Subphylum TUNICATA

Tunicata

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Subphylum TUNICATA

Compiler and date details

7 February 2006 - P. Kott, Queensland Museum, Brisbane, Queensland, Australia

1998 - P. Kott, Queensland Museum, Brisbane

Introduction

The Tunicata are grouped with Hemichordata and Cephalochordata as the Protochordata. Phylogenetic relationships of the protochordate taxa are not close, but they share certain chordate characteristics and possibly an ancestry with the Chordata (Berrill 1950). Among the shared features are a perforated pharynx, and a dorsal and sometimes hollow nerve cord derived by invagination from the ectoderm. In Tunicata the larval or adult tail (when it occurs), and in Cephalochordata a rod of cells resembling the embryonic notochord of chordate animals suggest a closer affinity with Chordata than with the invertebrate phylum Hemichordata. Also, in Tunicata and Cephalochordata but not in Hemichordata a median ventral mucus-secreting groove in the pharynx is known to take up iodine like the vertebrate thyroid gland (Godeaux 1989). Tunicates differ further from Hemichordata in their unique heart and linear circulatory system, the heart reversing the direction of blood flow from time to time to ensure the even distribution of oxygen and nutrients (Heron 1973, 1975) and in having tailed (rather than ciliated) larvae. A proposal that the tunicate anterodorsal ciliated opening of the neural gland into the pharynx might be an homologue of the vertebrate anterior pituitary has been found to be invalid (see Goodbody 1974). Secretory cells are not present and the cilia create a current into the duct and to the neural gland as part of the mechanism controlling blood volume (Ruppert 1990).

The group Tunicata Lamarck, 1816 initially contained ascidians and salps. Appendicularians and doliolids were added later by Huxley (1851). Although Lamarck believed the Tunicata to be an independent class between Alcyonaria and Vermes, they were more often thought of as a class (Molluscoides) of the Mollusca (see Cuvier 1817, 1830) until Kowalevsky (1867) demonstrated the chordate characteristics evident in their embryos (see Herdman 1882). Herdman (1882) eventually divided the Tunicata into the three classes known today (see also Garstang 1896 and Fenaux 1993). These are the sessile Ascidiacea (sea squirts), and the planktonic Thaliacea (salps, doliolids and purse salps) and Appendicularia (larvaceans).

There are marked differences between the three classes of the Tunicata. The larval tail, lost on metamorphosis in the Ascidiacea and Doliolidae (Thaliacea) and suppressed altogether in other Thaliacea, persists through life in Appendicularia. In Ascidiacea and Thaliacea the ectoderm of the body wall synthesises a substance known as tunicin that is similar (chemically) to plant cellulose. It forms a test or tunic around the body composed of an acellular, proteinaceous matrix containing fibres and blood and test cells (see Goodbody 1974). Ectodermal vessels extend into it from the body wall and in colonial forms the test continues around ectodermal vessels that connect colonial zooids, or it forms a matrix in which zooids (as well as test vessels) are partially or completely embedded. Generally Appendicularia lack a tunicin covering. Instead, a balloon-like mucous house is secreted by special (oikoplast) cells in the trunk epithelium and is inflated around the whole or part of the animal by the beating of its tail. Thaliacea and Ascidiacea have an atrial cavity invaginated from the external surface (and therefore lined with ectodermal epithelium) into which the pharyngeal perforations (stigmata) open. In Ascidiacea this cavity surrounds the sides and dorsum of the perforated part of the pharynx, maintaining its connection with the exterior through the dorsal opening (atrial aperture). In Thaliacea the atrial cavity is at the posterior end of the pharynx and its aperture is at the opposite end of the animal to the mouth. In Appendicularia there is no atrial cavity and only a single pair of pharyngeal perforations open directly to the exterior.

With the exception of certain ascidians from abyssal depths that have evolved a carnivorous habit, all three classes of the Tunicata are prodigious filter feeders, straining large quantities of water passing through their mucous filters. In Ascidiacea and Thaliacea the microorganisms and organic particles from the continuous sheet of mucus secreted by the endostyle and perforated by micropores is moved up over the inner wall of the perforated pharynx by cilia and rolled into a mucous rod that moves along the dorsal midline into the oesophagus. The beating of the cilia lining the stigmata draws a current of water in through the mouth (incurrent aperture), through the mucous-sheet and the stigmata of the pharyngeal wall and out through the atrial cavity and the atrial (excurrent) aperture. In Appendicularia water is moved through special filters in the mucous house by the beating of the tail. Food caught in these filters is washed into the mouth by an incurrent stream of water generated by cilia lining the pharyngeal perforations (known as spiracles) and as in the two other tunicate classes, and the food, trapped in mucus secreted by the endostyle, is moved down into the gut (see Deibel 1998; Flood & Deibel 1998). With very few exceptions, tunicates are hermaphrodite, each individual having gonads of both sexes. Male and female organs mature at different times and normally they are not self-fertile (Ryland & Bishop 1993).

