Pseudodistoma australe Kott, 1957

Kott, Patricia, 2008, Ascidiacea (Tunicata) from deep waters of the continental shelf of Western Australia, Journal of Natural History 42 (15 - 16), pp. 1103-1217 : 1130-1132

Pseudodistoma australe Kott, 1957

Pseudodistoma australis Kott 1957, p. 101 ; 1992a, p. 428 and synonymy; 2003, p. 625 and synonymy ( Pseudodistoma australis Kott, 1992 ; 1985 sic).

Distribution

Previous records (see Kott 2003): Western Australia (Exmouth Gulf to Cockburn Sound, off southern Australia to Port Phillip Heads); Tasmania (Forestier Peninsula); Queensland (Swain Reefs). New record: Western Australia CSIRO SS10 / 05 (Bald I., Stn 35, 157 m, 24.11.05, QM G328164 nine specimens; Bald I., Stn 39, 99 m, 25.11.05, QM G328103 ) .

An occurrence off the southern coast of Western Australia is consistent with the previously recorded range of this species, and is at the greatest depth yet recorded for this species.

Description

The colonies are upright, fleshy, translucent, naked cones to 12 cm, sometimes slightly irregular and branched. The basal half (to 3 cm diameter is only slightly narrower than the top and is one-quarter to three-quarters of the total length. Evenly distributed separately opening zooids are around the rounded head of the colony. Details of the structure of the zooids could not be determined, although short but broad thoraces, relatively narrow abdomina and long and narrow posterior abdomina can be seen. The identification of these specimens is provisional and is based on the form of the colonies, the absence of sand, the absence of common cloacal systems and the disposition of the zooids.

Family POLYCLINIDAE Milne Edwards, 1842

The family contains aplousobranch genera with gonads in the posterior abdomina of thread-like zooids, which are arranged in common cloacal systems, the atrial apertures opening into an internal common cloacal cavity. The relatively long, narrow, vertical gut loop consists of a long oesophagus and duodenum, and distinct sections of mid-intestine between the duodenum and posterior stomach and posterior stomach and rectum, respectively. The distal section of the mid-intestine is invariably in the pole of the gut loop and usually a rectal valve is at its junction with the long rectum that constitutes the ascending limb of the loop.

The most speciose genus is Aplidium Savigny, 1816 , distinguished by its longitudinally folded stomach wall and barrel-shaped stomach. Polyclinum , Aplidiopsis and Synoicum have characters in common, although Polyclinum displays significant differences from the other genera that may signify a polyphyletic family. The genera Polyclinum , Synoicum and Aplidiopsis are all distinguished from Aplidium by their lack of longitudinal gastric folds, similar shield-shaped stomachs with a smooth or mamillated wall and posteriorly directed strands of antero-dorsal and postero-ventral larval vesicles. Synoicum and Aplidiopsis are distinguished from Polyclinum by a long posterior abdomen, usually with testis follicles in a long single or double row, rather than the sac-like posterior abdomen with bunched testis follicles of Polyclinum . However Polyclinum and Aplidiopsis (but not Synoicum ) have a constriction separating the abdomen from the posterior abdomen (containing gonads) and the distal end of the gut loop is twisted, causing the gonoducts to loop around it. The proximal part of the ascending limb of the gut loop bends up in a loop crossing the distal part of the descending limb before extending anteriorly. The vas deferens is caught in this twist (see P. fungosum Kott, 1992a , Figure 25b).

Previously Polyclinum was the only genus of the Polyclinidae known to have branchial papillae on the transverse vessels. In the course of the present study these small protrusions have been found on the transverse vessels of S. rapum sp. nov. and S. intercedens ( Sluiter, 1909) . They are possible vestiges of the longitudinal branchial vessels usually present in taxa with larger zooids in what may be more primitive aplousobranch taxa (e.g. Cionidae , Diazonidae and some Protopolyclinidae ) and in the Phlebobranchia and Stolidobranchia .

