Trididemnum titanium, 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 : 1204-1205

Trididemnum titanium sp. nov.

( Figures 5C , 11C ) Trididemnum sibogae: Kott 2001, p 285 (part, as set out in ‘‘Previously recorded’’, below). Trididemnum nobile: Kott 2005a, p 2456 .

Distribution

Previously recorded (see Kott 2001, part, as T. sibogae , and Kott 2005a, as T. nobile ): Westernport, QM G300955 ( Kott 2001, Figure 132A); QM G300971 (2001, Plate 17B); Kingston, QM G302875 (2001, Plate 17F); Tasmania (Port Davey, QM G302884). New records: type locality (Tasmanian Canyons: Pieman Canyon, 174 m, paratype QM G323145; King I. Canyon, 250 m, holotype QM G323351); Tasmania (Banks Strait, 168 m, QM G328101); Tasmania (Bass Strait, QM G308816).

Description

Colonies are invariably three-dimensional trabeculae created by branching and anastomosing of parts of the colony, which sometimes are narrow or flattened lamellae, but often are more rounded. Irregular, dark patches and streaks of pigment are sometimes on the surface test of preserved specimens. Colonies are stiff and brittle with spicules crowded throughout. The specimen from Bass Strait (QM G308816) is an extensive array of thin irregular double-sided lamellae with spicules crowded throughout, especially in the central test where they are especially packed together to form a hard calcareous plate supporting the upright lamellae. Spicules are stellate to 0.17 mm diameter, with seven to nine and sometimes eleven sturdy conical rays in optical transverse section. The diameter of each ray expands towards its base; but the rays are not crowded.

Zooids are large, with posteriorly orientated atrial siphons and conspicuously lobed branchial siphons. A short retractor muscle was sometimes detected, but often was obscured by the contracted zooids. Nine coils of the vas deferens around a large beehiveshaped undivided testis were detected in one specimen ( QM G308816 ) from Bass Strait and in the holotype the remains of the proximal part of the vas deferens filled with sperm probably serves as a seminal vesicle. Gonads were not detected in the newly recorded specimens .

Larvae are present in the basal or central core of the test. The trunk is 0.1 mm long. Four lateral ampullae are on each side of the three anteromedian adhesive organs.

Remarks

The form of the complex colony, the black squamous ectoderm and the large, stellate spicules (to 0.15 mm diameter or more) with relatively short but pointed conical rays are characteristics of the species and help to distinguish it. Other Trididemnum spp. with complex three dimensional colonies from southeastern Australia have smaller spicules, namely T. amiculum Kott, 2001 (with seven coils of the vas deferens) and T. nobile Kott, 2001 (with nine coils of the vas deferens). Trididemnum lapidosum Kott, 2001 is a tropical species with similar large spicules but with longer and spikier rays, paddle-shaped branches of the colony and seven coils of the vas deferens. Trididemnum pigmentatum and T. sibogae have eight coils of the vas deferens although the former is a tropical species with smaller zooids and large spicules, but with more, and more crowded, spicule rays and less complex colonies. Trididemnum sibogae has smaller spicules with fewer rays. Similar large spicules in relatively complex colonies and zooids with posteriorly orientated atrial apertures are in the genus Leptoclinides , although in this genus the spicules very often have chisel-shaped tips, zooids tend to be larger than Trididemnum spp. and they lack the retractor muscle usually present in the latter genus.

Kott (2001) records T. sibogae from a range of temperate and tropical locations. The specimens have complex three-dimensional colonies and spicule diameters to 0.16 mm with 7–13 long spiky rays in optical transverse section. However, a revision of the scanning electron micrographs of spicules in the specimens assigned to this species reveals that they are in two groups. One group of specimens mainly from temperate locations with spicules to at least 0.12 mm, but often to 0.17 mm diameter, with 7–11 long, tapering points in optical transverse section and nine coils of the vas deferens appears to be the present species ( Trididemnum titanium sp. nov.). The other group, mainly from tropical locations but sympatric with the first group in the southern part of its range, appears to consist of specimens of T. sibogae (described above) with 9–13 spicule rays in optical transverse section, spicules to 0.1 mm diameter, and eight coils of the vas deferens.

Trididemnum nobile: Kott, 2005a (from Tasmania, Port Davey) appears to be a specimen of the present species, with large spicules to 0.135 mm ( Kott 2005a: 1.35 mm sic) diameter and seven to nine rays in optical transverse section and a similar number of coils of the vas deferens.

Leptoclinides grandistellus Kott, 2004a from Dongara (WA) has spicules of the same form, with 9–11 conical rays, but they are larger (to 0.22 mm diameter) and, although L. grandistellus has large posterior abdominal cavities, its colony does not form the threedimensional mass of the present species. Further, unlike the present species, its spicules become less crowded toward the base, but are never crowded throughout.

L. magnistellus Kott, 2001 also has spicules to 0.24 mm diameter, but they have 11–13 rays and are crowded and the colonies are brittle.

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).

Figure 11. Scanning electron micrographs. (A) Trididemnum pigmentatum (QMG308842); (B) Trididemnum sibogae (QM G308893); (C) Trididemnum titanium (QM G308816); (D) Lissoclinum capsulatum (QM G308804); (E) Lissoclinum reginum (QM G308871); (F) Didemnidae?genus?species (QM G308889); (G, H) Didemnidae?genus?species [(G) QM G308888; (H) QM G308900].