Ascidia callosa Stimpson, 1852

Sanamyan, Karen & Sanamyan, Nadya, 2017, Shallow-water Ascidians from Matua Island (central Kuril Islands, NW Pacific), Zootaxa 4232 (3), pp. 301-321 : 302-303

Ascidia callosa Stimpson, 1852

( Figure 2 )

Ascidia callosa Stimpson, 1852: 228 . Lambert & Sanamyan, 2001: 1772 (synonymy).

Material examined. Matua Island, Point Kluv, 16 m, one specimen (#177).

Description. The single collected specimen is small, about 10 mm long, attached by a whole left side ( Figure 2 A). The test is thick and soft, semitransparent, the body is clearly visible through it. Its surface in preservative is finely wrinkled, almost smooth, covered by filamentous diatoms, giving an impression of test hairs, but actually neither test hairs, nor papillae are present.

The body removed from the test is oval in outline, with the terminal branchial siphon and the atrial siphon placed in one-third of the body length distant from it along the dorsal side. The muscles form an irregular dense network on the right side of the body but absent on the left. The branchial tentacles are not numerous, about 17, in two size ranges. Prepharyngeal band is composed of two blades, it forms very shallow, almost not discernible V around minute dorsal tubercle, which is hard to detect. The branchial sac has 18 internal longitudinal vessels on the right and 16 on the left ( Figure 2 D). Intermediate branchial papillae are present but not everywhere. The dorsal lamina is high. The transverse branchial vessels continue to the dorsal lamina and project from its rim, giving it a ribbed appearance. The visceral mass occupies most part of the left side. The stomach is short, oval, located at postero-ventral part of the body. Voluminous intestine makes narrow closed loop along the whole ventral mid line, its pole is located almost behind the branchial siphon. The secondary loop is also narrow and closed. The rectum is straight, not long, and the anus is bi-lobed, with a plain margin. The ovary consist of a main tubule, running in the primary gut loop along its axis, and numerous, mostly perpendicular side branches spread over ascending and descending limbs of the gut loop ( Figure 2 C). Whole visceral mass is covered by numerous renal vesicles.

Remarks. Ascidia callosa is a single species of the family Ascidiidae occurring in diver-accessible depths (down to about 40 m) in a wide NW Pacific region from Commander Islands, through Kamchatka waters, where it is quite common, to at least central Kuril Islands . In NE Pacific, at Alaska coasts, A. callosa coexists with a similar, but distinct species A. columbiana (Huntsman, 1912) (see Lambert & Sanamyan, 2001).

Sanamyan (1998) reported Ascidia prunum Muller, 1776 from north Kuril Islands, but these specimens come from greater depths (118–370 m). The identity of that material needs confirmation but it is certainly not conspecific with A. callosa having much more internal longitudinal branchial vessels.

Figures

FIGURE 2. Ascidia callosa. A, intact specimen; B, specimen with test removed; C, specimen opened ventrally; D, branchial sac.