and bumpy.In some species, the tunic closely adjoins the ectoderm, while in others, it fuses with it only along the edges of the siphons.
Under the tunic lies a mantle or skin-muscular sac made of a single-layer skin epithelium (ectoderm) and two or three layers of longitudinal and transverse muscle bundles fused with it, lying in loose connective tissue. In the region of the siphons, there are special annular bundles of muscles that close and open these openings. The contraction and relaxation of the mantle muscles, along with the flickering of the cilia of the epithelium of the inner walls of the oral siphon, ensures the injection of water into the pharynx.
The oral siphon leads into the huge pharynx (Figure 1, 4 ), occupying most of the body of the ascidian. The border between the inner surface of the oral siphon and the walls of the pharynx is formed by a thickened annular ridge - the peribranchial or peripharyngeal groove, along which thin tentacles are invisible from the outside; some species have up to 30 pieces. The walls of the pharynx are pierced by many small gill openings - stigmas that open not outwards, but into the atrial cavity. A short esophagus departs from the bottom of the pharynx, passing into an extension - the stomach, followed by the intestine, which opens with an anus into the atrial cavity, near the cloacal siphon (Fig. 1, 14 ). The endostyle runs along the ventral side of the pharynx (Fig. 1, 7 ) - a groove lined with ciliated epithelium and having glandular fields, the mucus they secrete contains thyroid hormones. On the opposite side, a thin mobile fold protrudes into the pharyngeal cavity - the dorsal groove, or plate (Fig. 1, 8 ). The movements of the cilia of the ciliated epithelium, bordering the edges of the gill openings (stigmas), create a current of mucus secreted by the endostyle near the inner walls of the pharynx towards the dorsal plate. Thus, a continuously moving veil (“network”) of mucus arises, trapping food particles from the water that enters the pharynx through the oral siphon, flows out through the gill openings into the atrial cavity and out through the cloacal siphon. Mucus streams with trapped food particles at the dorsal plate turn into a mucous tourniquet that drains into the esophagus. In the stomach and intestines, food is digested and absorbed, and undigested residues are ejected through the anus into the atrial cavity and brought out with a stream of water. On the walls of the stomach, some species have folded or tuberculate protrusions, which are called hepatic outgrowths. However, they cannot be considered analogous to the liver of higher chordates. The tubular pyloric glands that secrete digestive enzymes are located in the wall of the stomach.
The pharynx also serves as a respiratory organ. The circulatory system of tunicates is peculiar. Heart (Fig. 1, 9 ) has the form of a short tube, from one end of which a vessel runs along the dorsal plate, branching in the walls of the pharynx; the vessels extending from the other end of the heart are directed to the internal organs (stomach, intestines, sex glands, etc.) and the mantle, where they pour blood into small cavities - lacunae. The heart sequentially, for several minutes, contracts first in one direction, then in the opposite direction. Therefore, the blood is directed either to the internal organs and the mantle, or to the walls of the pharynx, where it is saturated with oxygen. Thus, blood circulation is replaced by a pendulum movement of blood through the same vessels, alternately performing the function of either arteries or veins. This type of "circulation" seems to reduce the friction of the viscous fluid (blood) in the very complex network of vessels of the huge pharynx, while at the same time providing for the relatively low oxygen demand of these sessile animals.
The pharynx and most of the internal organs are surrounded by an atrial cavity, which opens outwards with a cloacal siphon (Fig. 1, 2 ); the walls of the atrial cavity are lined with ectoderm. Mesenteric adhesions develop between the body wall - the mantle - and the walls of the pharynx. The formation of the atrial cavity enhances the flow of water through the pharynx, intensifying both respiration and food production. On the wall of the mantle facing the atrial cavity, sometimes on the walls of the intestine there are small swellings - renal vesicles (in some species one large vesicle develops). In such "accumulation kidneys" uric acid crystals accumulate, which are not removed from the vesicles during the life of the individual. In some colonial ascidians, the products of nitrogen metabolism are excreted from the body into the environment in the form of ammonia (a property of many invertebrates), while uric acid nodules accumulate in the "renal vesicles".
Ascidians, like the rest of the tunicates, are hermaphrodites. Usually paired ovaries (Fig. 1, 13 ) in the form of long sacs filled with eggs lie in the cavity of the coelom and are attached to the walls of the mantle; short tubular oviducts open into the atrial cavity near the cloacal siphon. Some species have up to a dozen small, rounded ovaries. Testicles (Fig. 1, 12 ) in the form of numerous lobules or compact oval bodies are also located on the walls of the mantle; their short ducts open into the atrial cavity. Self-fertilization is prevented by the fact that in each individual the germ cells do not mature at the same time, and therefore it functions either as a male or as a female. Fertilization of eggs occurs in water outside the body or in the cloacal siphon, where spermatozoa penetrate with the flow of water through the gill openings. Fertilized eggs are carried out of the cloacal siphon and develop outside the body.
As a result of the development of a fertilized egg, a tailed larva is formed, which differs sharply in structure from adult ascidians (Fig. 2). the ascidian larva has the main characteristic features of chordates: a chord (Fig. 2, 11 ) located above it by the neural tube (Fig. 2, 9 ), pharynx with gill openings (Fig. 2, 19 ), but she is not eating.
The free-swimming larval stage lasts only a few hours. At the front end of her body, ectodermal outgrowths are formed.
you are papillae of attachment (Fig. 2, 1 ) that secrete sticky mucus. With their help, the larva, having found suitable soil, attaches itself to an underwater object (a stone, a large shell, etc.) and undergoes a regressive metamorphosis. The tail (notochord, neural tube, muscle cells) undergoes resorption and gradually disappears. The pharynx grows, in which the number of gill openings sharply increases; the intestinal tube is differentiated, and its end breaks into the overgrown atrial cavity. At the same time, the circulatory system is formed, gonads (sex glands) are formed, the oral and cloacal siphons move, and the body acquires a bag-like appearance characteristic of an adult ascidian. During metamorphosis, the pigment eye disappears (Fig. 2, 5 ) and statocysts (Fig. 2, 4 ), and the nerve cells of the walls of the cerebral vesicle are grouped into a compact nerve ganglion - the dorsal ganglion.
In addition to sexual reproduction, asexual reproduction is also widespread in ascidians. Developed from a fertilized egg, settled to the bottom and passed metamorphosis, the ascidia grows; then an outgrowth is formed in the lower part of her body - a kidney-shaped stolon (sometimes there are several of them), into which the processes of all internal organs grow. At the end of the stolon, swellings are formed - kidneys; in each of them, by complex differentiation, the organs of an adult sexual individual are formed. The animals formed as a result of budding either break away from the stolon, fall to the ground and attach themselves next to the mother organism (single sea squirts), or retain a close relationship with it (colonial sea squirts).
A specialized and short-lived phoretic larva, developing from a fertilized egg, gives the ascidians the opportunity, settling, to occupy parts of the seabed remote from their birthplace.
The class of ascidians combines three orders: single ascidians ( Monascidiae), complex sea squirts ( Synascidiae) and fireballs ( Pirosomata).
A separate position is occupied by pyrosomes, salp-like colonial ascidians (Fig. 3). The colony is formed by budding. From a fertilized egg, an ascidia-like zoocide develops - the founder of the colony. By budding, a group of four cross-shaped individuals lying in a common tunic arises. Kidneys are formed on their abdominal stolons, which, being transformed into zooids, break away from the stolon and occupy a certain position in the tunic. As a result, a colony appears in the form of a cone or a tube closed at one end (Fig. 3, A); it can include several hundred individual individuals - zooids (Fig. 3, B, 15 ). Their oral siphons open on the surface of the colony (Fig. 3, 2 ), and cloacal - in its internal cavity (Fig. 3, 9 ). Due to this arrangement of siphons, the colony is capable of jet propulsion. Near the oral siphon of each zooid, a finger-shaped outgrowth of the tunic is formed (Fig. 3, 1 ). There is no mobile dispersal larva. These animals are called fireworms because on the sides of the anterior part of the pharynx, each zooid has groups of cells in which symbiotic luminous bacteria live (Fig. 6 ).
