Didemnum incanum ( Herdman, 1899 )

Kott, Patricia, 2005, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (Part 3), Journal of Natural History 39 (26), pp. 2409-2479 : 2444-2446

Didemnum incanum ( Herdman, 1899)

( Figures 8A , 16A , 20C )

Leptoclinum incanum Herdman 1899, p 90 .

Didemnum incanum: Kott 2001, p 191 and synonymy; 2004b, p 2494.

Distribution

Previously recorded (see Kott 2001, 2004b): South Australia (Waterloo Bay SAM E3209, Gulf St Vincent, Cape Jervis); Victoria (Gabo I., Lorne); New South Wales (Illawarra, Port Jackson); Tasmania (Triabunna, SAM E2925); New Zealand (Wellington Harbour, Nelson, Picton Ferry Wharf, Pelorus Sound). New records: Tasmania (Port Davey, SAM E3237-8); South Australia (Kangaroo I., SAM E3252).

Description

The newly recorded specimens are parts of sheet-like encrusting colonies, crowded throughout with small (to 0.04 mm diameter) stellate spicules with five to nine conical and pointed rays in optical transverse section. In places, the surface of the colony has minute upright spicule-filled papillae between the stellate branchial openings which are outlined in spicules. Groups of thoraces have their ventral borders embedded in stands of test that connect the basal part of the colony to the surface test. Each clump of thoraces is surrounded by a deep common cloacal canal which sometimes penetrates between the thoraces separating them from one another, each in an independent sheath of test. Abdomina are embedded in the basal test. Zooids are small with a small comma-shaped thorax crossing the horizontal common cloacal cavity and the abdomina are embedded in the basal test. Six stigmata are in the anterior row of the branchial sac, the oesophageal neck is long and a retractor muscle extends from about halfway down the oesophageal neck. The gut forms a double loop. Nine coils of the vas deferens surround the undivided testis. Small larvae, the trunk 0.5 mm long, have four pairs of long finger-like ampullae each side of the three anteromedian adhesive organs and the tail is wound three-quarters of the way around it.

Remarks

The species is readily identified by its hard, sheet-like colonies, small zooids with small pointed branchial lobes and double gut loop, retractor muscle from halfway down the oesophageal neck, and especially by the small stellate spicules which have five to nine relatively long, pointed conical rays in optical transverse section and are never more than 0.04 mm diameter. The colony sometimes has a three-dimensional growth-form (see Kott 2001). Didemnum vestum Kott, 2000d from the Atlantic coast of North America has a three-dimensional growth form similar to some of the colonies of the present species. However, it has smaller spicules with significantly shorter rays, eight coils of the vas deferens and a larger larva with six pairs of ectodermal ampullae. Didemnum vexillum Kott, 2002b from New Zealand (which has erroneously been regarded as conspecific with the northern Atlantic species) has a similar growth form but is readily distinguished from both D. incanum and D. vestum by its larger spicules with more rays (see Kott 2004d).

Both Didemnum mantile Kott, 2001 and D. domesticum Kott, 2004c have similar small spicules with seven to nine long rays in optical transverse section, but, respectively, six and seven coils of the vas deferens. Didemnum madeleinae F. and C. Monniot, 2001 has larger spicules (to 0.05 mm diameter) with only five to seven conical spicule rays in optical transverse section and seven coils of the vas deferens. Didemnum perlucidum Monniot, 1983 from Guadeloupe has a larva similar to the present species and spicules with five to nine rays in optical section, but they are smaller than the spicules of the present species (to 0.03 mm diameter) and it has a sheet-like colony and only eight coils of the vas deferens. Other specimens assigned to D. perlucidum from the tropical western Pacific and Brazil (see Kott 2004d) are not sufficiently described for confident assignation. Other species, including D. granulatum Tokioka, 1954 , have seven to nine rays in optical transverse section but usually their spicule diameter is 0.06 mm or more, readily distinguishing them from the present species.

The Tasmanian record is the most southerly for this temperate species, that appears to be one of the few known trans-Tasman species.

Figures

Figure 8. (A) Didemnum incanum (SAM E3252): colony surface; (B) Didemnum jucundum (SAM E3236): thorax; (C) Didemnum microthoracicum (SAM E3256): surface view of a circle of thoraces surrounding a test connective. Scale bars: 1.0 mm (A); 0.1 mm (B, C).

Figure 16. Scanning electron micrographs of the calcareous spicules of: (A) Didemnum incanum (SAM E3238); (B) Didemnum leopardus sp. nov. (QM G308719, holotype); (C) Didemnum moseleyi (WAM 175.93); (D) Didemnum plebeium sp. nov. (WAM 155.93, syntypes); (E) Didemnum stragulum (WAM 162.93); (F) Didemnum ternerratum (SAM E3240); (G) Trididemnum amiculum (SAM E3263); (H) Trididemnum areolatum (QM G308746).

Figure 20. In situ images of: (A) Didemnum fragum (SAM E3266); (B) Didemnum fuscum (QM G308730); (C) Didemnum incanum (SAM E3252); (D) Didemnum jucundum (SAM E3236); (E) Didemnum leopardus (QM G308719, holotype); (F) Didemnum microthoracicum (SAM E3256); (G) Didemnum moseleyi (QM G308713); (H) Didemnum pellucidum (SAM E3271).