Trididemnum reticulatum, 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 : 765-766

Trididemnum reticulatum sp. nov.

( figure 19 )

Distribution. Type locality: Queensland (Slashers Reef—central Great Barrier Reef, reef slope 4 m, coll. AIMS Bioactivity Group, 15 April 1995, holotype QM G308535).

Description. The colony is a thin (about 3 mm thick), hard sheet, orange in life but dirty beige in preservative owing to a slightly translucent superficial spicule-free bladder cell layer which is about one-tenth of the colony thickness. Beneath the bladder cell layer (at branchial siphon level), spicules are crowded throughout the colony. Except for a small plug of spicules in the lining of each branchial siphon, they are absent over the anterior end of each zooid. When seen from the surface, this interruption of the crowded spicules by the zooids appears as a network pattern beneath the superficial bladder cell layer. Otherwise there are no special features on this flat, smooth-surfaced colony. The common cloacal cavity is shallow, at oesophageal level. Spicules are relatively small (to 0.04 mm diameter), some globular to burr-like, with flat, rounded or irregular ray tips and others with thicker rays and rosette-like groups of small bumps on the broad, flat tips. Occasionally broken spicules appear to be hollow.

Contracted zooids are small (to about 1 mm long). Branchial lobes are short and sharply pointed. Short, cylindrical atrial siphons project from the middle of each thorax. A fine, tapering retractor muscle projects from halfway down the oesophageal neck. The gut forms a double loop. The pointed conical testis has eight coils of the vas deferens around it.

Larvae, in the basal test, have a trunk 0.5 mm long, with the tail wound about three-quarters of the way around it. Four club-shaped ectodermal ampullae are along each side of only two antero-median adhesive organs on long cylindrical stalks.

Remarks. The species is unusual in its globular and burr-like spicules which are seldom found in this genus, the exceptions being the spicules of T. miniatum Kott, 1977 and T. spumosum Kott, 2001 which are smaller (to 0.02 and 0.03 mm diameter, respectively). Both these species have different colonies from the present species, although T. miniatum does has a similar arrangement of its spicules, which are not present over the anterior ends of the zooids. However, its colonies are small, soft cushions and it has green cell symbionts embedded in the test. Larvae also are unusual in having only two adhesive organs (as in the small larva of T. cyclops , see Kott, 2001).

Figures

FIG. 19. Trididemnum reticulatum (QM G308535): (A) thorax; (B) abdomen; (C) larva; (D) spicules. Scales: (A–C) 0.1 mm.