Salps are floating (pelagic) marine animals that have structural features in common with ascidians, but differ in the ability to jet propulsion - oral and cloacal siphons are located at opposite ends of the body (Fig. 4), surrounded by a thin, gelatinous, translucent tunic (Fig. 4, 1 ). The mantle is formed by a single-layer epithelium, to the inner surface of which muscle bands are adjacent, like hoops covering the body of an animal (Fig. 4, 6 ). Unlike adult ascidians, which have smooth muscles, in salps the fibers of the muscle bands are striated. Almost the entire body is occupied by the pharyngeal and atrial cavities, separated by a septum - the dorsal outgrowth. This septum is perforated by several gill openings - stigmas. A well-developed endostyle runs along the bottom of the pharynx (Fig. 4, 3 ). A short esophagus extends from the back of the pharynx, passing into the stomach (Fig. 4, 10 ); the intestine opens into the atrial cavity with the anus (Fig. 4, 11 ). The heart lies under the esophagus (Fig. 4, 12 ). In the front of the body on the dorsal side there is a nerve node - ganglion (Fig. 4, 5 ), to which the pigmented eye (the organ of light perception) sticks. Under the ganglion there is a neural gland, and at some distance from it lies the organ of balance - the statocyst, connected to the ganglion by a nerve (Fig. 4, 4 ).
Salps are characterized by alternation of sexual and asexual generations (metagenesis), usually associated with the formation of complex polymorphic colonies.
Representatives of the third class of tunicates - appendicularia - in appearance and structure are similar to ascidian larvae. These are small floating tunicates from a few millimeters to 1–2 cm long, without a true tunic and an atrial cavity; there is only one pair of gill openings in the pharynx. The notochord is surrounded by a thin connective tissue sheath. Above the chord lies the nerve trunk, and on its sides stretch two muscle strands formed by giant cells. The body is surrounded by a transparent house, the shape of which varies in different species. They reproduce only sexually. Thus, appendicularians have no alternation of generations, no asexual reproduction, and no distinct larval stage.
In adulthood, most ascidians lead a motionless, attached lifestyle (with the exception of colonial, free-floating pyrosomes;). In the adult state, the ascidians are devoid of a tail and a chord (both of these organs are present in the larva, but then are reduced;). The tunic is constant, reaches a considerable thickness and contains tunicin in its walls. The voluminous pharynx is riddled with many stigmas and surrounded by the atrial and cloacal regions. There is an atrial or cloacal opening. The muscle fibers of the mantle do not form rings or hoops. Reproduction occurs through characteristic budding. In addition, ascidia are characterized by sexual reproduction with complex transformation phenomena.
In asexually reproducing ascidians, a flask-shaped protrusion protrudes from the ventral side of the body, the so-called kidney-shaped stolon. It consists of the outer ectoderm, as well as continuations, in the form of protrusions, various systems organs: body cavities, heart sac, pharynx, etc. On the upper (dorsal) surface of the growing stolon, a rounded elevation appears - the future kidney, into which outgrowths of the organs listed above are also given. Through complex differentiation in a similar kidney, they are re-formed principal organs: intestines, pharynx with a mouth opening, cloaca, heart sac, etc. In solitary ascidians, the kidney soon breaks away from the stolon and turns into a sessile solitary form; in colonial ascidians, the kidney remains on the stolon and itself begins to multiply by budding. As a result of all these processes, a complex colony is formed.
Rice.1. Metamorphosis of ascidian larvae.
1-suckers; 2-periobranchial cavity; 3-chord; 4-ciliated fossa; 5-endostyle; 6-heart; 7-neural tube; 8-nerve node; 9 mouth; 10-rectum; I-brain bubble; 12 gillsholes; 13-stolon; 14-tail.
Eggs during sexual reproduction occur either in the atrial cavity of the animal, or in the external environment. Self-fertilization is relatively rare, more often cross-fertilization occurs.
The egg goes through stages of complete, almost uniform segmentation, which is somewhat disturbed at the stage of eight cells: namely, four cells lag behind the rest in size, the smaller ones are located on the future ventral side and represent the rudiment of the ectoderm, the larger ones form the endoderm (Fig. 1, 1, 2). The segmentation cavity is narrow; with the formation of an invaginated (invaginating) gastrula, the segmentation cavity disappears and a gastrula cavity is formed, called the primary intestinal cavity (archenteron).
The blastopore decreases in size and gradually moves, ending up on the dorsal side of the larva.
germ lengthens, its dorsal side becomes flatter, the ventral side is convex. The cells surrounding the blastopore are distinguished by their cubic shape and serve as the rudiment of the strand of the nervous system (Fig. 1, 3). The latter is laid initially in the form of the so-called medullary plate, located on the dorsal side of the embryo. On the sides of the plate appears on each side along the longitudinal ridge of the ectoderm; these ridges form medullary folds (Fig. 1, 7), and a medullary sulcus passes between them in a certain depression. Soon, the medullary folds grow towards each other, and as a result of their closure, a tubular canal of the nervous system is obtained, the bottom of which is formed by the medullary groove. The neural tube has an internal cavity, not like a lh, into which the blastopore opens from behind in the form of a narrow tubule (canalis neurentericus), connecting the neurocoel with the primary digestive cavity (archenteron). At the anterior end of the neurocoel, communication with the external environment is preserved for some time in the form of a small opening - the neuropore. The body of the embryo gradually lengthens. . From the dorsal wall of the endodermal intestinal tube, a notochord differentiates, which begins to grow rapidly in the longitudinal direction. The side walls of the endoderm intestinal tube give rise to a lateral, or lateral, outgrowth, each of which serves as the rudiment of the future mesoderm with an enterocele, or secondary cavitybody that develops in the mesoderm.
Rice. 2. The structure of the ascidian larva.
1-papilla attachment; 2 mouth; 3-endostyle; 4-brain vesicle; 5-eye; 6-cloac hole; 7-intestine; 8-nervous system; 9-chord; 10-heart.
As the body of the larva elongates and its posterior caudal region grows, two ectodermic outgrowths appear at the anterior end, with an attachment sedge (Fig. 2, 1).
What is the structure of the larva at this stage of its development? In appearance, such a larva resembles a tadpole with an elongated oval, laterally compressed body, with an elongated tail surrounded by a thin fin, an outgrowth of a tunic that dresses the entire body of the larva. A dorsal string runs along the axis of the tail (Fig.). The central one is more complicated than that of an adult ascidin: the anterior part of the neural tube is noticeably expanded, a small pigmented eye (Fig.) and a cerebral vesicle with an otocyst (Fig.) are formed here. A mouth located on the dorsal side of the body of the larva leads into the pharynx; the walls of the pharynx are permeated with stigmas, the number of which varies. The pharynx is surrounded on each side by an atrial, peribranchial sac with an atrial opening on the dorsal side of the larva. Intestine and differentiated. Such a larva in structure resembles appendicularia.
Rice. 3. Five stages of ascidian development. Gastrula formation.
1-ectoderm; 2-endoderm; 3-bookmark of the nervous system; 4-mesoderm; 5-neuropore; 6 medullary ridges; 7-neuroenteric channel; 8-bookmark chord.
In connection with the beginning of a sedentary life and attachment to the substrate with the help of the above-mentioned papillae in the body of the larva, as it turns into an adult form, interesting phenomena of regressive metamorphosis occur: the tail is gradually lost; the notochord is resorbed and disappears; oral and atrial openings change their position; the pharynx increases in size, the number of stigmas becomes large; simplifies, in particular, disappearspeephole and cerebral vesicle; the larva turns into a sessile ascidian, losing a number of internal organs and acquiring a more specialized structure (Fig. 3).
Previously, ascidians were divided into 3 superorders: simple, or single, ascidians (Monascidiae); complex, or colonial, sea squirts (Synascididae) and pyrosomes, or fireballs (Salpaeformes, or Pyrosomata). However, at present, the division into simple and complex ascidians has lost its systematic significance. Ascidians are divided into subclasses according to other characteristics.
