Ascidiacea

(invertebrate zoology) A large class of the phylum Tunicata; adults are sessile and may be solitary or colonial.

(Sea squirts)

Phylum: Chordata

Class: Ascidiacea

Number of families: 24

Thumbnail description
Benthic, solitary, and colonial species whose adults are sessile, almost exclusively fixed, firmly attached, or lying free on the sea floor; they usually filter feed; larvae are free swimming

Evolution and systematics

There are very few fossil organisms that can be interpreted as ascidians: ascidians have no hard skeleton elements or calcareous shells, making it unlikely that their soft bodies can be fossilized. The single known fossil species (from the Pliocene), Cystodytes incrassatus, belongs to the widely distributed recent genus Cystodytes, characterized by the presence of relatively large (up to 0.04 in, or 1 mm, in diameter) spicules of discoid form, allowing easy identification. Another, much more ancient (about 300 million years old) form, Jaekelocarpus oklahomensis, has been interpreted as a tunicate, but recent ascidian taxonomists do not support this view. Thus, in the absence of paleontological data, phylogenetic relationships of ascidians can be understood mainly on the basis of the morphological characteristics of the recent forms.

The nature and relationships of ascidians were not understood for a long time, although these common marine animals were known even to Aristotle, more than 2,300 years ago. Earlier authors, including Carl Linnaeus, placed colonial ascidians in Zoophytes, a compound group that contained many different unrelated taxa, while the solitary ascidians were regarded as members of the phylum Mollusca. This view was based mostly on a wrong opinion, that the tunic, or the test, covering the body of adult ascidians is a modification of the calcareous shell of mollusks; furthermore, solitary ascidians and bivalve mollusks both are filter feeders and have two siphons, intruding and extruding. In 1816 J. C. Savigny first recognized the common nature of the solitary and colonial ascidians, as well as some pelagic forms, and in the same year J. B. Lamarck created the group Tunicata. He believed that it was a distinct class between Alcyonaria and Vermes, although other authors still treated this group as a class Molluscoides of Mollusca. From 1867 to 1872 the Russian embryologist A. Kowalevsky published several works on larval development and morphological characteristics of ascidians and was the first to recognize the close relationship between ascidians and chordates. Kowalevsky showed that the development of ascidians was similar to that of lancelets, a small group of primitive chordate animals now placed in the subphylum Cephaolchrodata of the phylum Chordata.

There are three important features linking ascidians and other tunicates with the chordate animals. First there is the presence of a notochord, a rod of specialized cells in the tail of ascidian larvae. The notochord disappears during the metamorphosis from free-swimming larva to sessile adult ascidian. In lancelets it is well developed in adults. In adult vertebrates the chorda dorsalis is wholly retained only in some fishes or is represented by remnants between the vertebrae. Second, a dorsal hollow nerve chord also is present only in ascidian larvae and disappears in adults. It lies dorsal to the notochord, as in chordates, whereas it is always ventral in other phyla of invertebrate animals. Third, pharynx perforations, or gill slits, known in Ascidiacea as "stigmata," are present in all chordates, at least during embryonic development, but such structures are never present in invertebrate animals.

Class Ascidiacea belongs to the subphylum Tunicata and is divided into two orders: Enterogona (with two suborders, Aplousobranchia and Phlebobranchia) and Pleurogona (with one suborder, Stolidobranchia). The definition of the orders is based entirely on embryonic characters and reflects the origin of the atrial cavity—from a single or paired dorsal invaginations. For practical purposes, taxonomists use mostly suborders, saying that the ascidians are divided into three main groups, Aplousobranchia, Phlebobranchia, and Stolidobranchia. This subdivision is based mostly on the morphological features of adults, in particular, the structure of the branchial sac, the position of the gut, and some other features. This subdivision does not reflect colonial organization: Stolidobranchia and Phlebobranchia contain mostly solitary species but also include colonial species, and most Aplousobranchia are colonial, but some species are solitary.

