Trididemnum nobile Kott, 2001

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 : 763-764

Trididemnum nobile Kott, 2001

( figure 17 B–D)

Trididemnum nobile Kott, 2001: 272 and synonymy.

Trididemnum vermiforme Kott, 2001: 289 and synonymy.

Distribution. New records: South Australia (Kangaroo I., SAM E2901–2, E2918); Tasmania (Triabunna, SAM E2904). Previously recorded (see Kott, 2001): Western Australia (Port Hedland, Cervantes); South Australia (Great Australian Bight, Sir Joseph Banks Is, Reevesby I. West I., Pearson I., Port Bonython, Gulf St Vincent, Investigator Strait, Willunga Reef); Victoria (Phillip I., Portsea, Lorne); Queensland (Caloundra).

Description. The colonies are the usual hemispherical masses with narrow branches fusing with other parts of the surface forming a complex reticulum enclosing secondary spaces lined with parts of the outer surface. The common cloacal cavity is vast, usually surrounding a central test core, but sometimes the colony lobes lack a central core and have an uninterrupted, open, central cavity. The secondary spaces inside the colony together with the large posterior abdominal cavities result in sponge-like colonies riddled with cavities of one sort or another. One specimen (SAM E2901) is aspiculate and translucent but the others have patches of sparsely distributed spicules in the surface and throughout the colony. Zooids, with black squamous epithelium especially on the anterior part of the thorax, show through the white spicules. Occasionally some sand is included in the test. Spicules are to 0.09 mm diameter, and have seven to nine conical and occasionally truncated rays in optical transverse section.

Zooids have 10 rectangular stigmata in the anterior row, and 10 coils of the vas deferens around the spherical undivided testis. A large circular lateral organ is opposite the dorsal end of the third row of stigmata. Larvae are in the test behind the zooids (in the central core when that is present). The trunk (0.65–0.7 mm long) has three long, slender spoon-tipped lateral ampullae each side of the three anteromedian adhesive organs. The concavity of the spoon is directed outwards. A long external horizontal ampulla projects back across the posterior end of the oozooid thorax on the left. Ten stigmata are in the anterior row of the three in the oozooid. A large spherical yolk mass is in the usual position for this genus, in front of the oozooid.

Remarks. Kott (2001) did not discuss the differences between aspiculate colonies of the present species and the other aspiculate species of the genus, T. pseudodiplosoma: Kott, 2001 . The latter species sometimes has complex, translucent colonies similar to the present species, but the larvae are dramatically different, those of the present species being simple without blastozooids and with only three pairs of ectodermal ampullae. Also, T. pseudodiplosoma has only eight coils of the vas deferens and lacks the extensive posterior abdominal cloacal cavities of the present species.

Trididemnum vermiforme Kott, 2001 has the same complex colony, same sized larvae with three pairs of larval ectodermal ampullae with spoon-shaped tips, similar zooids with dark squamous epithelium especially anteriorly, 10 coils of the vas deferens and stellate spicules with seven to nine conical rays in optical transverse section. Kott (2001) distinguished this species from T. nobile by its larger spicules and their presence throughout the colony and the presence of a central common cloacal cavity without a central test core. The larger spicules (0.075–0.09 mm diameter) were detected in one of the newly recorded specimens (SAM E2904). The presence of central common cloacal cavities and central test core is variable, as is the distribution of spicules through the colony. On re-examination of the type specimens of both T. nobile and T. vermiforme , 10 stigmata have been found in the anterior row in both larval and adult thoraces. Kott (2001) appears to have been mistaken in reporting eight for T. nobile .

Trididemnum caelatum Kott, 2001 has similar but larger larvae than T. nobile , with the same spoon-shaped lateral ampullae, but it has a thin sheet-like colony and more spicule rays.

Figures

FIG. 17. Trididemnum discrepans (WAM 183.93): (A) larva. Trididemnum nobile (SAM E2902): (B) thorax; (C) larva; (D) spicules. Scales: (A–C) 0.1 mm.