Recent reviews of aspects of the Ascidiacea are on physiology (Goodbody 1974), microscopic anatomy (Burighel & Cloney 1997) and biology and functional anatomy (Millar 1971; Kott 1985, 1990a, 1992a, 2001: Annotated Glossaries; Kott 1989). The biology of the Thaliacea and Appendicularia is reviewed in Bone (1998).

Ascidiacea is the most diverse class, with about 700 extant species in the Australian fauna. Thaliacea, although prolific and with a high biomass in coastal waters, are much less diverse, having less than 100 species worldwide. Appendicularia also are not diverse, with only about 60 species known worldwide. Most species of the planktonic Thaliacea and Appendicularia, carried by ocean currents, have wide geographic ranges that include most of the oceans of the world.

Table 1. Arrangement of higher level taxa followed in this work.

Class: APPENDICULARIA
Fritillariidae
Kowalevskiidae
Oikopleuridae
Bathochordaeinae
Oikopleurinae

Class: ASCIDIACEA
Order: Enterogona
Suborder: APLOUSOBRANCHIA
Cionidae
Clavelinidae
Diazonidae
Didemnidae
Euherdmaniidae
Holozoidae
Polycitoridae
Polyclinidae
Protopolyclinidae
Pseudodistomidae
Pycnoclavellidae
Ritterellidae
Stomozoidae

Suborder: PHLEBOBRANCHIA
Agneziidae
Ascidiidae
Corellidae
Perophoridae
Plurellidae

Order: Pleurogona
Suborder: STOLIDOBRANCHIA
Hexacrobylidae
Molgulidae
Pyuridae
Styelidae
Styelinae
Botryllinae
Polyzoinae

Class: THALIACEA
Doliolidae
Pyrosomatidae
Pyrosomatinae
Pyrostremmatinae
Salpidae
Salpinae
Cyclosalpinae

Acknowledgements

Much of the information on the Ascidiacea in this work is the result of the author's research on the class. However, for information on Thaliacea and Appendicularia she has relied on the work of Harold Thompson (1945); and on more recent publications and comments of Dr Robert van Soest of the Zoological Museum, University of Amsterdam (Thaliacea) and Dr Robert Fenaux of the Station Zoologique of the Université de Paris, Villefranche-sur-Mer (Appendicularia). She is especially grateful to Dr van Soest and Dr Fenaux for their advice.

In the published Catalogue, the illustrations used in the ascidian family introductions are reproduced from Kott (1985, 1990a, 1990b, 1992a, 1992). To maintain a consistent style, Figs 23-25 (Thaliacea) were redrawn from the original publications (as indicated) by F. Knight. Figure 26 (Appendicularia) was reproduced from Alldredge (1978), with permission.

Compilation of this section of the Zoological Catalogue of Australia database, including this edition, was partially funded by the Australian Biological Resources Study. The continuing support of the Director and the Board of the Queensland Museum, and the assistance of its administrative staff staff, are acknowledged with gratitude as is the assistance of the author's research assistant (under an ABRS grant for work on the taxonomy of the Australian Ascidiacea) whose contribution to the compilation of this revision has been substantial. The author is grateful also for assistance from the editorial staff of the ABRS, and the diligence of six anonymous referees whose constructive suggestions on the original database contributed to its accuracy and comprehensiveness.

Database Notes

The information on the Australian Faunal Directory site for the Tunicata is derived in part from the Zoological Catalogue of Australia database compiled on the Platypus software program. The file has been updated by Kott to include all published work to 27 September 2005, including Kott (2005).

Arrangement of Higher Level Taxa
In this work, families are treated in alphabetical order within their class and ordinal groupings as outlined in Table 1.

Geographic Scope
The Tunicata of the waters surrounding Macquarie Is., the Australian Antarctic Territory, Heard and McDonald Ils are not included in this work. Species known from these areas are treated in monographic works by Lohmann (1905, 1908), Lohmann & Buckmann (1926), Garstang (1933) Garstang & Georgeson (1935), Kott (1954, 1969, 1971), Millar (1960) Tokioka (1964), C. Monniot (1978), Monniot & Gail (1978) and Monniot & Monniot (1983).

Limital Area

Distribution data in the Directory is by political and geographic region descriptors and serves as a guide to the distribution of a taxon. For details of a taxon's distribution, the reader should consult the cited references (if any) at genus and species levels.

Australia is defined as including Lord Howe Is., Norfolk Is., Cocos (Keeling) Ils, Christmas Is., Ashmore and Cartier Ils, Macquarie Is., Australian Antarctic Territory, Heard and McDonald Ils, and the waters associated with these land areas of Australian political responsibility. Political areas include the adjacent waters.

Terrestrial geographical terms are based on the drainage systems of continental Australia, while marine terms are self explanatory except as follows: the boundary between the coastal and oceanic zones is the 200 m contour; the Arafura Sea extends from Cape York to 124 DEG E; and the boundary between the Tasman and Coral Seas is considered to be the latitude of Fraser Island, also regarded as the southern terminus of the Great Barrier Reef.

Distribution records, if any, outside of these areas are listed as extralimital. The distribution descriptors for each species are collated to genus level. Users are advised that extralimital distribution for some taxa may not be complete.