Didemnum tapetum, Kott, 2008

Kott, Patricia, 2008, Ascidiacea (Tunicata) from deep waters of the continental shelf of Western Australia, Journal of Natural History 42 (15 - 16), pp. 1103-1217 : 1188-1190

Didemnum tapetum sp. nov.

( Figures 14C, D ; 18E )

Distribution

Type locality: Western Australia CSIRO SS10 / 05 (Bald I., Stn 39, 118.623E 35.1791S, 99 m, 24 November 2005, holotype WAM Z27522 , QM G328016 ) .

Description

The colony is an extensive, thin, brittle sheet with spicules crowded throughout. Its maximum diameter is about 24 cm and the upper and the under surfaces are smooth and even. The undersurface is hard and white with some ripple marks. The upper surface of the preserved colony is a creamish-yellow colour, the colour becoming more intense around what may be regularly spaced sessile common cloacal apertures about 15 mm apart. An extensive, shallow, horizontal common cloacal cavity is at thorax level. At low magnifications the spicules are almost globular, usually to 0.055 mm diameter and occasionally to 0.08 mm, with short, conical to rounded or irregular-tipped rays crowded around the circumference of the spicules. At higher magnifications the spicules are seen to be burr-shaped with narrow rod-like rays with pointed to flat-tipped rays.

Zooids are robust, with a relatively large thorax, four rows of stigmata, a conspicuous branchial siphon, and a distinct retractor muscle. The vas deferens coils eight times around the large undivided testis. Larvae have an almost spherical trunk with the tail wound most of the way around it. A large corona of about 17 lateral ampullae per side surround the three antero-median adhesive organs, ocellus and otolith and three rows of stigmata are in the larval thorax.

Remarks

The short, crowded rays and relatively small and almost globular spicules of this thin, brittle colony are generally of similar size and form to the Indo-West Pacific Didemnum chartaceum Sluiter, 1909 which is distinguished from the present species by its superficial aspicular layer of bladder cells over a layer of spicules, lack of spicules through much of the remainder of the spongy, gelatinous colony, without the larger spicules found in the present species and with nine coils of the vas deferens around an undivided testis. The spicules are smaller and have more rays than those of D. arancium Kott, 2001 , which also is distinguished by having only six coils of the vas deferens. Polysyncraton scorteum Kott, 2001 has similar but larger spicules than the present species and fewer vas deferens coils and P. doboense Sluiter, 1913 from the Aru Is. has similar spicules, although they have fewer rays than the present species.

Larvae of the present species are relatively small, although they resemble the larvae of both Didemnum ossium and D. multiampullae (both described from the present collection) in the large number of lateral ampullae in the corona that surrounds the anterior end of the trunk.

The holotype is one of most extensive encrusting sheet-like didemnid colonies yet reported.

Didemnum velum sp. nov

( Figures 14E, F ; 18F )

Distribution

Type locality: Western Australia CSIRO SS10 / 05 (Albany, Stn 47, 118.2990E 35.3546S, 179 m, 25 November 2005, holotype WAM Z27528 , QM G328120 ) .

Description

The holotype is an encrusting sheet growing on a sponge. It has a conspicuous superficial aspicular bladder cell layer over a thin layer of spicules at the level of the short branchial siphons. A single layer of less crowded spicules is on the base of the colony. The remainder of the test between these two layers of spicules is completely aspicular. Spicules are small (to 0.03 mm diameter) with 15–17 rays in optical transverse section. They are of two types, either with pointed conical rays, or with rounded, blunt-tipped rays. A shallow, horizontal, common cloacal cavity is at thoracic level.

Zooids are small and contracted. The thorax is almost completely spherical. A small stalked lateral organ with the shallow concavity directed postero-ventrally projects slightly from about two-thirds of the way down each side of the thorax. The branchial siphon is a short cylinder with six small lobes around the margin of the opening. A sessile, horizontal atrial opening interrupts the dorsal surface of the thorax. A retractor muscle extends out into the test from about halfway down the narrow oesophageal neck. The four rows of stigmata have about seven stigmata in each half row. The gut forms a simple loop. The stomach is large with a thick epithelial lining. A relatively long duodenum opens into a small posterior stomach (in the loop of the gut) and is separated from the wide proximal part of the rectum by a short mid-intestine. The rectum extends anteriorly, gradually narrowing to a straight cylindrical tube. Gonads were not detected in this specimen. Well-developed thoracic oesophageal buds have four rows of long, oval stigmata, up to seven in each row. Up to four stolonic vessels extend out from the ventral concavity of the gut loop.

Remarks

The spherical form of the small contracted thoraces contrasts with the contracted thoraces of many didemnid species with large sessile atrial apertures which become slightly curved into a comma-shape around a dorsal concavity in the vicinity of the atrial opening. In the present species the dorsal border of contracted zooids is not concave and the spherical outline of the contracted thorax resembles some of the smaller zooids in the genus Trididemnum . Indeed many species of the genus Trididemnum have a similar superficial layer of bladder cells over a layer of spicules and another, but thinner, layer of spicules on the base of the colony as in the present species (see T. areolatum (Herdman, 1906) and T. caelatum Kott, 2001 ). However Trididemnum has only three rows of stigmata in the adult and larval pharynges and in the oesophageal buds, but four in the adult and buds of the present species. Leptoclinides and Polysyncraton both have four rows of stigmata in oesophageal buds but the former genus never has a retractor muscle. In Polysyncraton species with an aspicular bladder cell layer overlying a layer of spicules (which are absent from a large part of the colony) often occur (see P. rica , P.otuetue , P. purou , P. robustum and P. dromide ). However the present species has smaller zooids and smaller spicules than are usually found in Polysyncraton and it lacks the atrial lip usually characteristic of the latter species. Therefore, although gonads were not detected, the present specimen appears to be a new species of Didemnum .

Species of Didemnum with a similar distribution of spicules are D. chartaceum (larger spicules and some globular ones), D. dolium sp.nov. and D. sucosum (both with a complex three-dimensional cloacal system and branching colonies also with some globular spicules) and are clearly distinguished from the present new species.

Figures

Figure 14. Didemnum moseleyi (QM G328455): A, larva. Didemnum multiampullae sp. nov. (QM G328057): B, larva. Didemnum tapetum sp. nov. (QM G328016): C, abdomen showing gut loop and testis; D, larva. Didemnum velum sp. nov. (QM G328120): E, zooid; F, thorax showing lateral organs. Trididemnum grandistellatum sp. nov. (QM G328007): G, thorax; H, abdomen. Scales: A–H, 0.2 mm.

Figure 18. Scanning electron micrographs of didemnid spicules: A, Didemnum multiampullae, QM G328057: B, Didemnum ossium, QM G328055; C, Didemnum perplexum, QM G328034; D, Didemnum plebeium, QM G328451; E, Didemnum tapetum, QM G328016; F, Didemnum velum, QM G328120; G, Didemnum sp 1, QM G328160; H, Didemnum sp. 2, QM G328166.