Trididemnum nebula, Kott, 2007

Kott, Patricia, 2007, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (part 4), Journal of Natural History 41 (17 - 20), pp. 1163-1211 : 1194-1196

Trididemnum nebula sp. nov.

( Figure 5A, B )

Distribution

Type location: Lord Howe I. (Lagoon Beach: 1.5 m, sandy bottom, coll. Aquenal, 17 February 2006, syntypes QM G328001 ; sheltered lagoon, boat ramp, 2 m, QM G328002 ) .

Description

Colonies are small (to 2 cm maximum diameter) soft, low cushions. The test is cloudy, a brownish colour in preservative. Spicules are in irregular thin patches in the surface test and line the floor of the thoracic common cloacal canals. They are globular, to 0.05 mm diameter. However, spicules are present only in some colonies, others are aspiculate.

Zooids are small, the thorax and abdomen about equal lengths. The branchial aperture is divided into six rounded, conspicuous lobes on a short branchial siphon. The atrial aperture is on a short siphon halfway down the thorax and directed horizontally. Although the exact number is obscured by contraction, about 10 stigmata per half row are in the three rows halfway down the pharynx. A strong, tapering retractor muscle of variable length projects from the top half of the oesophagus, but depending on the contraction of the zooid, it may appear to originate from the posterior end of the thorax.

The gut forms an open loop slightly flexed ventrally over the undivided testis surrounded by eight coils of the vas deferens. The zooids are surrounded with black squamous epithelium, which obscures their structure. An endostylar cap was not detected.

Small, spherical larvae are being incubated in the basal test. The tail is wound threequarters of the way around the 0.4 mm diameter trunk. Three lateral ampullae are on each side of the three anteromedian adhesive organs.

Remarks

Superficially, the small, soft, cloudy colonies resemble those of Trididemnum miniatum Kott, 1977 and T. clinides Kott, 1977 . However, the cloudiness of the test is not indicative of embedded symbionts, nor are there any symbiotic cells in the common cloacal cavity, and the larval trunks lack the coat of the symbiotic cells present in the former two species. Further, T. miniatum has a sessile atrial aperture rather than the laterally orientated atrial siphon of the present species and it has smaller spicules. Trididemnum clinides Kott, 1977 is distinguished by its large spicules. Species that, like the present one, have a small midthoracic, horizontal atrial siphon, three pairs of larval lateral ampullae, black squamous epithelium, 10 or more stigmata per half row, and eight coils of the vas deferens are T. areolatum Herdman, 1906 (distinguished by its large stellate spicules) and T. caelatum Kott, 2001 (also distinguished by its large stellate spicules).

Spicule distribution, the vas deferens and atrial siphon in T. pusillum Kott, 2004a are similar to the present species. Further, although Kott (2004a) remarks that larvae for T. pusillum are not known, a larva from the holotype is shown in Kott (2004a, Figure 18d) and this also is similar to the present species. Despite these similarities, these species are distinguished by their spicules, which, in T. pusillum , have relatively few pointed spiky rays.

In T. tectum Kott, 2001 , globular spicules are in the same position, in the floor of the cloacal cavity, as the present species, but spicules and zooids are larger, zooids have fewer coils of the vas deferens and the larvae have more lateral ampullae. Trididemnum nobile Kott, 2001 is occasionally aspiculate, has a similar (but larger) larval trunk, a posteriorly orientated atrial siphon and its spicules (when present) are stellate.

The present species, with its small soft cloudy colonies, occasional small globular spicules around the common cloacal cavity, black squamous epithelium, horizontally orientated atrial siphon, small zooids with relatively numerous vas deferens coils and stigmata, and small larvae with three pairs of lateral ampullae appears to be a previously undescribed species.

Figures

Figure 5. (A, B) Trididemnum nebula (QM G328001 syntype): (A) thorax; (B) larva. (C) Trididemnum titanium (QM G323351 holotype): colony. (D, E) Lissoclinum capsulatum (QM G308804 holotype): (D) thorax; (E) abdomen. Scale bars: 0.1 mm (A, B, D, E), 1 cm (C).