Polysyncraton calculum, Kott, 2008

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 : 1171-1172

Polysyncraton calculum sp. nov.

( Figures 16E, F )

Distribution

Type locality: Western Australia CSIRO SS10 / 05 (Bald I., Stn 35, 118.645E 35.197S, 157 m, 24 November 2005, holotype WAM Z27529 , QM G328140 ) .

Description

The colony is a thin encrusting one growing around a sponge. It is cream in preservative, the colour resulting from the yellow zooids. Common cloacal apertures are about 5 mm apart on slightly elevated rounded prominences on the upper surface. A large horizontal common cloacal cavity is at oesophageal level. The basal layer of the test is thin and contains abdomina and larvae. The large spicules (to 0.14 mm diameter), crowded throughout the colony, are stellate with seven to nine conical pointed rays in optical transverse section. Zooids are large, but in this specimen they appear to have been dried out. The large more or less rectangular thorax is evident and the branchial aperture is on a short tulip-shaped siphon with six pointed lobes around its rim. The ventral margins of the thoraces lie partially embedded in the surface layer of test while a wide open, sessile atrial aperture on the dorsum exposes most of the branchial sac to the oesophageal common cloacal cavity. A long atrial lip with a bidentate tip protrudes from the anterior rim of the aperture. The branchial sac has four rows of stigmata and about 10 stigmata are in each halfrow. A long, tapering retractor muscle projects from about halfway down the oesophageal neck.

Larvae, present in the basal test, have a trunk 0.76 mm long, the tail wound twothirds of the distance around it, four rows of stigmata in the pharynx of the oozooid and eight lateral ampullae on each side of the three antero-median adhesive organs. A large horizontal ampulla is on the left projecting back from just behind the adhesive array and a single thoracic blastozooid was detected.

Remarks

The specimen lacks the atrial siphon of Leptoclinides and Trididemnum and the fine club-shaped larval ampullae of Lissoclinum . However it has the retractor muscle of Didemnum and Polysyncraton and with four rows of stigmata in the larval oozooid it can be confidently assigned to Polysyncraton , despite the absence of gonads.

The large spicules are a conspicuous character of this species. They are similar to, but have slightly fewer rays and are significantly larger than, the spicules of P. pseudorugosum and P. lodix (which has less acutely pointed rays). Also, the former species is distinguished by its circular common cloacal canals and slightly quilted upper surface. Polysyncraton sideris Kott, 2001 has similar large, stellate spicules but is distinguished from the present species by the large vesicles that surround each zooid, more numerous larval lateral ampullae and the lack of a larval blastozooid. Similar large stellate spicules in P. tasmanensis Kott, 2001 are distinguished from the present ones by the occasional presence of chisel-shaped ray tips.

Figures

Figure 16. Scanning electron micrographs of didemnid spicules: A, Atriolum tubiporum, QM G328004; B, Leptoclinides durus, QM G328158; C, Leptoclinides fungiformis, QM G328444; D, Leptoclinides vesica, QM G328458; E, F, Polysyncraton calculum, QM G328140; G, Polysyncraton dealbatum, QM G328119; H, Polysyncraton fistulum, QM G328446.