Class ASCIDIACEA

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Class ASCIDIACEA

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P. Kott, Queensland Museum, Brisbane, Queensland, Australia

Introduction

The Ascidiacea, the largest class of the Tunicata, are fixed, filter feeding organisms found in most marine habitats from intertidal to hadal depths. The class contains two orders, the Enterogona in which the atrial cavity (atrium) develops from paired dorsal invaginations, and the Pleurogona in which it develops from a single median invagination. These ordinal characters are not present in adult organisms. Accordingly, the subordinal groupings, Aplousobranchia and Phlebobranchia (Enterogona) and Stolidobranchia (Pleurogona), are of more practical use at the higher taxon level.

In the earliest classification (Savigny 1816; Milne-Edwards 1841) ascidians-including the known salps, doliolids and later (Huxley 1851), appendicularians-were subdivided according to their social organisation, namely, solitary and colonial forms, the latter with zooids either embedded (compound) or joined by basal stolons (social). Recognising the anomalies this classification created, Lahille (1886) used the branchial sacs to divide the group (now known as Tunicata) into three orders: Aplousobranchia (pharynx lacking both internal longitudinal vessels and folds), Phlebobranchia (pharynx with internal longitudinal vessels but lacking folds), and Stolidobranchia (pharynx with both internal longitudinal vessels and folds). Subsequently, with thaliaceans and appendicularians in their own separate classes, Lahille's suborders came to refer only to the Class Ascidiacea, and his definitions were amplified by consideration of the position of the gut and gonads relative to the branchial sac (Harant 1929).

Kott (1969) recognised that the position of the gut and gonads are linked with the condition and function of the epicardium. These are significant characters and are informative of phylogenetic relationships. However, although generally conforming with Lahille's orders, the new phylogeny cannot be reconciled with a too rigid adherence to his definitions based solely on the branchial sac. Definitions of Lahille's taxa have been adjusted to confer the flexibility needed to accommodate the various evolutionary trends evident in this diverse group of animals. The presence of uninterrupted longitudinal branchial vessels in Ciona and some diazonids indicates a relationship with Phlebobranchia but is not the only indicator of affinity at subordinal rank. Rather, internal longitudinal branchial vessels are primitive, occurring in all suborders. They tend to be reduced in numbers with size reduction and simplification and are lost altogether in Aplousobranchia, in which evolution is dominated by replication, size reduction and simplification (Kott 1990a).

Ascidians have evolved in two major lines, one (Aplousobranchia) primarily associated with replication and the development of colonial systems, progressive size reduction and simplification of zooids, increasing colony organisation and the relatively small ovaries associated with internal fertilisation and viviparity. In the other line (Phlebobranchia and Stolidobranchia) evolution generally is associated with growth of the individual (rather than development of colonies), sophistication of various organs which enhance the efficiency of solitary individuals (including digestive glands) and the large ovaries associated with external fertilisation and oviparity. Occasionally replication and a colonial habit also occur in certain families in the latter two suborders, and when this occurs, the adaptive changes (including size reduction of zooids, development of colonial systems, internal fertilisation and viviparity) parallel the changes associated with colony development in Aplousobranchia. In the most highly organised colonies (in Stolidobranchia as well as Aplousobranchia), the replicate zooids are entirely embedded in common test material and their excurrent apertures open into large internal cloacal spaces (canals and cavities). In less well organised colonial species (in all suborders), the excurrent apertures open directly to the exterior (as they do in solitary forms). In less well developed colonies in all suborders, zooids are only partially embedded or they are separate and connected only by basal stolons.

The only exceptions to these generalisations are, in Aplousobranchia, the primitive and solitary Ciona intestinalis and some taxa in Diazonidae that form simple colonies which are fertilised externally; and in the invariably solitary stolidobranch genera Polycarpa (Styelidae) and Molgula (Molgulidae), a few species are internally fertilised and brood their embryos.

This subordinal phylogeny and classification followed in this work is based on Kott's revisions (Kott 1985, 1990a, 1992a, 2001). In Aplousobranchia (including the Cionidae and Diazonidae), the gut loop and gonads are posterior to the pharynx; and persistent endodermal epicardial sacs generally participate in regeneration of tissues, especially in replication. Only in clavelinids does mesodermal tissue generate replicates. In Phlebobranchia and Stolidobranchia, the gut loop and gonads are in the parietal body wall beside the pharynx; and the epicardium is modified for an excretory role, replication (when it occurs) being effected by the ectoderm.

The Australian Ascidian Fauna
The known Australian ascidian fauna numbers about 700 species, although this number is expected to increase as new species are found. The diversity of the fauna is greater than is known for other geographic areas of comparable size (Kott 1985, 1990, 1992, 2001)). Several factors may explain this, namely:

- The northern Australian coast is in the middle of the vast Indo-West Pacific tropical coralline region. The Australian continental shelf fauna acquires recruits and gains genetic diversity from that region.

- The Australian continent forms a bridge between the tropical and temperate waters and provides a diversity of habitats for colonisation and speciation.