The atrial siphon in Synoicum is of particular interest, demonstrating a minimal departure from the six-lobed aperture of Ritterella , a genus distinguished by its longitudinally folded stomach and separately opening six-lobed branchial and atrial apertures. The similarity of the atrial siphons of some Synoicum spp. and Ritterella is emphasized by a median papillum posterior to the atrial aperture that occurs in certain species of Ritterella , Synoicum and Polyclinum (see Ritterella compacta Kott 1992, p. 442 ).

However, the atrial apertures of Synoicum spp. display a great deal of variation. Sometimes this is as a result of contraction, but occasionally they may represent significant specific morphological characters. The simple, short siphon with a circular sphincter and almost smooth atrial rim of S. vesica sp. nov., S. durum Kott, 1992 and S. bowerbanki Millar, 1963 appear to be only marginally different from the condition observed in S. intercedens ( Sluiter, 1909) and S. suareum Kott, 1992 in which the aperture is produced well out from the zooid. A similar but more conspicuous extension of the atrial siphon has been observed in S. sacculum Kott, 1992a and in S. rapum sp. nov. The atrial apertures of S. implicatum sp. nov, S. castellatum Kott, 1992a , S. concavitum Kott, 1992a and S. erectum Kott, 1992a have significantly longer pointed bifid or trifid lips projecting from the anterior rim of the opening. Synoicum bucccinum Kott, 1992a and S. chrysanthemum Kott, 1992a have three or four similar long atrial lips arising from the anterior border of the aperture, but the opening itself is long and sessile, exposing much of the branchial sac to the common cloacal cavity. It appears to represent a consistent genetically controlled character shared by a small group of species in this genus. It differs significantly from excurrent openings in the majority of species in this genus, which are more restricted circular openings.

Synoicum spp. are diverse in the material collected from the western Australian continental shelf. The genus is represented by six species all with sandy colonies, of which three are new to science and another ( S. laboutei ) was known previously only from the West Indian Ocean. Of the species taken, S. chrysanthemum is readily distinguished by its stalked colony, double rows of zooids around zooid-free patches of test. Double series of zooids lining the sides of circular cloacal canals also occur in S. vesica sp. nov., S. longistriatum Kott, 1992a and S. macroglossum ( Hartmeyer, 1919) but are not as regular as in S. chrysanthemum , while in S. laboutei the zooids are on each side of straight canals that extend parallel to one another down the sides of a vertical conical colony. Synoicum rapum sp. nov. has turnip-shaped colonies with a thin hard layer of sand externally like S. vesica but its zooids appear to be arranged in circles around central common cloacal apertures like S. durum Kott, 1992a and, together with its pouched, mulberry-like stomach walls, this arrangement of zooids distinguishes S. rapum from others with a similar colony shape. The hard, external layer of sand in S. rapum and S. vesica sp. nov. is distinctive. Despite these distinctive characters that separate species in each genus from one another, there are aspects of the form of their systems and zooids shared by species in both Synoicum and Aplidium . As well as the absence of folds in the stomach wall, the form of the atrial siphon, with the anterior rim produced out into a conspicuous tongue, seemed to separate Synoicum from Aplidium (see Kott 1992a). Nevertheless, the latter character is not exclusive to Synoicum (occurring also in Aplidium spp. ) and is not always present (absent from S. macroglossum ). Examination of material in the present collection suggests that a clear separation between these genera on presently recognized characters is not readily sustained (see Synoicum pseudogrisiatum sp. nov., S. sphinctorum Kott, 2006 ). Many Aplidium spp. have five stomach folds and muscular atrial lips extending out from the body wall anterior to the siphon. However, another group of Aplidium spp. has a large atrial lip projecting out from the upper rim of the atrial opening that resembles the condition in Synoicum spp. Kott’s (1992a) statement that there is a difference in entry of the oesophagus into the stomach in Synoicum appears to be related to the state of contraction of the zooid and cannot be substantiated. Their similar larvae also suggest a close phylogenetic relationship between Synoicum and Aplidium . However, similarity in the form of colonial systems, and the adaptations of the atrial apertures associated with these systems, could be the result of convergence rather than a significant indication of affinity. The only reliable character presently known to distinguish these two genera is the condition of the stomach wall, careful examination sometimes revealing that what appear to be folds are artefacts caused by collapse of the stomach rather than true structural folds.