The structure of ascidia. Ascidians are benthic animals leading an attached lifestyle. Many of them are single forms. The size of their body averages a few centimeters in diameter and the same in height. However, some species are known among them, reaching 40-50 cm, for example, the widespread Ciona intestinalis or deep sea Bathypera gigantea. On the other hand, there are very small ascidians, less than 1 mm in size. In addition to solitary ascidians, there are a large number of colonial forms in which individual small individuals, a few millimeters in size, are immersed in a common tunic. Such colonies, very diverse in shape, overgrow the surfaces of stones and underwater objects. Most of all, single ascidia look like an oblong, inflated bag of irregular shape, growing with its lower part, which is called the sole, to various solid objects. Two holes are clearly visible on the upper part of the animal, located either on small tubercles or on rather long outgrowths of the body, resembling the neck of a bottle. These are siphons. One of them is oral, through which the ascidia sucks in water, the other is cloacal. The latter is usually somewhat shifted to the dorsal side. Siphons can be opened and closed with the help of muscles - sphincters. The body of ascidians is dressed in a single-layer cell cover - the epidermis, which highlights on its surface a special thick shell - a tunic that performs supporting and protective functions. The outer color of the tunic is different. Ascidians are usually colored in orange, reddish, brown-brown or purple tones. However, deep-sea ascidians, like many other deep-sea animals, lose color and become off-white. Sometimes the tunic is translucent and through it the insides of the animal shine through. Often the tunic forms wrinkles and folds on the surface, overgrown with algae, hydroids, bryozoans and other sedentary animals. In many species, its surface is covered with grains of sand and small pebbles, so that the animal can be difficult to distinguish from surrounding objects. Tunic is gelatinous, cartilaginous or jelly-like consistency. Its unique feature is that the fiber-like substance that makes up the tunic (tunicin) is present in it in large quantities and makes up more than 60% of its mass. The thickness of the tunic can reach 2-3 cm, but usually it is much thinner. In an aberrant deep-sea genus Situla the tunic is represented by a thin film (less than 0.1 mm). In the thickness of the tunic, tubular vessels of ectodermal origin pass through which blood circulates. In addition, it can be inhabited by wandering amoeba-like cells that perform a protective phagocytic function, penetrating here from the blood. This is possible only due to the gelatinous consistency of the tunic. In no other group of animals do cells inhabit formations of a similar type.
Under the tunic lies the actual wall of the body, or mantle, which includes, in addition to the single-layer ectodermic epithelium covering the body, a connective tissue layer with muscle fibers. The outer muscles consist of longitudinal, and the inner of the annular fibers. Such muscles allow ascidians to make contractile movements and, if necessary, to throw water out of the body. The mantle covers the body under the tunic so that it lies freely inside the tunic and fuses with it only in the region of the siphons. In these places are sphincters - muscles that close the openings of the siphons.
There is no solid skeleton in the body of ascidians. Only some of them have small calcareous spicules of various shapes scattered in different parts of the body.
The alimentary canal of ascidians begins with a mouth located at the free end of the body on the introductory, or oral, siphon. Around the mouth is a corolla of tentacles, sometimes simple, sometimes quite strongly branched. The number and shape of the tentacles are different in different species, but there are never less than 6 of them. A huge pharynx hangs inward from the mouth, occupying almost the entire space inside the mantle. The pharynx of ascidians forms a complex respiratory apparatus. Along its walls, gill slits are arranged in strict order in several vertical and horizontal rows, sometimes straight, sometimes curved so that a kind of gill basket is obtained. Often the walls of the pharynx form 8-12 rather large folds hanging inward, located symmetrically on its two sides and greatly increasing its inner surface. The folds are also pierced by gill slits, and the slits themselves can take on very complex shapes, twisting in spirals on cone-shaped outgrowths on the walls of the pharynx and folds. The gill slits are covered with cells bearing long cilia. In the intervals between the rows of gill slits, blood vessels pass, also correctly positioned, which are formed due to the invagination of the walls of the gill basket. There can be up to 50 of them on each side of the pharynx. Here the blood is enriched with oxygen. Sometimes the thin walls of the pharynx contain small spicules to support them. Two protrusions in the form of thin-walled sacs, or epicardiums, depart from the posterior wall of the pharynx. In many ascidians, they merge into an unpaired organ. The epicardium plays an important role in the asexual reproduction of ascidians. In some cases, they can perform the functions of the kidneys, in which waste products of the body accumulate.
The gill slits, or stigmas, of ascidians are invisible when viewed from the outside, having removed only the tunic. From the pharynx they lead into a special cavity lined with ectoderm and consisting of two parts fused on the ventral side with the mantle. This cavity is called peribranchial, atrial or peribranchial. It lies on each side between the pharynx and the outer wall of the body. Part of it forms a cloaca. This cavity is not an animal body cavity. It develops from special protrusions of the outer surface into the body. The peribranchial cavity communicates with the external environment through the cloacal siphon.
A thin dorsal plate hangs from the dorsal side of the pharynx along its entire length, sometimes dissected into thin tongues, and a special sub-gill groove, or endostyle, passes along the ventral side, which has cells of two genera - glandular and ciliary, arranged in a series of longitudinal zones. By beating the cilia on the stigmas, the ascidian drives water so that a direct current is established through the mouth opening. Further, water is driven through the gill slits into the peribranchial cavity and from there through the cloaca to the outside.
Passing through the cracks, water gives oxygen to the blood, and various small organic residues, unicellular algae, adhere to the mucus secreted by the endostyle. This mucus, by means of the movement of the cilia of the epithelium, is continuously transmitted to the walls of the gill basket in the form of a kind of mucus trapping network. Then, passing to the dorsal plate, it is formed into a mucus tourniquet with food particles adhering to it. In the form of such a kind of "roll" food enters the short esophagus. Curving to the ventral side, the esophagus passes into a swollen stomach, from which the intestine emerges. The intestine, bending, forms a double loop and opens with an anus into the cloaca. Excrement is pushed out of the body through the cloacal siphon. Thus, the digestive system of ascidians is very simple, but the presence of an endostyle, which is part of their hunting apparatus, attracts attention. It turns out that the endostyle is a homologue of the vertebrate thyroid gland and secretes an organic substance containing iodine. Apparently, this substance is close in composition to the thyroid hormone. Some ascidians have special folded outgrowths and lobed masses at the base of the walls of the stomach. This so-called liver. It is connected to the stomach by a special duct.
The circulatory system of ascidians is not closed. The heart is located on the ventral side of the animal's body. It looks like a small elongated tube and consists of two layers: the inner one - the myocardium and the outer one - the pericardium. Between them there is a cavity surrounding the heart - the pericardial sac. From two opposite ends of the heart departs along a large blood vessel. From the anterior end, the gill artery begins, which stretches in the middle of the ventral side and sends numerous branches from itself to the gill slits, giving small side branches between them and surrounding the gill sac with a whole network of longitudinal and transverse blood vessels. From the back, dorsal side of the heart, the intestinal artery departs, giving branches to the internal organs. Here, blood vessels form wide lacunae - spaces between organs that do not have their own walls, very similar in structure to the lacunae of bivalve mollusks. Blood vessels also enter the wall of the body. Even the tunic has its own epidermal tubular vessels through which blood circulates. The entire system of blood vessels and lacunae opens into the gill-intestinal sinus, sometimes called the dorsal vessel, to which the dorsal ends of the transverse gill vessels are also connected. This sinus is significant in size and stretches in the middle of the dorsal part of the pharynx. All tunicates, including sea squirts, are characterized by a periodic change in the direction of blood flow, since their heart alternately contracts for some time, either from back to front, then from front to back. When the heart contracts from the dorsal region to the abdominal region, the blood moves through the gill artery to the pharynx, or gill sac, where it is oxidized and from where it enters the branchio-intestinal sinus. The blood is then pushed into the intestinal vessels and back to the heart, just as it is in all vertebrates. With the subsequent contraction of the heart, the direction of the blood flow is reversed, and it flows, as in most invertebrates. Thus, the type of circulation in tunicates is transitional between the circulation of invertebrates and vertebrates. The blood of ascidians is acidic due to the high content of sulfuric acid in it. There are lemon-yellow, orange and colorless blood cells, which contain a lot of vanadium. He takes part in redox processes in blood cells, as well as in the formation of a tunic. Previously, it was assumed that vanadium performs the function of iron in blood hemoglobin and is an oxygen carrier, but this has not been confirmed. It has now been established that the blood of ascidians contains no more oxygen than sea water. Apparently, oxygen enters the blood of these animals by simple diffusion.
