Trididemnum pusillum, Kott, 2004

Kott, Patricia, 2004, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (part I), Journal of Natural History 38 (19), pp. 731-774 : 764-765

Trididemnum pusillum sp. nov.

( figure 18 )

Distribution. Type locality: Western Australia (W of Port Hedland 40 m, coll. T. S. Ward, 5 December 1982, holotype NTM E32).

Description. The newly recorded colonies are fleshy plates, spicules being present at the surface but sparse elsewhere, although a thin layer lines the common cloacal cavity. The posterior abdominal common cloacal cavity isolates zooids from the fleshy basal or central test (in some elevated domes or ridges). Spicules are stellate to 0.08 mm diameter, with 9–11 conical rays (sometimes with chisel-shaped tips) in optical transverse section.

Zooids are small, but have a long thin retractor muscle, black squamous epithelium and an endostylar pigment cap. Branchial siphons have six pointed lobes around the aperture and atrial siphons are postero-dorsal, with a plain rimmed aperture. Eight coils of the vas deferens surround the undivided testis. Larvae are not known.

Remarks. The particularly small zooids and the spicules resemble those of T. vahaereere Monniot and Monniot, 1987 , although chisel-shaped tips have not been reported on the spicule rays of either T. vahaereere or the related T. tomarahi Monniot and Monniot, 1987 . Both these species are further distinguished from the present one by their brittle colonies with spicules crowded throughout and the absence of posterior abdominal common cloacal cavities.

Other Trididemnum species with spicules mainly in a surface layer and lining a posterior abdominal common cloacal cavity are the tropical T. areolatum (Herdman, 1906) with more spicule rays, the temperate T. amiculum Kott, 2001 with larger spicules to 0.1 mm and larger zooids, and T. nobile Kott, 2001 with large complex colonies and larger zooids.

Figures

FIG. 18. Trididemnum pusillum (NTM E32): (A) thorax; (B) abdomen; (C) testis; (D) larva; (E) spicules. Scales: (A–D) 0.1 mm.