Didemnum ossium Kott, 2001

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 : 1186

Didemnum ossium Kott, 2001

( Figure 18B )

Didemnum ossium Kott, 2001, p. 217 and synonymy.

Distribution

Previously recorded (see Kott 2001): Western Australia (Montebello Is, Bonaparte Archipelago); Northern Territory (English Company Is); New Caledonia; Philippines. New records: Western Australia CSIRO SS10/05 (Jurien Bay, Stn 82, 85 m, 2.12.05, QM G328055).

Description

In preservative, the pale pinkish-beige basal surface of the thin, brittle sheet-like colony is applied closely to the basal surface of a large colony of Polysyncraton millepore (QM G328033) forming a frond-like structure that probably stands up from the sea floor on its edge. The colonies appear to support each other, each with its fairly evenly spaced branchial aperture exposed to the exterior on opposite sides of the frond. The common cloacal cavity is a shallow horizontal cavity at thorax level. Sessile common cloacal apertures are fairly evenly spaced over each exposed surface. Spicules are crowded throughout the colony. Spicules are mostly stellate, to 0.05 mm diameter, with 11–15 relatively long pointed rays in optical transverse section, but a few are globular with flat-tipped rays.

Zooids are robust. Although they are contracted in this specimen, the branchial siphon is conspicuous and relatively long with six rounded lobes on the rim of the aperture. A wide sessile atrial aperture opens directly into the shallow, horizontal common cloacal cavity. A narrow anterior atrial lip, bifid at the tip, projects across in the roof of the common cloacal cavity. A retractor muscle projects from the upper part of the oesophagus. About eight stigmata are in each of the four rows. A large, undivided testis follicle has nine coils of the vas deferens around its outer surface.

The larval trunk is 0.9 mm long. It has about 16 pairs of finger-like lateral ampullae and a long horizontal lateral ampulla (about one third of the length of the trunk) projects back from behind the ring of ampullae on the left side of the trunk.

Remarks

The zooids of the present material are crushed and contracted but in life they probably were large and robust. A similar branchial siphon and atrial tongue, nine coils of vas deferens, crowded spicules with 11–15 conical rays and some globular spicules with flat-tipped rays, 17 larval lateral ampullae per side, and a large horizontal ampulla on the left are reported for the type material of this species. The hard colony surface, without an aspiculate superficial layer, is also distinctive.

Known tropical species with similar numbers of larval lateral ampullae but with fewer vas deferens coils are Didemnum elongatum (which has a similar branchial aperture and a short, narrow atrial lip like the present species but less robust zooids) and D. clavum which has different spicules. Didemnum paa C. and F. Monniot, 1987 and D. astrum Kott, 2001 have 10 coils of the vas deferens and fewer larval lateral ampullae than the present species. Didemnum chartaceum has aspiculate layers of test, more crowded conical spicule rays and other distinctive characters, although its larval ampullae and vas deferens coils are similar to the present species. Didemnum levitas Kott, 2001 has similar spicules and zooids but the centre of the colony is aspiculate and it has only seven coils of the vas deferens. Didemnum inveteratum Kott, 2001 from north-western Australia has a hard, rigid colony, shallow thoracic common cloacal canals with zooids along each side (rather than the shallow horizontal canals of the present species) and only five coils of the vas deferens. Didemnum multiampullae sp. nov. and D. tapetum sp. nov. described in the present work have similar numbers of lateral ampullae surrounding the anterior end of the larval trunk, but they are distinguished from the present species by their spicules, the former having larger spicules with more rays in transverse optical section and the latter having almost globular spicules.

Figures

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.