The nervous system in adult ascidians is extremely simple and less developed than in the larva. Simplification of the nervous system occurs due to the sedentary lifestyle of adult forms. The nervous system consists of the supraesophageal, or cerebral, ganglion, located on the dorsal side of the body between the siphons. From the ganglion, 2-5 pairs of nerves originate, going to the edges of the mouth opening, pharynx and to the insides - the intestines, genitals and to the heart, where there is a nerve plexus. Between the ganglion and the dorsal wall of the pharynx there is a small paranervous gland, the duct of which flows into the pharynx at the bottom of the fossa in a special ciliated organ. This piece of iron is sometimes considered a homologue of the lower appendage of the brain of vertebrates - the pituitary gland. Sensory organs are absent, but probably the mouth tentacles have a tactile function. Nevertheless, the nervous system of tunicates is not essentially primitive. Ascidian larvae have a spinal tube lying above the notochord and forming a swelling at the anterior end. This swelling, apparently, corresponds to the brain of vertebrates and contains larval sensory organs - pigmented eyes and an organ of balance, or statocysts. In some species, a pressure-sensing organ has now been discovered. When the larva develops into an adult animal, the entire posterior part of the neural tube disappears, and the cerebral vesicle, together with the larval sense organs, disintegrates; due to its dorsal wall, the dorsal ganglion of the adult ascidian is formed, and the abdominal wall of the bladder forms the paranervous gland. As noted by V.N. Beklemishev, the structure of the nervous system of tunicates is one of the best evidence of their origin from highly organized mobile animals. The nervous system in ascidian larvae is more developed than in the lancelet, which lacks a brain bladder.
Ascidians have no special excretory organs. Probably, the walls of the alimentary canal take part in the excretion to some extent. However, many ascidians have special so-called scattered accumulation buds, consisting of special cells - nephrocytes, in which excretion products accumulate. These cells are arranged in a characteristic pattern, often clustered around the intestinal loop or gonads. The reddish-brown color of many ascidians depends precisely on the excretions accumulated in the cells.
Only after the death of the animal and the decay of the body, the waste products are released and go into the water. Sometimes in the second knee of the intestine there is an accumulation of transparent vesicles that do not have excretory ducts, the nature of which is not yet clear and in which concretions containing uric acid accumulate. The members of the family Molgulidae the accumulation kidney - a modified epicardium - becomes even more complicated and turns into a large isolated sac, the cavity of which contains concretions. The great originality of this organ lies in the fact that the kidney sac of species of this family and some other ascidians always contains symbiotic fungi that do not even have distant relatives among other groups of lower fungi. Fungi form the thinnest threads - micelles, braiding concretions. Among them there are thicker formations of irregular shape, sometimes sporangia with spores are formed. These lower fungi feed on urates, the products of excretion of ascidians, and their development frees the latter from accumulated excretions. Apparently, these fungi are necessary for ascidians, since even the rhythm of reproduction in some forms of ascidians is associated with the accumulation of excretions in the kidneys and with the development of symbiotic fungi. How fungi are transferred from one individual to another is unknown. Ascidian eggs are sterile in this respect, and young larvae do not contain fungi in the kidney, even when the excretions are already accumulating in them. Apparently, young animals are again "infected" with fungi from sea water.
Ascidians are hermaphrodites, that is, the same individual has both male and female gonads, or gonads, at the same time. The ovaries and testes lie one or several pairs on each side of the body, usually in a loop of intestine. Their ducts open into the cloaca, so that the cloacal opening serves not only for the exit of water and excreta, but also for the excretion of sexual products. Self-fertilization in ascidians does not occur, since eggs and sperm mature at different times. Fertilization most often occurs in the peribranchial cavity, where the spermatozoa of another individual penetrate with a current of water. Rarely is it outside. Fertilized eggs exit through the cloacal siphon, but sometimes eggs develop in the peribranchial cavity and already formed floating larvae emerge. Such a live birth is especially characteristic of colonial ascidians. In addition to sexual reproduction, ascidians also reproduce asexually by budding. In this case, various colonies of ascidians are formed.
The structure of an ascidiozooid, a member of a colony of complex ascidians, does not differ in principle from the structure of a single form. But their dimensions are much smaller and usually do not exceed a few millimeters. The body of the ascidiozooid is elongated and divided into two or three sections, in the first, thoracic, section there are the pharynx and peribranchial cavity, in the second - the loop of the intestine, and in the third - the outgrowths of the epicardium, the gonads and the heart. Sometimes different organs are located somewhat differently.
Ascidiozooids can either be scattered in the common tunic of the colony, and then both their oral and cloacal openings come out, or they are arranged in regular figures in the form of rings or ellipses. In the latter case, the colony consists of groups of individuals with independent mouths, but having a common cloacal cavity with one common cloacal opening, into which the cloacae of individual individuals open.
The internal organs of ascidians - the intestines, sex glands, heart - lie in the primary cavity of the body. This cavity in the process of embryonic development of the embryo originates from the cavity of the blastula. However, they also have a secondary body cavity, or coelom, but it is greatly reduced. The coelomic formations include the above-mentioned small cavity of the pericardial sac and the cavity of the epicardium. The ascidian coelom is probably holologous to the second (collar) pair of coelomic sacs of hemichordates and echinoderms. It is formed in the enterocele way, i.e., by protrusion of blind pockets from the endodermal intestine.
Reproduction and development of ascidia. The development of ascidia occurs in a more complex way. Ascidian eggs are usually rich in yolk. Their fragmentation is clearly bilateral in nature, which is typical for chordates. When a larva emerges from the egg shell, it is quite similar to an adult appendicular. It, like the appendicular, resembles a tadpole in appearance, the elongated oval body of which is somewhat compressed from the sides, and the tail is elongated. The larva is soon surrounded by a tunic, which lies in two layers on the body of the animal, in one layer on the tail, and forms a thin fin along the dorsal and ventral sides of the tail. In the tail of the larvae of ascidians, the plan of the structure of chordates is especially pronounced. A chord runs along its axis, formed by a row of elastic cylindrical cells arranged one behind the other, about 40 in number. The nervous system of the larva lies above the chord in the form of a neural tube, which forms a cerebral vesicle at the anterior end of the body. It contains the larval sense organs - a rather complex eye and a unicellular balance organ, or statocyst, in which lies a solid granule - statolith. In the larvae of some ascidians, a peculiar sensitive organ develops, combining both the function of the organ of balance and the photoreceptor function, the photolith. It consists of two parts - a statocyst with a dark statolith, which plays the role of a pigment bowl, and a bundle of light-sensitive cells located nearby. Recently in the brain vesicle larvae Cnemidocarpa finmarkensis another sensitive formation that perceives pressure was identified.
Under the chord in the tail stretches a thin cord - a rudiment of the primary intestine, which quickly disappears during the development of the larva. According to the observations of A. O. Kovalevsky, as soon as the larvae begin to swim, the cells of the "caudal intestine" separate, round and turn into future blood cells. On the sides of the notochord are two muscle bands, consisting of a small number of cells. On the anterior part of the dorsal side of the body of the larva there is a mouth leading to the pharynx, the walls of which are pierced by several rows of gill slits. But, unlike appendicularia, the gill slits, even in ascidian larvae, do not open directly outward, but into a special peribranchial cavity, the rudiments of which in the form of two sacs protruding from the surface of the body are clearly visible on each side of the body. They are called peribranchial invaginations. At the anterior end of the body of the larva, three sticky attachment papillae are visible.
Initially, the larvae swim freely in the water, moving with the help of oscillatory movements of the tail, and at the same time rotate around their main axis. Their body sizes reach one or several millimeters. Special observations have shown that the larvae swim in the water for a short time - from 2 hours to 5 days. They don't eat. During this time, they can cover distances up to 1 km, although most of them settle to the bottom relatively close to their parents. However, even in this case, the presence of a free-swimming larva contributes to the dispersal of immobile ascidians over considerable distances and helps them to spread throughout all seas and oceans.
Settling to the bottom in a suitable place, the larva attaches itself to various solid objects with the help of sticky papillae. Thus, the larva sits down with the front end of the body, and from that moment it begins to lead an immobile, attached way of life. In this regard, there is a radical restructuring and a significant simplification of the structure of the body. The tail retracts very quickly - in some species in 6-15 minutes. The remnants of the notochord, statocysts and eyes disappear gradually - within a few days. Instead of the cerebral vesicle, only the nerve ganglion and the paranervous gland remain. The body takes on a sac-like shape. Both peribranchial invaginations begin to grow strongly on the sides of the pharynx and surround it. The two openings of these cavities gradually converge on the dorsal side and merge into one cloacal opening. The newly formed gill slits open into this cavity. The intestine also opens into the cloaca.
