Didemnum roberti Michaelsen, 1930

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 : 758

Didemnum roberti Michaelsen, 1930

Didemnum roberti Michaelsen, 1930: 516 ; Kott, 2001: 230 and synonymy.

Distribution. New records: Western Australia (Montebello Is, WAM 153.93; Port Hedland, WAM 205.87, 482.88, 214.90, 1170.89). Previously recorded (see Kott, 2001): Western Australia (Rottnest I., Shark Bay, Montebello Is, Bonaparte Archipelago, Dampier Archipelago, Cape Lambert), Northern Territory (Parry Shoals, Bathurst I., Gulf of Carpentaria), Queensland (Cockburn Reef).

Although often recorded from the north-western Australian coast, the species is not yet known from the north-eastern coast or the Great Barrier Reef; nor is it yet recorded from Indonesia.

Description. One of the newly recorded colonies (WAM 1170.89) has rounded lobes to 3 cm diameter with terminal common cloacal apertures protruding vertically from a basal sheet. Intense, irregular, black flecks of pigment are evenly scattered in the surface test except where they are crowded into a black circle inside the whiterimmed common cloacal apertures. The other colonies are large, irregular sheets to 4 mm thick, with sand, shell and weed debris on the base. Usually, spicules with the characteristic attenuated spiky rays are moderately crowded at the level of the branchial siphons, are sparse at thoracic level and form a thin layer lining the posterior-abdominal common cloacal canals. The basal test, including the central test core of the vertical lobes, is usually almost aspiculate, except for a thin layer on the base of the colony. However, in one of the newly recorded colonies (WAM 205.87) spicules are present throughout. The ray length / spicule diameter ratio is 0.375 and the maximum spicule diameter is 0.08 mm.

Zooids have moderately long branchial siphons, long and narrow thoraces, a narrow anterior atrial tongue, about nine stigmata in the anterior row and a long ribbon-like retractor muscle projecting from about halfway down the oesophageal neck. Abdomina are bent up behind the thorax. Gonads were not detected in the newly recorded material. Conspicuous pointed columnar cells project from around the thorax.

Embryos (including larvae) are in the newly recorded colony (WAM 1170.89) collected in October.

Remarks. This large, dramatic, conspicuous, relatively common and wellcharacterized species is known from the northern half of the western Australian coast, the southern part of the Timor Sea, the Gulf of Carpentaria and Cockburn Reef off the eastern coast of northern Cape York. It has not been found either further to the north in Indonesian waters, or to the south along the tropical coast of Queensland; and it has not been recorded from the Great Barrier Reef.