Didemnum multiampullae, 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 : 1184-1185

Didemnum multiampullae sp. nov

( Figures 14B ; 18A )

Distribution

Type locality: Western Australia CSIRO SS10 / 05 (Kalbarri, Stn 102, 113.311E 27.8134S, 96 m, 5 December 2005, holotype WAM Z27526 , QM G328057 ) .

Description

In preservative, colonies are thin white encrusting sheets with spicules crowded throughout. The stellate branchial apertures, lined with spicules, are evenly distributed over the surface and in some areas some small, pointed, spicule-filled surface papillae are scattered between the branchial apertures. Stellate spicules (to 0.08 mm diameter) usually have 15–17 sharply pointed conical rays in optical transverse section but a few have rounded ray tips and some are almost globular with rounded to flat-tipped rays.

Zooids are relatively small, although the branchial siphons are large and tulipshaped. About eight stigmata are in each half row. Often a large yellow egg is against the posterior aspect of the ventrally flexed gut loop and the distal coil of the vas deferens extends tightly over the surface of the egg. The vas deferens coils eight times around the undivided testis. A strong retractor muscle projects from the top of the oesophageal neck. A branched test vessel with terminal ampullae projects from the concavity created by the ventral flexure of the distal end of the gut loop.

Large larvae, the trunk 0.95 mm long with the tail wound about halfway around it, are being incubated in the holotype colony. A corona of small, cylindrical ampullae, about 20 on each side, surrounds the antero-median adhesive organs, the latter on short, thick, cylindrical stalks. Other structures were not detected in these mutilated specimens.

Remarks

This species has large larvae with especially numerous lateral ampullae and zooids with large branchial siphons, small and narrow atrial tongues and a large, yellow egg tightly applied to the posterior end of the gut loop with the vas deferens crossing over its outside surface. These characters can occur in species of the genus Polysyncraton as well as in Didemnum . However, the relatively large number of vas deferens coils in the present species is more often found in Didemnum than in Polysyncraton and it has three rows of stigmata in the larval pharynx and an undivided testis, both characteristic of Didemnum . Further, species of the genus Polysyncraton known to lack larval blastozooids and to have horizontal thoracic common cloacal spaces, numerous pairs of lateral larval ampullae and six or more coils of the vas deferens are readily distinguished from the present species by characters additional to the generic characters. In this regard, Polysyncraton dromide Kott, 2001 differs from the present species in the distribution of its spicules and P. millepore Vasseur, 1969 has fewer coils of the vas deferens. Polysyncraton palliolum Kott, 2001 has eight coils of the vas deferens (like the present species) but although its larvae are not known, it can be distinguished by the arrangement of its zooids along each side of the common cloacal canals that form a network in the surface test. Polysyncraton sideris Kott, 2001 also has numerous larval lateral ampullae but is distinguished by its larger spicules and other characters of its colony and zooids including the seven coils of the vas deferens.

In the genus Didemnum , D. ossium Kott, 2001 (a tropical species from north-western Australia and the Northern Territory and newly recorded from the north-western coast) and D. tapetum sp. nov. (a new species from the southern coast of Western Australia) have larvae with numerous lateral ampullae, spicules crowded throughout the colony and the same number of vas deferens coils as the present species. However, D. tapetum sp. nov. has almost globular spicules and D. ossium spicules are stellate but significantly smaller and with fewer rays than those of the present species.

Figures

Figure 14. Didemnum moseleyi (QM G328455): A, larva. Didemnum multiampullae sp. nov. (QM G328057): B, larva. Didemnum tapetum sp. nov. (QM G328016): C, abdomen showing gut loop and testis; D, larva. Didemnum velum sp. nov. (QM G328120): E, zooid; F, thorax showing lateral organs. Trididemnum grandistellatum sp. nov. (QM G328007): G, thorax; H, abdomen. Scales: A–H, 0.2 mm.

Figure 18. Scanning electron micrographs of didemnid spicules: A, Didemnum multiampullae, QM G328057: B, Didemnum ossium, QM G328055; C, Didemnum perplexum, QM G328034; D, Didemnum plebeium, QM G328451; E, Didemnum tapetum, QM G328016; F, Didemnum velum, QM G328120; G, Didemnum sp 1, QM G328160; H, Didemnum sp. 2, QM G328166.