There are 24 families, and, at present, about 185 (from about 240 described) genera are treated as valid. The number of valid species is more difficult to count; the estimated number is 2,500–2,800. The interesting peculiarity of the ascidian classification is the relatively small number of genera in relation to the number of species. More than half of all known species belong to 10 very large genera listed here in decreasing order of number of species: Aplidium, Didemnum, Molgula, Polycarpa, Ascidia, Styela, Eudistoma, Pyura, Cnemidocarpa, and Synoicum. Most genera are well separated and can be identified easily, but species identification is often much more difficult. In general, the current ascidian classification seems to be close to the ideal, or "natural," system; that is, it appears to reflect the real phylogenetic relationships between taxa.

Physical characteristics

Usually, ascidians are described as sessile organisms with sac-like bodies firmly attached by the posterior end to the substratum. In reality, the shape of different ascidian species may be very diverse, and sometimes it is even difficult to recognize them as belonging to Ascidiacea. Although they grow side by side, is hard to imagine that the large, upright, cylindrical, solitary Ciona species belong to the same order as the Didemnum, which look like thin, encrusting stones and resemble a sponge colony.

The body is always covered with the test, a protective layer of cellulose-like material secreted by the epithelium. The test may be clear and often brightly colored, as in the spectacular Pacific species Halocynthia aurantium, or it may be covered by various kinds of spines, as in the northern Boltenia echinata; it even may contain calcareous spicules, as in snow-white Bathypera ovoida. Among species living on a sandy or muddy bottom, the test often has long, thin outgrows, or test hairs, and sometimes is covered by a dense layer of sand grains, making the specimen cryptic, like many species of the family Molgulidae.

The body muscles typically are arranged in transverse and longitudinal bands. Sometimes they are numerous, thick, and strong, forming a solid muscular wall; in many species, however, the muscles are represented by only a few thin fibers. There are two openings on the body: an oral or branchial opening and an atrial opening. These openings may be sessile or set on the ends of siphons of various lengths, the atrial siphon always dorsal to the branchial siphon. The oral siphon leads to the voluminous pharynx, called in ascidians the "branchial sac," The branchial sac occupies most the space in the body of solitary ascidians. The space inside the branchial sac is the branchial cavity, and the space between the branchial sac and the body wall is the atrial cavity. The atrial cavity opens to the exterior through the atrial siphon, and the branchial and atrial cavities are filled with seawater. The seawater, with food particles and oxygen, is drawn into the branchial sac through the branchial siphon and a circle of oral tentacles and passes from the branchial to the atrial cavity through numerous perforations in the wall of the branchial sac, where food items are sieved; then filtered water moves out from the body through the atrial siphon.

Perforations of the wall of the branchial sac generally are small and have ciliated margins called stigmata, The shape of the stigmata is an important taxonomic character: they may be straight and arranged in transverse or sometimes longitudinal rows, or they may be spiral figures, sometimes forming high funnels protruding into the branchial cavity. The wall of the branchial sac has transverse and longitudinal branchial vessels crossing each other and forming rectangular meshes. In some deepwater ascidians true stigmata are absent, the wall of the branchial sac is reduced, and the branchial sac is represented only by wide rectangular meshes formed by crossing longitudinal and transverse branchial vessels. The branchial sac is bilaterally symmetrical, with its mid-dorsal line marked by a fold termed the "dorsal lamina." The midventral line has an endostyle, a groove lined with ciliated glandular epithelium secreting mucus. The mucus constantly moves from the endostyle to the dorsal lamina and then, with the filtered food particles, to the esophagus, to which the bottom of the branchial sac opens. The esophagus typically is short and always is much narrower than the branchial sac. It leads to the stomach and then to the intestine. The intestine makes a loop and opens into the atrial cavity.

The position of the gut loop in relation to the branchial sac is an important taxonomic character of the suborder and family levels: the gut loop may be on the left or, in one family, on the right side of the branchial sac; between the branchial sac and the body wall; or, mostly in colonial ascidians, under the branchial sac. Gonads are hermaphroditic and situated in the gut loop, under the gut, or on the body wall on the sides of the branchial sac. Gonoducts always open into the atrial cavity, and only in one highly specialized genus do they penetrate the test and open directly to the exterior. The nervous system, as in all sessile animals, is simple and represented by an elongated ganglion situated on the dorsal side between the siphons and a few nerves running from it. The heart is a thin-walled tubular organ on the ventral side of the body or, in colonial ascidians, in the bottom of elongated zooids.

