Ascidia escabanae Monniot, 1998

Sanamyan, Karen & Sanamyan, Nadya, 2007, Poorly known Ascidiacea collected in the vicinity of the Commander Islands and East Kamchatka, NW Pacific, Zootaxa 1579 (1), pp. 55-68 : 60

Ascidia escabanae Monniot, 1998

( Figure 3 )

Ascidia escabanae Monniot, 1998: 544 .

Ascidia clementea: Sanamyan and Sanamyan, 1998: 212 . Not Ritter, 1907: 32.

Material examined: RV Keldish, cruise 22, st. 2316, 4294– 4200m, 55°36.8'N, 167°23.04'E – 55°35.0'N, 167°24.4'E, 5–6 August 1990, one specimen .

Previous records: NE Pacific, Bering Sea, Escabana Trough of the southern Gorda Ridge ( Monniot 1998, Sanamyan and Sanamyan 1998)

Description. The specimen is flattened laterally, oval in outline, 3.5 cm high. The colourless, translucent and almost smooth test, with minute sparse conical papillae in places, is thicker on the left side. It contains blood vessels, which are, however, not as conspicuous as originally described for this species and do not differ much from those found in other Ascidia spp. Apertures are on low siphons, the branchial siphon is terminal and the atrial halfway along the dorsal mid-line. The contracted body removed from the test is 2 cm long. Body musculature consists of weak circular siphonal muscles and a network of thin fibers on the right which terminate in a band of parallel transverse fibers along the ventral region. Muscles are not present on the left. A branchial velum is not present. About 25 long, thin tentacles are attached directly to the body wall. The prepharyngeal band of two thin lamellae is in a circle just behind the ring of tentacles. It is not indented dorsally. Papillae were not detected on the body wall between the tentacles and prepharyngeal band. A large, more than 1 mm long, ganglion is just behind a small simple dorsal tubercle. The dorsal lamina has two blades anteriorly which fuse near the middle of the ganglion, and continue posteriorly as a rather high single lamella with minute spaced indentations on the margin ( Fig. 3A ). The branchial sac is thin and delicate with much reduced tissue. It has more than 60 transverse vessels, and about 50 thread-like internal longitudinal vessels on each side supported on short papillae. Minute intermediate papillae are invariably present on internal longitudinal vessels. Parastigmatic vessels have not been detected in any part of the branchial sac. The stigmata are large, two to four per mesh. The gut forms a simple open, more or less J-shaped arc, its proximal limb shorter than the distal limb. The small stomach has shallow internal longitudinal plications and the border between its wider pyloric end and the intestine is obscure. The junction between the stomach and intestine is in the curve of the J, the stomach occupying a large part of the proximal limb. The rectum opens at the atrial opening in a smooth-rimmed anus. The ovary is spread over the intestine as a set of branched tubules. Male gonads and gonoducts were not detected. The whole gut loop excepting distal end of the rectum is embedded in a mass of renal vesicles, which are, however, transparent and may be easily overlooked.

Remarks. The only previous records of this species are from one location in the NE Pacific (see above). The general shape of the body, position of siphons, shape of the gut loop, branched tubular ovary on but not inside the gut loop, absence of the branchial velum, and large neural ganglion are all as originally described for the species (see Monniot, 1998). However, papillae were not detected in the prebranchial region, two blades of dorsal lamina in Escabana specimens fuse far behind the ganglion (at the level of the ganglion in the present specimen), and the present specimen is smaller and has only about 50 internal longitudinal vessels (80 in the original description). Several other specimens from the same station have been wrongly identified by Sanamyan and Sanamyan (1998) as Ascidia clementea . They are larger than the present specimen and have more (up to 70) internal longitudinal vessels. Ascidia clementea appears to be a closely related but distinct species, the main differences are the shape of the gut loop and position of the ovary within the gut loop. Further, the branchial sac of A. escabanae appears to be much thinner in comparison to that figured for A. clementea by Ritter (1907, Plate 4, Fig. 34) and Monniot and Monniot (1989, Fig. 2C ) although this might be only a subjective impression.

Figures

FIGURE 2. Ciona gelatinosa (KBPIG 1127/1). A, body removed from the test; B, rectum and genital papilla; C, position of the specimen on the sea floor (from a photo).

FIGURE 3. Ascidia escabanae. A, dorsal lamina; B, detail of the branchial sac; C, specimen opened along ventral midline, branchial sac removed; D, internal view of antero-dorsal region.