Trididemnum grandistellatum, 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 : 1192-1193

Trididemnum grandistellatum sp. nov.

( Figures 14G, H ; 19A )

Distribution

Type locality: Western Australia CSIRO SS10 / 05 (Bald I., Stn 35, 118.645E 35.1907S, 157 m, 24 November 2005, holotype WAM Z27520 , QM G328007 ) .

Description

The colony is sheet-like, with a layer of spicules at thorax level beneath a superficial aspicular layer. Spicules are only sparse in the clear, transparent test of the remainder of the colony. The spicules are large (to 0.186 mm diameter) with 13–15 conical pointed rays in optical transverse section. They break up readily. The common cloacal canals are at thorax level and they encircle blisterlike areas of zooid-free test. From the surface, the stellate, six-lobed branchial apertures are seen opening to the surface along each side of these circular canals. Atrial apertures are circular sessile openings surrounded by a sphincter muscle and exposing only a small part of the branchial sac to the common cloacal canals.

Zooids are robust and have black squamous epithelium on the body wall. Although the branchial sac is obscured by contraction, there appear to be about 12 stigmata in each of three rows. A retractor muscle, free from the top of the oesophagus, is of varying length, depending on its state of contraction (from short and stumpy to long and finely tapering). A small circular lateral organ is on each side of the thorax, ventral to the atrial aperture. The gut loop is open and rounded. Seven coils of the vas deferens surround the undivided testis and in some zooids the male duct is seen to act as a seminal vesicle. Up to four short stolonic vessels with rounded terminal ampullae project from the ventral concavity of the gut loop.

Remarks

Although the actual number of rows of stigmata is obscured, the black squamous epithelium, position of the atrial opening and its sphincter, the position and size of the small circular lateral organs and the large spicules confined to a single layer beneath the surface together suggest that this is a Trididemnum species rather than a species of Didemnum . Although Polysyncraton spp. usually have a roomy gut loop, they differ from Trididemnum in having numerous testis follicles and four rather than three rows of stigmata in larvae, replicates and adults and a sessile open atrial aperture rather than the posteriorly orientated atrial siphon. Leptoclinides spp. also have multiple testis follicles and lack a retractor muscle.

The colony with its large spicules confined to a layer beneath the surface resembles species of the (Trididemnum) savignyi group (see Kott 2001). Trididemnum titanium Kott, 2007 forms a more complex three-dimensional mass than the present specimen, has large spicules (up to 0.17 mm diameter) but with only 9–11 rays in optical transverse section crowded throughout the colony and more (nine) coils of the vas deferens.

The only other known species in this group with such large spicules is T. pigmentatum although the rays are more attenuated and spikey. Trididemnum savignyi has a similar colony but smaller spicules.

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 19. Scanning electron micrographs of didemnid spicules: A, Trididemnum grandistellatum, QM G328007; B, Trididemnum sibogae, QM G328421; C, Lissoclinum maculatum, QM G328462.