In the case of colonial ascidians, the test forms the so-called common test, a mass in which individuals, called "zooids," are completely or partly embedded. The general plan of the structure of the zooids is the same as in solitary ascidians, but details may differ significantly. The body may be undivided or divided into two (thorax and abdomen) or three (thorax, abdomen, and post-abdomen) regions. The thorax contains the branchial sac, the gut loop is in the abdomen, and the gonads are either in the abdomen or the post-abdomen. Branchial siphons of all zooids in a colony open directly to the exterior, but atrial apertures in many species open into the cloacal cavity within the common test. The colony may contain single or several isolated cloacal cavities, each exposed to the exterior through one or several openings. Zooids connected with one cloacal cavity form a system. The shape of the systems varies; zooids may be arranged in circular systems around the single cloacal opening in the center, or they may form long double rows along cloacal canals. The form of the systems usually can be recognized easily on living colonies; the systems make a characteristic pattern on the surface of the colony, which, in certain cases, helps to identify the species.

The size of most ascidians varies from 0.04 to 0.4 in (1–10 mm), rarely 0.6 in (15 cm). Certain species, however, are much larger; in favorable conditions some solitary species may reach 19.7 in (50 cm) in height, and thin, encrusting colonies of certain species of didemnids grow to 9.8 sq ft (3 sq m). The largest species is the colonial Antarctic Distaplia cylindrica; its long, sausage-shaped colonies may be up to 23 ft (7 m) in length and 3 in (8 cm) in diameter. The smallest species is the deepwater Minipera pedunculata, whose diameter is only 0.02 in (0.5 mm).

Many ascidians are brightly colored, usually red, brown, and yellow or, rarely, blue. The coloring of colonial ascidians is especially diverse: that of the zooids and the common test may be different, resulting in complex, often very beautiful patterns on the colony surface. Some species, such as Clavelina, have a transparent body, often marked with variously colored spots and lines.

Many ascidians have a peculiar feature: they are able to accumulate vanadium. More primitive species tend to have higher levels of vanadium in their tissues.

Distribution

Ascidians are distributed widely in all oceans and seas, from the intertidal zone to the abyssal depths. In exceptional cases, species can survive in brackish or freshwater. Several species have expanded outside their original range and propagated all over the world. For example, Molgula manhattensis, naturally distributed on the Atlantic coast of North America from Maine to Louisiana, was found in large numbers on the Pacific coast in San Francisco Bay in the 1950s, and later it was introduced into Japan on ships' hulls. In 1975 it was recorded in Australia, and in 2000 it was recorded in large numbers on the Russian coast of the Sea of Japan.

Habitat

Ascidians cover a wide range of benthic habitats. Most species require a hard substratum and firmly attach themselves to rocks, stones, shells, algae, and so on. Other species are adapted for living on soft, muddy or sandy bottoms; such species are not firmly attached but often have thin hairlike outgrows on the test, anchoring them in the mud. There is a group of several interesting interstitial species: these rarely recorded minute species live between gravel grains; some of them are not fixed and can move. Ascidians occur from the intertidal zone to abyssal depths and have been recorded in all deepwater trenches; the maximal recorded depth is 27,560 ft (8,400 m). Most species inhabit shallow waters, some of them, such as the thin calcified colonies of certain didemnids, can survive on the open shores under strong wave actions.

Behavior

Ascidians are sessile, usually firmly fixed to the substratum. In response to an external stimulus, they can only slowly contract the body and close the siphons. Some of the very few interstitial species can actively move between gravel particles. Very slow movement of the colony also has been recorded in one or two tropical colonial species. Abyssal Situla and Megalodicopia species can quickly close their enormously large, bilobed branchial siphons in attempt to catch small swimming invertebrates, such as copepods.

Feeding ecology and diet

Almost all ascidians are filter feeders. The cilia lining the stigmata on the pharyngeal wall constantly pump water with suspended organic particles through the branchial sac. This feeding method is called active filtration: an ascidian expends energy to create a steady flow of water through the branchial sac. With the filtration membrane, a constantly moving mucous sheet on the inner surface of the branchial sac, ascidians are able to capture very fine organic particles including bacteria and phytoplankton. The species living in water with higher concentrations of suspended particles usually have more complex and dense branchial sacs; the stigmata in these species typically are small and numerous, and high branchial folds increase the branchial surface.