Sitting on the bottom with the front part, on which the mouth is located, the ascidian larva finds itself in a very disadvantageous position in terms of capturing food. Therefore, in the settled larva, another important change occurs in the general plan of the body structure: its mouth begins to slowly move from bottom to top and, in the end, is located at the very upper end of the body. The movement of the mouth occurs along the dorsal side of the animal and entails the displacement of all internal organs. The moving pharynx pushes the cerebral ganglion in front of it, which eventually lies on the dorsal side of the body between the mouth and the cloaca. This completes the transformation, as a result of which the animal turns out to be completely different in appearance from its own larva.
Ascidians have a highly developed ability to restore lost body parts. For example, the lower part of the body can restore the lost upper part, internal organs can reappear, or even the whole organism can be recreated after a deep degeneration of individuals resulting from adverse environmental conditions from a mass of cellular material. Apparently, this feature led to the emergence of a wide variety of forms of asexual reproduction in ascidians, either by transverse division or by different methods of budding, which ensured their success in the struggle for existence.
Ascidians have a simple transverse fission in two, but it is very rare, while multiple division, or strobilation, is very characteristic of complex ascidians. In these ascidians, the body is divided into two or three sections - thoracic (thoracic), abdominal (abdominal) and post-abdominal. The transverse division of the body into separate fragments occurs only in the abdomen and post-abdomen - either separately, or in both parts together. Of great importance in this case is the outgrowth of the epicardium, which enters the posterior parts of the body of the ascidiozooid and, during division, gives rise to most of the internal organs of the new organism. At the beginning of strobilation, the abdominal part of the body is greatly elongated, it accumulates nutrients, which are obtained as a result of more or less decay of the thoracic part of the mother. Then there is a division of the abdominal region into several fragments, usually called kidneys, from which new individuals arise. The separated chest fragment at the same time restores the lower part of the body. At amaroecium soon after the larva settles on the substrate, the posterior part of its body greatly increases and develops, and a post-abdomen is formed, into which the heart is displaced. When the length of the postabdomain begins to greatly exceed the length of the body of the larva, it separates from the maternal individual and divides into 3-4 parts, from which young buds are formed - blastozooids. They move forward and are located next to the mother's body, in which the heart is re-formed. The development of blastozoids occurs unevenly, and when some of them have already completed it, others are just beginning to develop.
The ascidia formed from the larva can also reproduce in another asexual way - through budding. In this case, different parts of the body of the mother organism can give rise to new individuals, and depending on this, four different types of budding are distinguished. The most common is the so-called vascular budding, when new zooids develop on the vessels of the tunic. At the same time, branched stolons creeping along the substrate develop, extending from the soles of budding individuals. They are formed by the epidermal blood vessels of the tunic and are dressed in a thin layer of it. The ends of such tubular stolons form vine-like extensions, inside which blood cells accumulate. They separate from the stolons and turn into kidneys. At the same time, the cellular mass of the kidney is organized into an internal vesicle covered with an epithelial layer of cells, which gives rise to all the internal organs of a new organism developing from the kidney - the blastozoid. In the integument of the body of the kidney, two invaginations are formed - oral and cloacal siphons, which break through, respectively, into the pharynx and into the cloaca. This happens, in particular, Clavelina lepadiformis. It does not form colonies. The individuals resulting from budding are completely independent, but live in close groups, intergrowths, very characteristic of sea squirts. In other species, the kidneys are not separated from the stolon, and new zooids that have developed from them are connected by a single blood circulation. This is already a real colony, it is, for example, in Ecteinascidia tortugensis.
In addition to vascular budding, other types of budding are also possible - pyloric, stolonial and pallial. During pyloric budding, which occurs only in species of the Didemnidae family, two buds develop at once in the abdominal part of the maternal ascidiozooid, separated from each other. They include the skin, epicardium and intestines of the mother's body. One of the kidneys is the rudiment of the thoracic region. It develops a pharynx, penetrated by gills, and a nervous system. The second kidney gives rise to the abdominal region with the esophagus, stomach and intestines, as well as the heart. After the kidneys have developed, the mother divides so that her thoracic region joins with the new abdominal region developed from the kidney, and, conversely, the new thoracic region joins with the maternal abdomen. Two half-new individuals are obtained. In other cases, each kidney develops into a full-fledged individual, completing the missing part. Such budding can begin even during embryonic development, and the larva that has emerged from the egg already contains two or four daughters. Finally, stolons can develop from the outer epidermis of the maternal individual, into which outgrowths of the posterior pharyngeal wall - the epicardium, as well as a large number of mesenchymal cells, enter. Such a bud-bearing stolon begins to divide even during embryonic development and gives up to 9-14 buds, which, having become isolated, immediately grow, and together with the maternal individual - an oozooid, form a colony of 10-15 zooids. After a certain period of time, the oozooid dies, and the blastozooids begin asexual reproduction. At the same time, some kidneys develop into sexually mature blastozoids, and some continue to remain asexual and give rise to the next generation of kidneys. Such budding is called stolonial and is characteristic only for species of the same family - Polycitoridae. Sometimes, with this method of budding, complex life cycles. So, adult zooids of ascidians from the genus Distaplia can only reproduce sexually. Even in the embryonic state, the oozoid, which developed from the egg, separates from itself the primary kidney, which, while still completely immature, again divides into three - secondary - kidneys. Of these buds, one develops into an adult blastozoid with sexual rudiments, and the other two, remaining asexual, divide again and give three buds of the third generation of blastozoids, of which one also grows into an adult. This continues many times, and the number of individuals in the colony increases. Only under certain conditions can sexually mature blastozoids begin to reproduce sexually. If such conditions are absent, the role of adults is reduced to feeding the colony. In general, ascidian life cycles are simple, and they still do not have a clearly defined alternation of generations with different methods of reproduction, and oozooids and blastozooids are very similar in structure.
There is another way of budding - pallial, or mantle, which is found in more highly organized ascidians, including solitary ones. As a rule, adults bud. In this case, long cylindrical processes of the epidermis are formed on the sides of the animal's body, which include tubular outgrowths of the outer wall of the peribranchial cavity, and individual cells accumulate between them. The small ascidians that develop on these stolons remain connected to the mother organism, and the stalks of their kidneys turn into blood vessels, forming a single network in the colony. However, in some cases, detachment of the kidneys and the formation of independent single ascidians may occur.
So, the processes of budding in ascidians are extremely diverse. Sometimes even close species of the same genus have different ways of budding. Some ascidians are able to form dormant, stunted buds that allow them to survive adverse conditions, in particular, overwinter. As we have seen, in different species of ascidians from different families, the kidney can either break away from the stolon and give rise to a single daughter ascidia (in single species), or it remains sitting on the stolon, grows, begins to bud again, and eventually a new colony is formed ( in colonial species). If the blastozoids in it are weakly connected, only with the help of a stolon, they reach much larger sizes (but usually smaller than single ascidians) than in the case of their close connection, when the size of the blastozoid does not exceed a few millimeters.
It is interesting that the buds in colonial forms with a common gelatinous tunic always separate inside it, but do not remain in the place where they were formed, but move through the thickness of the tunic to the final place. The kidney always makes its way to the surface of the tunic, where its mouth and anus open. In some species, these openings open independently of the openings of other kidneys, in others only one mouth opens outward, while the cloacal opening opens into a cloaca common to several zooids. Sometimes this can form long channels. In many species, the zooids form a tight circle around the common cloaca, and those that do not fit in it are pushed away and give rise to a new circle of zooids and a new cloaca. Such an accumulation of zooids forms the so-called cormidium.
Sometimes cormidia are very complex and even have a common colonial vascular system. Cormidium is surrounded by an annular blood vessel, into which two vessels flow from each zooid. In addition, such vascular systems of individual Cormidia communicate with each other, and a complex general colonial system arises. vascular system, so that all ascidiozooids are interconnected. As you can see, the connection between individual members of colonies in various complex ascidians can be very simple, when individual individuals are completely independent and immersed in a common tunic, and the kidneys also have the ability to move around in it. Or it can become more or less complicated, up to a high degree of integration of colonies, with a single circulatory system, synchronously occurring life processes (maturation, reproduction, death of blastozooids), with mechanisms of internal regulation of the number of individuals and generations in its composition. In the latter case, a kind of colonial organism arises.
When budding in ascidians, the following is observed interesting phenomenon. As is known, in the process of embryonic development, various organs of the animal organism arise from different, but completely specific parts of the embryo (germ layers) or layers of the body of the embryo that make up its wall at the very first stages of development.
