Ascidia thompsoni Kott, 1952

Kott, Patricia, 2006, Observations on non-didemnid ascidians from Australian waters (1), Journal of Natural History 40 (3 - 4), pp. 169-234 : 211-212

Ascidia thompsoni Kott, 1952

( Figure 7E )

Ascidia thompsoni Kott 1952, p 312 ; 1985, p 56 and synonymy.

Ascidia challengeri: Kott 1985, p 32 .

Distribution

Previously recorded (see Kott 1985): Tasmania (Great Taylor Bay, Port Davey ); South Australia ( St Vincent Gulf ). New records: Tasmania ( Port Davey , SAM E2889 ); Tasmanian Canyons ( Pieman Canyon , 472.8 m, QM G308827 ) .

Description

The newly recorded individuals are 10 and 15 cm long, respectively, narrow anteriorly and increasing to about 4 cm wide posteriorly. The surface is even but rough and the test is firm, even and translucent. The body is laterally flattened. The terminal branchial aperture has about eight lobes and the margin is fringed (although this is obscured in this specimen). The sessile atrial aperture is two-thirds of the body length along the dorsal surface. About 100 long, curved branchial tentacles are crowded around the base of the branchial siphon. The dorsal ganglion can be seen about halfway between the dorsal tubercle (behind the tentacular ring) and the atrial siphon. The slit on the dorsal tubercle is basically U-shaped with the opening directed anteriorly. The horns are turned in or out and sometimes it is interrupted or slightly convoluted and sometimes the dorsal tubercle slit is pulled out into sharp angles (see QM G9325 from St Vincent Gulf and the newly recorded specimen QM G308827). A meshwork of muscles is on the right side of the body. The branchial sac has about six stigmata per mesh. The inside curve of the tight secondary gut loop is swollen with mud. The long stomach with its fine longitudinal fold occupies at least half of the outer (ascending) limb of the primary loop, and is overlaid by this mud-filled portion of the gut. The distal part of the rectum is bent back postero-dorsally to terminate at the base of the short atrial siphon. Shallow lobes are on the anal border. Gonads are usually in the primary loop of the gut, although they appear to be senescent in the large specimen from Pieman Canyon. Small intermediate branchial papillae are only at the junctions of parastigmatal with internal longitudinal vessels.

Remarks

The species is distinguished from Ascidia sydneyensis (which has a mud-filled gut and translucent test) by its numerous branchial tentacles, body musculature, the position of the dorsal ganglion and the relatively simple U-shaped slit on the dorsal tubercle, and the postero-dorsal bend of the distal part of the rectum. With the exception of its musculature, these characters distinguish the species from A. saccula n. sp. and the Antarctic A. challengeri Herdman, 1882 (which both have translucent tests and mud-filled gut).

Figures

Figure 7. (A–D) Ascidia saccula (holotype QM G308839): (A) external view showing pointed surface papillae around apertures; (B) body wall muscles; (C) internal view showing gut loop; (D) branchial tentacles, dorsal tubercle, neural ganglion, and dorsal lamina. (E) Ascidia thompsoni (QM G308827): internal view showing gut loop. Scale bars: 2 mm (A, D); 1 cm (B, C); 2 cm (E).