- The temperate waters of the southern coast contain Gondwanan elements and recruits from the Southern Ocean, as well as the indigenous species that appear to be isolated in these habitats.

The oviparous larvae of solitary species are free-swimming for periods of one to 24 hours, and prior to hatching have been floating free in the water for up to three days (Anderson et al. 1975). Accordingly, they are subject to dispersal which could contribute to gene flow and remove risks of isolation. However, dispersal could also affect population maintenance, reducing the success of fertilisation for these sessile organisms. The viviparous larvae of colonial species are seldom free-swimming for more than one hour (Berrill 1950) and failure to effect recruitment to neighbouring populations could be the cause of isolation of populations in temperate rocky coastal habitats. The profusion of suitable habitats in coral reef environments may prevent such isolation in the tropics.

In Australian waters, most of the known ascidian families are well represented relative to their diversity in other parts of the world, although certain families, especially colonial ones, appear to have radiated more than others. Of particular interest is the family Holozoidae which is represented by a number of genera that only occasionally are represented in waters north of the equator. Some of these are diverse (Sigillina Savigny, 1816, Hypodistoma Tokioka, 1967 and Sycozoa Lesson, 1832) and others are not so diverse (Hypsistozoa Brewin, 1953, Neodistoma Kott, 1990 and Polydistoma Kott, 1990). In this family, only the genus Distaplia Della Valle, 1881 is cosmopolitan. The polycitorid genus Eudistoma Caullery, 1909 and two polyclinid genera, Aplidium Savigny, 1816 and Polyclinum Savigny, 1816, are particularly well represented. Some taxa are restricted in both diversity and range, e.g. in the Clavelinidae the monotypic Nephtheis fascicularis (Drasche, 1882) is indigenous to the West Pacific tropical region. The family Didemnidae is especially diverse, with a rich temperate indigenous fauna as well as a component of tropical species from the wider Indo-West Pacific (see Kott 2005).

Diversity in growth-form and choice of habitat occurs in most ascidian families and even genera. For example, a suite of species in the genus Clavelina Savigny, 1816 demonstrates a range of different levels of colony organisation including solitary species, species with individuals joined basally, and others with the zooids entirely embedded in the common test. The related cosmopolitan family Pycnoclavellidae displays a similar range in colony form and has a range of species in Australian waters which exploit a similar variety of habitats. Other aplousobranch taxa with prolific replication, probably rapid colony growth that is not constrained by the existence of cloacal systems (namely, Ritterellidae, Pseudodistomidae, Protopolyclinidae and Euherdmaniidae), have a range of different colonies from solitary zooids to branching, plate-like or compact colonies and also are found in a range of habitats.

In the Phlebobranchia, Ascidia Linnaeus, 1767 contains numerous and diverse species, and some (e.g. Ascidia thompsoni Kott, 1952 and Ascidia scaevola (Sluiter, 1904)) are unusual in having sand embedded in the test. Ascidia scaevola is especially well adapted to a sandy sea floor habitat, as are many species of the Agneziidae which use their body muscles to close folds of test across their apertures. The Plurellidae, one of the few western Pacific indigenous families also show remarkable adaptations for sandy sea-floor habitats by having the gonads in a projection from the body wall that is actually embedded in the sandy test.

In the Stolidobranchia, which usually have a characteristically tough, leathery test, a number of stalked species in Styelidae and Pyuridae favour habitats where currents change direction. The pachydermatina group in the genus Pyura Molina, 1782, includes species with especially long, narrow stalks. It has one representative in New Zealand, but other species are indigenous in Australian temperate waters. However, temperate Australian species are often indigenous possibly having speciated following isolation from their tropical sister groups. This occurs also at genus level, in the case of the genus Polycarpa Heller, 1877, which is conspicuous in Australian temperate as well as tropical waters. It appears to have tropical affinities, and does not occur south of the subtropical convergence.

Investigations on the Australian Ascidian Fauna
The early exploration of the Australian ascidian fauna was by European-mounted expeditions visiting these waters. The French corvettes la Coquille (1822-25), Uranie and la Physicienne (1817-20), and the Astrolabe (1826-29) and the British Challenger (1873-76) expeditions took material from temperate as well as tropical Australian waters (Lesson 1830-32; Quoy & Gaimard 1824, 1833, 1834; Herdman 1882, 1886). Later, the Swedish Expedition (Dr E. Mjöberg) to north-western Australia (Hartmeyer 1919) and the German expedition to south-western Australia (Hartmeyer & Michaelsen 1928, Michaelsen 1930) sampled the fauna of the western side of the continent. Along the more populated eastern coast, the Australian Museum and the NSW Fisheries Research Vessel Thetis made collections reported on by Herdman (1899) and Herdman & Riddell (1913). Various collections from Australian locations made by Professor Schmarda (Heller 1878) also added significantly to the knowledge of the ascidian fauna, as did Danish expeditions to the western Pacific (including New Zealand and Australian waters) in the Dana, and later the Galathea, reported on by Michaelsen (1922, 1924) and Millar (1975).