Most organisms have three germ layers: the outer or ectoderm, the inner or endoderm, and the middle or mesoderm. In the embryo, the ectoderm covers the body, while the endoderm lines the internal intestinal cavity and provides nutrition.
The mesoderm provides a link between them. In the process of development from the ectoderm, as a general rule, the nervous system, skin integuments are formed, and in ascidians, peribranchial sacs; from the endoderm - the digestive system and respiratory organs; from the mesoderm - muscles, skeleton and genital organs. With various methods of budding in ascidians, this rule is violated. For example, during paleal budding, all internal organs (including the stomach and intestines arising from the endoderm of the embryo) give rise to an outgrowth of the peribranchial cavity, which is ectodermic in origin. And, conversely, in the case when the kidney contains an outgrowth of the epicardium, which, apparently, should be considered as a whole, most of the internal organs, including the nervous system and peribranchial sacs, are formed as a derivative of the mesoderm.
Pyros building. Pyrosomes, or fireballs, are free-floating colonial pelagic tunicates. They got their name because of the ability to glow with bright phosphorescent light. Pyrosomes are divided into two groups according to the method of formation of a colony - Pyrosomata fixata and Pyrosomata ambulata. They are represented by only two genera and a dozen different species.
Of all planktonic forms of tunicates, pyrosomes are closest to ascidians. With the exception of one benthic species, these are colonial ascidians floating in the water. Each colony consists of many hundreds of individual individuals - ascidiozooids, enclosed in a common, often very dense tunic. In pyrosomes, all zooids are equal and independent in terms of nutrition and reproduction. The colony has the shape of a long elongated cylinder with a pointed end, having a cavity inside and open at the wide rear end. Outside, the pyrosome is covered with small, soft, spiny outgrowths. The most important difference between a colony of pyrosomes and a colony of sessile ascidians lies also in the strict geometric regularity of the shape of the colony. Individual zooids stand perpendicular to the wall of the colony body. Their mouth openings are turned outward, and the cloacal openings are on the opposite side of the body and open into the common cavity of the colony. Separate small ascidiozooids capture water with their mouths, which, having passed through their body, enters the body cavity. The movements of individual individuals are coordinated among themselves, and this coordination of movements occurs mechanically, in the absence of muscle, vascular or nerve connections. In the tunic, mechanical fibers are stretched from one individual to another by pyros, connecting their motor muscles. The contraction of the muscle of one individual pulls the other individual with the help of the fibers of the tunic and transmits irritation to it. Contracting simultaneously, small zooids push water through the cavity of the colony. In this case, the entire colony, similar in shape to a rocket, having received a reverse push, moves forward. Thus, pyrosomes have chosen for themselves the principle of jet propulsion. This method of movement is used not only by pyrosomes, but also by other pelagic tunicates.
The pyrosom tunic contains such a large amount of water (in some tunicates, water is 99% of body weight) that the entire colony becomes transparent, as if glassy and almost invisible in the water. However, there are also pink-colored colonies. Such gigantic pyrosomes - their length reaches 2.5 or even 4 m, and the diameter of the colony is 20-30 cm - have been repeatedly caught in the Indian Ocean. The tunic of these pyrosomes has such a delicate texture that, getting into plankton nets, the colonies usually break up into separate pieces. In 1969, a pyrosome was photographed near New Zealand Pirostemma spinosum more than 20 m long, its diameter was 1.2 m. Half of the body length fell on a long process extending from the end of the common cloacal opening. Usually, the dimensions of pyrosoma are much smaller - from 3 to 10 cm long with a diameter of one to several centimeters. A new genus and species of pyrosomes, Propyrosoma, has been described based on materials from the research vessel Vityaz. vitjasi. The colony of this species also has a cylindrical shape, up to 0.5 m in size. Through the pinkish mantle of this pyrosome, as dark brown (or rather, dark pink in living specimens) inclusions, the insides of individual ascidiozooids shine through.
The mantle has a semi-liquid consistency, and if the surface layer is damaged, its substance spreads in the water in the form of viscous mucus, and individual zooids disintegrate.
The structure of the ascidiozooid pyrosome is not much different from the structure of a single ascidian, except that its siphons are located on opposite sides of the body, and are not brought together on the dorsal side. The sizes of ascidiozooids are usually 3-4 mm, but in giant pyrosomes - up to 20 mm. Their body may be laterally flattened or oval. The mouth opening is surrounded by a corolla of tentacles, or only one tentacle may be present on the ventral side of the body. Often the mantle in front of the mouth opening, also on the ventral side, forms a small tubercle or a rather significant outgrowth. The mouth is followed by a large pharynx, cut through by gill slits, the number of which can reach 50. These slits are located either along or across the pharynx.
Approximately perpendicular to the gill slits are blood vessels, the number of which also varies from one to 3-4 dozen. The pharynx has an endostyle and dorsal tongues hanging down into its cavity. In addition, in the anterior part of the pharynx, on the sides, there are luminous organs, which are accumulations of cell masses. In some species, the cloacal siphon also has luminous organs. The luminous organs of the pyrosom are inhabited by symbiotic luminous bacteria, under the pharynx lies the nerve ganglion, the back of which plays the role of a light-sensitive organ. There is also a near-nervous gland, the canal of which opens into the pharynx. The muscular system of ascidiozooids pyrosomes is poorly developed. The annular muscles located around the oral siphon and the open ring of muscles near the cloacal siphon are rather well expressed. Small bundles of muscles - dorsal and abdominal - are located in the corresponding places of the pharynx and radiate along the sides of the body. In addition, there are also a couple of cloacal muscles. Between the dorsal part of the pharynx and the body wall there are two hematopoietic organ, which are elongated clusters of cells. Propagating by division, these cells turn into various elements of the blood - lymphocytes, amoebocytes, etc.
The digestive section of the intestine consists of the esophagus extending from the back of the pharynx, stomach and intestines. The intestine forms a loop and opens with an anus into the cloaca. On the ventral side of the body lies the heart, which is a thin-walled sac.
The testes and ovaries also open with their ducts into the cloaca, which can be more or less elongated and opens with a cloacal siphon into the common cavity of the colony. In the region of the heart, ascidiozooids pyrosomes have a small finger-like appendage - the stolon. It plays an important role in the formation of colonies. As a result of the division of the stolon in the process of asexual reproduction, new individuals bud from it.
Reproduction and development of pyrosomes. Pyrosomes, like ascidians, have a method of reproduction by budding. But in them, budding occurs with the participation of a special permanent outgrowth of the body - the kidney-shaped stolon. It is also characterized by what happens at very early stages of development. Pirosom eggs are very large - up to 0.7 mm and even up to 2.5 mm and are rich in yolk. In the process of their development, the first individual is formed - the so-called cyatozooid. The cyatozooid corresponds to the ascidian oozooid, i.e., it is an asexual maternal individual that developed from an egg. It stops developing very early and collapses. The entire main part of the egg is occupied by a nutritious yolk, on which the cytozooid develops. At Propyrosoma vitjasi on the yolk mass there is a cyatozooid, which is a fully developed ascidian with an average size of about 5.5 mm. There is even a small mouth opening that opens outwards under the egg shell. There are 10-13 pairs of gill slits and 4-5 pairs of blood vessels in the pharynx. The intestine is fully formed and opens into the cloaca, the siphon has the shape of a wide funnel. There is also a nerve ganglion with a paranervous gland and a heart that pulsates vigorously.
By the way, all this speaks of the origin of pyrosom from ascidians. In other species, during the period of maximum development of the cyatozooid, only the rudiments of the pharynx with two gill slits, the rudiments of two peribranchial cavities, the cloacal siphon, the nerve ganglion with the paranervous gland, and the heart can be distinguished. The mouth and digestive intestine are absent, although the endostyle is outlined. A cloaca with a wide opening is also developed, opening into the space under the egg membranes. At this stage, even in the egg shell, pyrosomes already begin the processes of asexual reproduction. At the posterior end of the cyatozooid, a stolon is formed - the ectoderm gives rise to an outgrowth into which the continuations of the endostyle, the pericardial sac, and the peribranchial cavities enter. From the ectoderm of the stolon in the future kidney, a nerve cord arises, independent of the nervous system of the cyatozooid itself.
