Didemnum fibriae, 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 : 750-751

Didemnum fibriae sp. nov.

( figure 11 A–D)

Distribution. Type locality: Western Australia (Woodman’s Point, Cockburn Sound, 18 m, grey mud; coll. L. Marsh and S. Slack-Smith, 20 February 1991, holotype WAM 175.91).

Description. The colony is an extensive firm, encrusting sheet. The test is fibrous with a felt-like consistency and generally is aspiculate, although here and there are small patches of spicules in the surface test around each branchial aperture, sometimes a sparse layer lines the common cloacal cavity, and occasionally spicules are distributed sparsely through the colony. Spicules are small (to 0.036 maximum diameter) with 11–13 not very compact, blunt-tipped rays in optical transverse section. The ray length / spicule diameter ratio is 0.2–0.25, the common cloacal cavity is three-dimensional, with deep primary canals occasionally becoming posterior abdominal separating clumps and rows of zooids from one another. Although zooids generally are crowded, common cloacal spaces penetrate amongst them at thorax level.

Zooids are small, with a cylindrical branchial siphon. A sessile, open, atrial aperture exposes the branchial sac directly to the common cloacal cavity. A long, tapering retractor muscle is present. About eight stigmata are in the anterior row, a large brown egg is present, and the vas deferens coils 11 times around the large, undivided testis. The gut loop is capacious. Larvae, present in the basal test, have a 1.25 mm long trunk with the tail wound halfway around it, six to eight pairs of ectodermal ampullae and a thoracic and an abdominal bud.

Remarks. The felt-like consistency of the test is unusual. Like D. jedanense Sluiter, 1909 , zooids and larvae are large, the latter with eight pairs of ectodermal ampullae and blastozooids, and colonies occasionally are entirely aspiculate. However, D. jedanense has more spicule rays and only eight coils of the vas deferens. Didemnum effusium Kott, 2001 , the other known aspiculate Didemnum , is recorded only from southern Australian locations, has a similar number of vas deferens coils (10) but its larva is smaller, has only four pairs of ectodermal ampullae and lacks blastozooids. The species is characterized by the large zooids and larvae, the form and distribution of its spicules, the fibrous test and the numerous coils of the vas deferens.

Figures

FIG. 11. Didemnum fibriae (WAM 175.91): (A) thorax; (B) abdomen; (C) larva; (D) spicules. Didemnum grande (NTM E36): (E) spicules. Scales: (A–C) 0.2 mm.