At great depths, water contains very little organic matter. Ascidians having the usual type of branchial sac, with ciliated stigmata, are very small in size there. Some deepwater species, such as Culeolus, have lost true ciliated stigmata; the branchial sac in such species resembles a loose net made by the crossing vessels, without tissue between them. These branchial sacs have less resistance to water flow than do the dense filters of shallow-water ascidians. These species typically have very large, widely opened branchial siphons oriented to the water current, allowing so-called passive filtration: water passes through the branchial sac without any muscular action or beating of the cilia, thus saving energy. Such highly adapted abyssal species often are large. Ascidians of the family Octacnemidae have a mixed diet. Some of them apparently can catch small swimming invertebrates with their large, bilobed branchial siphons.

Reproductive biology

All ascidians are hermaphroditic. All colonial species of the suborder Aplousobranchia are viviparous; in these species ova are fertilized internally and developed within the parent colony, in the atrial cavity of zooids or in special outgrowths of the zooid body wall, termed brood pooches, or in the common test of the colony. Viviparous species release swimming larvae. Solitary species are either viviparous or oviparous, releasing ova. In viviparous solitary species, larvae are incubated in the atrial cavity. Ascidian larvae are called tadpole larvae. Larvae have an oval trunk, usually less than 0.04 in (1 mm) long, and a tail that is longer than the trunk. The larval trunk contains the larval organs, including the cerebral vesicle with a light receptor and statocyte and adhesive papillae on the anterior end, and a rudiment of the future adult. In many colonial species this rudimentary ascidian within the larval trunk is rather well developed and resembles a zooid of the adult ascidian. Sometimes, as in colonial Diplosoma, the larval trunk may contain two or more future zooids. Ascidian larvae never feed; they swim for a short time and then attach to the substratum and start metamorphosis. During metamorphosis all larval organs degenerate, and an ascidian develops from the rudiment located in the larval trunk. In the case of solitary ascidians only one specimen develops from each larva, whereas in colonial ascidians individuals (zooids) replicate themselves by asexual reproduction (budding) and form a colony.

Conservation status

No species of Ascidiacea are listed by the IUCN.

Significance to humans

Ascidians have no direct and important significance to humans. Like many other marine organisms, they are a potential source for bioactive chemicals that may be used in pharmacology. Also, several species are consumed as food in some countries, for example, Japan. Some species may cause problems in fish farming operations, growing on the bottoms of ships and negatively affecting operations such as mussel and oyster cultivation.

Species accounts

Resources

Kott, P. "The Australian Ascidiacea." Part 1, "Phlebobranchia and Stolidobranchia." Memoirs of the Queensland Museum 23 (1985): 1–440. ——. "The Australian Ascidiacea." Part 2, "Aplousobranchia (1)." Memoirs of the Queensland Museum 29, no. 1 (1990): 1–226. ——. "The Australian Ascidiacea." Part 3, "Aplousobranchia (2)." Memoirs of the Queensland Museum 32, no. 2 (1992): 375–620. ——. "The Australian Ascidiacea." Part 4, "Aplousobranchia (3), Didemnidae." Memoirs of the Queensland Museum 47, no. 1 (2001): 1–407.

Van Name, W. G. "The North and South American Ascidians." Bulletin of the American Museum of Natural History 84 (1945): 1–476.

[Article by: Karen Sanamyan, PhD]

A class of Tunicata which occurs as solitary zooids or, by a process of asexual budding, develops into colonies.

Zooids vary in length from about 0.1 to 10 in. (a few millimeters to 25 cm). Individuals or colonies are invested by a protective covering, the tunic or test, made of polysaccharide material structurally close to cellulose. Beneath the test is the body wall or mantle. Each zooid has two apertures: inhalant (oral) and exhalant (atrial). Water currents, created by cilia on the margins of stigmata in the pharyngeal wall, draw water into the branchial sac, where it is filtered and passed out through the exhalant aperture. Digestive enzymes are secreted into the stomach, and a pyloric gland, of unknown function, enters at the junction of stomach and intestine. Gonads are hermaphroditic, and may be situated in the loop of the intestine or in the mantle wall.

Three orders of ascidian are recognized. Ascidians occur throughout the seas of the world and at all depths, including the abyssal region. Most species feed on minute particulate matter, but a few are carnivores and engulf small zooplankters. See also Tunicata.