At this time, the stolon is divided by transverse constrictions into four sections, from which the first buds develop - blastozoids, which are already members of the new colony, i.e., ascidiozooids. The stolon gradually becomes transverse to the axis of the body of the cytozooid and the yolk and twists around them. Moreover, each kidney becomes perpendicular to the axis of the body of the cytozooid. As the kidneys develop, the maternal individual - the cyatozooid - is destroyed, and the yolk mass is gradually used to feed the four kidneys - ascidiozooids - the ancestors of the new colony. Four primary ascidiozooids take a geometrically correct cruciform position and form a common cloacal cavity. This is a real little colony. In this form, the colony leaves the mother's body and is released from the egg shell. Primary ascidiozooids, in turn, form stolons at their posterior ends, which, lacing up, give rise to secondary aspidiozooids, etc. As soon as the ascidiozooid is isolated, a new stolon is formed at its end, and each stolon forms a chain of new buds, which separate from its end as it matures. The colony is progressively growing. Each ascidiozooid becomes sexually mature, has male and female gonads, and is able to reproduce sexually.
In one group of pyrosomes, ascidiozooids retain their connection with the parent individual and remain in the place where they originated. In the process of kidney formation, the stolon lengthens and the kidneys are connected to each other by cords. The ascidiozooids are arranged one after another towards the closed, anterior end of the colony, while the primary ascidiozooids move towards its rear, open part.
In another group of pyrosomes, which includes most of their species, the kidneys do not remain in place. Once they reach a certain developmental stage, they separate from the stolon, which never elongates. At the same time, they are picked up by special cells - phorocytes. Phorocytes are large, amoeba-like cells. They have the ability to move in the thickness of the tunic with the help of pseudopodia, or pseudopodia, in the same way as amoebas do. Picking up the kidney, phorocytes carry it through the tunic covering the colony to a strictly defined place under the primary ascidiozooids, and as soon as the final ascidiozooid breaks off from the stolon, the phorocytes carry it along the left side to the dorsal part of its producer, where it is finally established in such a way that the old ascidiozooids move farther and farther to the top of the colony, and the young find themselves at its rear end.
Each new generation of ascidiozooids is transferred with geometric correctness to a strictly defined place in relation to the previous generation and is arranged in floors. After the formation of the first three floors, secondary floors begin to appear between them, then tertiary, etc. floors. Primary floors have 8 ascidiozooids each, secondary - 16 each, tertiary - 32 each, etc. geometric progression. The diameter of the colony increases. However, with the growth of the colony, the clarity of these processes is disturbed, some ascidiozooids get confused and fall into other people's floors. The same individuals in the pyrosomal colony that reproduced by budding later develop gonads and proceed to sexual reproduction. As we already know, each of the many ascidiozooid pyrosomes formed by asexual budding develops only one large egg.
According to the method of formation of colonies, namely, whether ascidiozooids maintain a connection with the mother's organism for a long time or not, pyrosomes are divided into two groups - Pyrosoma fixata and Pyrosoma ambulata. The former are considered more primitive, since the transfer of kidneys with the help of phorocytes is a more complex and later acquisition of pyrosomes.
The formation of a primary colony of four members was considered so constant for pyrosomes that this character was even included in the characterization of the entire order Pyrosomida. However, new data on the development of pyrosomes have shown, for example, that Propyrosoma vitjasi a kidney-bearing stolon can reach a very long length, and the number of buds simultaneously formed on it is about 100. Such a stolon forms irregular loops under the egg membrane. Unfortunately, it is still unknown how the colony is formed in pyrosomes.
Lifestyle of ascidians and millet. Ascidians are bottom animals. Adult forms spend their whole lives motionless, attaching to some solid object at the bottom and driving water through the gill-pierced pharynx in order to filter out the smallest phytoplankton cells and particles of organic matter from it, which the ascidians feed on. They cannot move. Only when frightened of something or swallowing something too large, the sea squirt can shrink into a ball. In this case, water is ejected with force from the siphon.
Single ascidians sometimes form large aggregates that grow into whole druses and settle in large clusters. This gives them a number of biological advantages - in reproduction, nutrition, protection from enemies. As already mentioned, many species of ascidians are colonial. More often than others, massive gelatinous colonies are found, individual members of which are immersed in a common rather thick tunic. Such colonies form crusty outgrowths on stones or "are found in the form of peculiar balls, cakes and outgrowths on legs, sometimes resembling mushrooms in shape. In other cases, individual individuals of the colonies can be almost independent.
As a rule, sea squirts simply adhere to stones or other hard objects with the lower part of the tunic. But sometimes their body can rise above the ground on a thin stalk. Such a device allows animals to "catch" a larger volume of water and not drown in soft ground. It is especially characteristic of deep-sea ascidians, which live on thin silts that cover the ocean floor at great depths. So, the rounded body of the ascidian genus Culeolus with very wide open siphons, not protruding at all above the surface of the tunic, sits on the end of a long and thin stalk, with which the animal can attach itself to small stones, spicules of glass sponges and other objects at the bottom. The stalk cannot withstand the weight of a rather large body, and it probably floats above the bottom, carried away by a weak current. Its coloration is whitish-gray, colorless as in most deep-sea animals.
In order not to sink in the ground, the sea squirts may have another adaptation. Tunic appendages, by which ascidians are usually attached to stones, grow and form a kind of "parachute" that keeps the animal on the bottom surface.
Such "parachutes" can also appear in typical inhabitants of hard soils, usually settling on stones, when they transition to life on soft, silty soils. Root-like outgrowths of the body allow individuals of the same species to enter a new and unusual habitat for them and expand the boundaries of their range, if other conditions are favorable for their development.
However, very peculiar ascidians are described, capable of swimming short distances above the bottom. They are members of the genus Octanemus. (Octacnemus)- only 4 species - colorless, translucent animals with a diameter of no more than 7 cm. Their thin tunic forms around. oral siphon 8 wide lobes - tentacles. In the zone of attachment to the substrate on the tunic there are only thin hair-like outgrowths. Octanemuses are inhabitants of the great depths of the ocean. They are found in tropical regions at a depth of 2000-4000 m.
Some scientists tend to consider them as secondarily settled to the bottom of the salps. These are predatory animals capable of catching small crustaceans, nematodes, etc. In addition to them, other predatory ascidians have now become known. Their unusual way of life led them to develop six strong, muscular tentacles capable of grabbing the small invertebrates they feed on, and the introductory siphon thus turned into a trapping organ. The pharynx became narrow and short. It lacks true gill slits covered with ciliated epithelium, but communicates with the cloacal cavity through a small number of openings. This includes less than a dozen species of small ascidians belonging to the genera Hexacrobylus, Sorbera, Oligotrema and Gasterascidia. These are also deep-sea animals, living mainly in the abyssal zone of the ocean at a depth of 3000-5000 m. Their systematic position is unclear, but some French experts on the tunicate fauna tend to consider them even as a separate class of tunicates.
Ascidians were also found among a very specific fauna inhabiting the thinnest passages between grains of sand. Such fauna is called interstitial. Now about 10 species of ascidians are already known, which have chosen such an unusual biotope as their habitat. These are extremely small animals - their body diameter is 0.8 - 2 mm. Some of them are mobile.
Ascidians are widespread both in cold seas and in warm ones. They are found in the Arctic Ocean and in the waters of the Antarctic. They were even found directly on the coast of Antarctica during the survey by Soviet scientists of one of the fiords of the "oasis" of Bunger. The fiord was fenced off from the sea by heaps perennial ice, and the surface water in it was heavily desalinated. On the rocky and lifeless bottom of this fiord, only lumps of diatoms and threads: green algae were found.
However, in the very kutu of the bay, the remains of a starfish and a large number of large, up to 14 cm long, pinkish-transparent gelatinous ascidians were found. The animals were torn off the bottom, probably by a storm and washed up here by the current, but their stomachs and intestines were completely filled with a green mass of partially digested phytoplankton. They probably fed shortly before they were fished out of the water close to the shore.
Ascidians are especially diverse in the tropical zone. There is evidence that in the tropics there are about 10 times more species of tunicates than in temperate and polar regions. Interestingly, in the cold seas, ascidians are much larger than in warm ones, and their settlements are more numerous. They, like other marine animals, obey the rule that a smaller number of species live in temperate and cold seas, but they form much larger settlements and their biomass per 1 m 2 of the bottom surface is many times greater than in the tropics.
