Trididemnum amiculum 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 : 760-761

Trididemnum amiculum Kott, 2001

( figure 16 )

Trididemnum amiculum Kott, 2001: 256 .

Distribution. New record: New South Wales (Cronulla, AM Z5137). Previously recorded (see Kott, 2001): Tasmania (Devonport); New South Wales (E of Coogee).

Description. The newly recorded colony is large, consisting of upright cylindrical to flattened lobes, up to 10 cm high, with white-rimmed, terminal common cloacal apertures. Dark pigment is in the surface test around the anterior end of each lobe. Adjacent lobes are joined to one another here and there along their contiguous surfaces or by flat connecting bridges. Large rigid common cloacal cavities and canals surround the central test core and form a sponge-like, three-dimensional network of posterior abdominal canals. A hard, supporting, branched skeleton of crowded spicules is in the very centre of the test core of each of the vertical lobes. Otherwise spicules are not crowded, although they are present throughout the colony. They are in the superficial layer of test on the upper surface and lining the common cloacal cavity and these surfaces feel raspy to the touch. Spicules are large (to 0.12 mm in diameter) with 9–11 pointed rays in optical transverse section.

Zooids are narrow, with black squamous epithelium, especially around the branchial siphons and the anterior part of the body. Branchial siphons are about onefifth of the total length of the thorax with six small points around the rim of the opening. The atrial siphon is short, generally laterally directed, at the posterior end of the thorax. Eight rectangular stigmata are in the anterior row in the branchial sac, the number reducing to six in the last row. A large circular saucer-shaped lateral organ is conspicuously depressed into each side of the body just antero-ventral to the base of the atrial siphon. A short tapering retractor muscle projects from the posterior end of the thorax. The testis is undivided and the vas deferens is coiled seven times around it.

Remarks. The colony resembles the holotype, except in the distribution of spicules, the rigid skeleton of spicules in the centre of each lobe was not detected in the holotype, nor were spicules found throughout the colony. In previously recorded specimens spicules are in a thick surface layer and a basal layer but are sparse elsewhere. Further, the branchial siphon is not as long in relation to the thorax as in the holotype, although this difference may be a result of contraction, or even an intraspecific variation, as the colony and other aspects of these narrow zooids (with the retractor muscle from the posterior end of the thorax) suggest that they are conspecific.

Kott detected only five of the vas deferens coils although seven are now known to occur in this species. The key character Kott (2001) used to separate this species from others in the genus (the length of the branchial siphon) is inappropriate, the length of the branchial siphon in the newly recorded colony demonstrating its variability.

Other species with complex colonies and fewer than 13 spicule rays in optical transverse section are T. nobile (which differs in its 10 vas deferens coils), T. sibogae (with larger spicules up to 0.16 mm diameter) and T. lapidosum, Kott, 2001 (with similar colonies and spicules also to 0.16 mm diameter but without posterior common cloacal cavities). The colonies superficially resemble those of Didemnum roberti Michaelsen, 1930 .

Figures

FIG. 16. Trididemnum amiculum (AM Z5137): (A) thorax; (B) testis; (C) spicules. Scales: (A, B) 0.1 mm.