The Australian tropical fauna, nevertheless, was largely unexplored until the second half of the 20th century, the only exception being the small collection made during the Great Barrier Reef Expedition (Hastings 1931). However, the Dutch exploration of Indonesian waters by the Siboga (Sluiter 1904, 1909), other works by Sluiter (1885, 1890, 1895) and Pizon (1908), as well as the American Albatross collection (Van Name 1918) from the Philippines, continuing European and Japanese exploration (Tokioka 1950-1975; C. Monniot 1987-1991; F. Monniot 1987-1995; C. & F. Monniot 1987; F. & C. Monniot 1996, 2001; Monniot et al. 1991; and Nishikawa 1984, 1986) and a report on the Fijian fauna (Kott 1981) have continued to reveal a diverse tropical western Pacific fauna containing many species with ranges that include tropical Australian waters.

Savigny's (1816) work on collections from the Red Sea, and Herdman's (1906) on a small collection from Sri Lanka indicating that the tropical Indian Ocean ascidian fauna has much in common with the western Pacific fauna were confirmed by the later reports on German collections from the western Indian Ocean of Hartmeyer (1912) and Michaelsen (1918a, 1918b, 1919, 1920) and more recent works of C. & F. Monniot and F. & C. Monniot (1997). Indeed, many tropical species have been shown to have a range from the West Pacific to the eastern coast of the African continent and the northern Australian fauna appears to be the middle of the geographic range of this Indo-West Pacific fauna.

Investigations on the Japanese fauna by Oka (from the end of World War I), and Tokioka (from the end of World War II), followed by Nishikawa, and a survey in the South China Sea (Kott & Goodbody 1982) demonstrate a temperate fauna not closely related to the Australian fauna, the northern limits of the tropical fauna generally being in the Tokhara Islands.

Indo-West Pacific affinities of the Australian tropical ascidian fauna were confirmed recently, with the study by Kott (2008) on a collection from the Arafura Sea off north-eastern Arnhem Land in northern Australia.

Indigenous studies of the Australian ascidian fauna began after World War II (Kott 1952, 1957 et seq.). Before that, it is estimated, only about one fifth of the currently known Australian ascidians had been described. The taxonomy and biology of the Australian fauna are gradually being revised (Kott 1985; 1990a, b; 1992a, b; 1998; 2001; 2002a-c; 2003; 2004a-c; 2005).

Type Specimens of the Australian Ascidian Fauna
The type material that represents the initial (European) phase (which extended up to World War I) in the investigation of the Australian ascidian fauna is in European museum collections. In Paris (Muséum National d'Histoire Naturelle), some (though not all) of the type material of Quoy and Gaimard is retained, and also the modern collections of C. and F. Monniot. Unfortunately, none of Savigny's collection remains (Bouchet & Danrigal 1982). Collections from the Challenger Expedition and from Sri Lanka are in the Natural History Museum, London (BMNH)-formerly the British Museum (Natural History). Collections from north-western Australia (Hartmeyer 1919) are in the Swedish Natural History Museum. Large collections from south-western Australia (Hartmeyer & Michaelsen 1928) and Michaelsen (1930) are held in the Zoologisches Museum Hamburg and the Museum für Naturkunde, Berlin. Some South African material (Hartmeyer 1912) is in the Natural History Museum, Vienna. Material collected by Professor Schmarda and reported on by Heller (1878) is also in Hamburg and in the Natural History Museum, Vienna. Specimens from the Danish Dana and Galathea expeditions are in the Zoological Museum of the University of Copenhagen. The most comprehensive European collection of types relating to the Australian fauna is in the Zoological Museum of the University of Amsterdam, where most of Sluiter's collections from Indonesia are carefully curated and preserved (see Spoel, 1969). Station locations of the Siboga expeditions are recorded in Tydeman (1902). Pizon's (1908) types are held in the Muséum d'Histoire Naturelle, Geneva.

Japanese collections are held in the University of Tsukuba (Oka's material), in the Biological Laboratory of the Imperial Household of Japan (Tokioka 1953), in the National Science Museum (Tokyo) and in the Seto Marine Biological Laboratory (Tokioka 1949 et seq. and Nishikawa 1984, 1986: see Harada, 1991).

Although the material reported on by Herdman (1899) was lodged in the Australian Museum (Sydney), no other ascidian material of any significance was held in any Australian museum until after World War II — it was all in Europe. The established Australian State museums now hold significant collections of type and other material representing the ascidian fauna of each respective geographic region of the Australian coast. Types held in the Australian Museum (the NSW State Museum) are set out in Rowe & Marshall (1979).

In this Catalogue, the registration numbers in parentheses refer to a sampled portion of the preceding specimen held in another museum.

Published	As part of group	Action Date	Action Type	Compiler(s)
14-Dec-2012		14-Dec-2012	MODIFIED
		12-Feb-2010	(import)

Australian Biological Resources Study

Australian Faunal Directory