Most ascidians live in the most superficial littoral or tidal zone of the ocean and in the upper horizons of the continental shelf, or sublittoral, to a depth of 200 m. With increasing depth, the total number of their species decreases. Currently, about a hundred species of ascidians are known deeper than 2000 m. The maximum habitat depth at which these animals were found is 8430 m. At this depth, ascidians were discovered during the work of the Soviet oceanographic expedition on the Vityaz research vessel in the Kuril-Kamchatsky Trench. They were representatives of a new deep-sea genus Situla- unusually large, up to 35 cm high, ascidians, which turned out to be a new species and genus Situla pelliculosa from the family Ostacnemidae. Subsequently, a closely related species was found by a French expedition in Atlantic Ocean in the abyssal zone at a depth of 2115-4690 m and two more species were found by our research vessel "Akademik Kurchatov" in the South Sandwich Trench at a depth of 5650 m in the south of the Atlantic. A characteristic feature of these transparent and thin-walled animals, sitting on concretions on short and thick legs, as thin-walled as their body, is the presence of a huge introductory siphon, the diameter of which is almost equal to the diameter of the body. It was possible to take photographs of these ascidians in a solution of table salt, in which the membranous body Situla, which did not retain a permanent shape, straightened out and took a natural position. Their gill sac completely lost its sac-like shape and turned into a trapping net stretched along with the peripharyngeal zone in the same plane in the form of a plate tightening the opening of the introductory siphon. In the center of the plate is the entrance to the esophagus, the intestinal loop and the gonad are suspended from behind. From top to bottom in the middle of the complexly perforated gill plate, the dorsal plate passes, and from bottom to top to the opening of the esophagus - the endostyle. All these organs are open from the outside. About a hundred strongly rudimentary tentacles are located along the edge of the oral siphon. Thus, the entire mantle complex turns out to be open and stretched into a peculiar, very thin trapping net, through which, apparently, passive filtration of water occurs. The integument of the body is extremely delicate, in places turning into the thinnest colorless films up to 0.05 mm thick. Only the intestines are painted in a pale olive-green color due to the thinnest silt shining through its walls, and the gonad is visible as a dense yellowish spot behind the central part of the gill plate. In general, the animal is very similar to a spoon or ladle with a thick bottom and a short handle.
Ascidians avoid desalinated areas of the seas and oceans. The vast majority of them live at normal oceanic salinity of about 350/00.
As already mentioned, largest number Ascidian species lives in the ocean at shallow depths. Here they also form the most massive settlements, especially where there are enough suspended organic particles in the water column that serve as food for them. Ascidians settle not only on stones and other hard natural objects. Their favorite place of settlement is also the bottoms of ships, the surface of various underwater structures, etc. Sometimes settling in large quantities along with other fouling organisms, sea squirts can cause great harm au pair. It is known, for example, that, settling on the inner walls of water conduits, they develop in such numbers that they greatly narrow the diameter of pipes and clog them. With mass extinction in certain seasons of the year, they clog the filtration devices so much that the water supply can stop completely and industrial enterprises suffer significant losses.
One of the most widely distributed sea squirts is Ciona intestinalis, overgrowing the bottoms of ships, can settle in such huge quantities that the speed of the ship is significantly reduced.
However, the ability of ascidians to form massive settlements may be of some interest to people, given the remarkable feature of these animals, namely the ability to accumulate vanadium and other metals in the body, mainly in the blood.
Vanadium, a microelement of great practical importance, is dissolved in sea water in very small quantities. In the body of an ascidian, vanadium can make up more than 1% of the animal's ash mass, or up to 0.65% of the dry body weight. This is about 10,000 times the concentration of vanadium in seawater. The accumulation of metals by ascidians is a very interesting phenomenon. They have a pronounced ability to selectively accumulate in huge quantities not only vanadium, but also some other microelements. They found compounds of various - about one and a half dozen - metals, the content of which is hundreds and thousands of times higher than in the environment. Among them are such trace elements as tantalum, niobium, titanium. The enrichment factor for the latter can reach 100,000. Vanadium and titanium are among the biologically active substances, and their physiological role in organisms is being carefully studied. It should also be remembered that the ascidian tunic contains another valuable substance - cellulose. Its quantity, for example, in one copy of the most widespread species Ciona intestinalis is 2-3 mg. These ascidians sometimes settle in huge numbers. The number of individuals per 1 m 2 of the surface reaches 2500-10000 copies, and their wet weight is 140 kg per 1 m 2. There is an opportunity to discuss how it is possible to practically use ascidians as a source of valuable substances. Not everywhere there is wood from which cellulose is extracted, and deposits of vanadium are few and scattered. If you arrange underwater "sea gardens", then large quantities of sea squirts can be grown on special plates. It is estimated that from 1 hectare of the sea area occupied by ascidians, one can obtain from 5 to 30 kg of vanadium and from 50 to 300 kg of cellulose.
Pyrosomes live in the ocean. Pyrosomes, like salps, are most sensitive to cold waters and prefer not to leave the tropical zone of the ocean, where they are very widespread. Like other planktonic forms of pyrosome tunicates - inhabitants of the surface layers of water, they apparently do not occur deeper than 1000 m. However, there are indications in the literature that pyrosomes are located at a depth of 3000 m.
Pyrosomes do not form such large clusters as salps. However, in some marginal regions of the tropical region, their accumulations were also found. In the Indian Ocean at 40-45 ° S. sh. during the work of the Soviet Antarctic expedition, a huge number of large pyrosomes were encountered. Pyrosomes were located on the very surface of the water in spots. In each spot, there were from 100 to 400 colonies, which glowed brightly with blue light. The distance between the spots was 100 m or more. On average, there were 1-2 colonies per 1 m2 of water surface. Similar accumulations of pyrosomes have been observed off the coast of New Zealand.
Pyrosomes are known as exclusively pelagic animals. However, relatively recently, in the Cook Strait near New Zealand, it was possible to obtain several photographs from a depth of 160-170 m, in which large accumulations of Pyrosoma atlanticum, whose colonies simply lay on the bottom surface. Other individuals swam in close proximity above the bottom.
It was daytime, and the animals may have gone to great depths to hide from direct sunlight, as many planktonic organisms do. Apparently, they felt good, as the environmental conditions were favorable for them. In May, this pyrosome is common in the surface waters of the Cook Strait. Interestingly, in the same area in October, the bottom at a depth of 100 m is covered with dead, decaying pyrosomes. Probably, this mass extinction of pyrosomes is associated with seasonal phenomena. To some extent, it gives an idea of how many these animals can be found in the sea.
It should be noted that one benthic pyrosome has been described so far - Pyrosoma benthica. Four specimens of this new species were caught at a depth of 185 m off the Cape Verde Islands. The length of the colony did not exceed 6 cm, its structure is typical for pyrosomes, the colony has the ability to glow. These animals should be regarded as secondarily sedentary on the ocean floor.
Pyrosomes, which in translation into Russian means "fireballs", got their name from their inherent ability to glow. It was found that the light that occurs in the cells of the luminous organs of pyrosomes is caused by special symbiotic bacteria. They settle inside the cells of the luminous organs and, apparently, multiply there, since bacteria with spores inside them have been repeatedly observed. Luminous bacteria are passed down from generation to generation. By blood flow, they are transferred to the eggs of the pyrosomes, which are at the last stage of development, and infect them. Then they settle between the blastomeres of the crushing egg and penetrate into the embryo. Luminous bacteria penetrate along with the blood stream and into the kidneys with pyrosoms. Thus, young pyrosomes inherit luminous bacteria from their mothers. However, not all scientists agree that pyrosomes glow thanks to symbiont bacteria. The fact is that the luminescence of bacteria is characterized by continuity, and pyrosomes emit light only after some kind of irritation. There is interesting description how, during the expedition on the Challenger, the team members had fun by signing with their fingers on pyrosomes, and the luminous trace of painting with a bright line remained for several seconds on the body of animals. The light of ascidiozooids in a colony can be surprisingly intense and very beautiful, it is usually blue, but in tired, aerated and dying animals it becomes orange and even red.
However, not all pyrosomes can glow. The giant pyrosomes described above from the Indian Ocean, as well as a new species Propyrosoma vitjas do not have luminous organs. But, it is possible that the ability to glow in pyrosomes is unstable and is associated with certain stages of development and colonies.
Pyrosomes, as well as salps, can obviously feed on those fish that eat jellyfish and ctenophores. For example, pyrosomes have been found in the stomachs of swordfish. And from the stomach of another fish - mupusa - 53 cm in size, 28 pyrosomes were once extracted.
