Octacnemus kottae, Sanamyan & Sanamyan, 2002

Sanamyan, K. E. & Sanamyan, N. P., 2002, Deep-water ascidians from the south-western Atlantic (RV Dmitry Mendeleev, cruise 43 and Academic Kurchatov, cruise 11), Journal of Natural History 36 (3), pp. 305-359 : 322-325

Octacnemus kottae sp. n.

(®gures 11, 12A±C)

Material examined. St. 874, 3700±3910 m, one specimen. HOLOTYPE KIE 1/1027.

Description. The species resembles an enlarged zooid of Octocorallia, rather than an ascidian (®gure 11A). It has a distinct cylindrical peduncle of uniform diameter 8±9 mm and about 3 cm long, and a ¯attened oral disk (5branchial siphon) with a crown of eight triangular lobes. The test is thin and transparent. The peduncle has a tuft of crowded, unbranched hair-like outgrowths at its proximal end to which sand particles are attached. The oral disk is more or less symmetrical. Its central part is almost completely circular, about 15±16 mm in diameter and all eight lobes are of about the same length (14±18 mm), thus the total diameter of the oral disk is about 4.5 cm. The elongate triangular oral lobes have pointed tips, their bases are about half their length. Eight low elevations of the test are on the central part of the disk close to the base of each lobe and with attached sand grains. No structures corresponding to these elevations are on the body wall. The lobes have lateral pinnules, as in O. ingol W Madsen, 1947 , about 25±20 on each side.

The branchial aperture is close to the dorsal margin of the oral disk. The sessile atrial aperture is a small oval opening on the dorsal surface of the distal half of the peduncle. The body and the test around this aperture are damaged and cannot be examined properly, but apparently there are no atrial lobes or other test outgrowths in this region.

The arrangement of muscles on the oral disk is the same as in other species of the genus. The circular muscles are con®ned to the oral lobes and regularly spaced. The radial muscles form short thick bunches between the lobes. There are few, thin and sparse radial muscles on the oral lobes. Most of the surface of the central disk lacks muscles. Thick, short transverse muscles are present on the dorsal side of the body wall between branchial and atrial apertures, and a bunch of thinner longitudinal muscles is present on the right side of the atrial aperture (®gure 11B). Fine circular muscles are around the margin of the atrial opening. The branchial opening continues into the short thick-walled siphon, which opens into the branchial cavity through a high velum. The thick circular muscles of this siphon (®gure 12A) correspond to the circular muscles of the branchial siphon of other ascidians. The test on the internal surface of this siphon has thin but pronounced longitudinal folds.

The numerous, crowded, simple branchial tentacles are on the free edge of the velum. The prepharyngeal band is a distinct groove on the epithelium, it is far from the tentacles and makes a well-marked dorsal V. The large oval dorsal tubercle is far anterior from the tip of the dorsal V and has a large oval opening. The small triangular ganglion is close to the dorsal tubercle and anterior to it. The ganglion is relatively super®cial in the body wall and it is seen more readily from the outside the body wall. The dorsal lamina is absent and is replaced by numerous raised, crowded papillae. Similar papillae are also present on both sides of the endostyle and the retropharyngeal groove, and a few are present between the prepharyngeal band and the perforated zone of the branchial sac. The papillae are not present between the prepharyngeal band and the tentacles, or on the perforated part of the branchial sac. The perforated zone occupies a large part of the branchial sac. The perforations are large, 0.25 ±0.5 mm, and formed by a net of intersecting branchial vessels. The vessels are high, ¯attened laterally and more or less straight. They intersect at right angles, and therefore the perforations are more or less rectangular (®gure 12B).

Internal organs form a compact visceral mass with a short posterior extension. They are in a poor condition and cannot be examined in detail. In the ®xed specimen the visceral mass occupies only the upper half of the peduncle.

Remarks. The present species most closely resembles O. ingol W, especially in the presence of lateral pinnules on the oral lobes. The latter species was originally described from one somewhat damaged specimen from south Greenland. To save that specimen, Madsen (1947) did not dissect it and his description referred mostly to external features, while the internal organs were described only by inspection through the transparent test. Monniot and Monniot (1973) assigned two specimens (one of which was juvenile) from the north-east Atlantic to O. ingol W and provided a detailed description. The Monniots’ specimen corresponds closely to the present one in many features, but diOEers in the structure of the dorsal lamina, which is a main feature distinguishing the two species. This organ was described and ®gured by Monniot and Monniot (1973) as a low continuous plain-edged lamina, and resembles the dorsal lamina of many other ascidians. In the present species such a dorsal lamina is not present, and the mid-dorsal area of the branchial sac is covered by papillae, which are also present on other parts of the branchial sac but were not present in the Monniots’ specimen. There are some diOEerences in the shape of the oral lobes, the shape of the stigmata, the presence of atrial lobes in the north Atlantic specimens, but none of these seem to be really signi®cant. Octacnemus ingol W was further recorded from the north and south Atlantic, Indian Ocean and from the south-western Paci®c, but none of these descriptions referred to internal anatomy of the specimens, and it is possible, that some of these records belong to other species.

All the other species of Octacnemus and Polyoctacnemus have no pinnules on the oral lobes and therefore are obviously distinct from O. kottae . Among them O. zarcoi Monniot and Monniot, 1984 resembles the present species in the absence of the dorsal lamina and presence of papillae on the mid-dorsal line of the branchial sac. The structure of the branchial perforations of O. zarcoi is similar, but more complex than the present species. Similar papillae are distributed over the whole inner branchial wall of O. alatus Monniot and Monniot, 1985 but are not present in either O. bythius Moseley, 1876 or O. vinogradovae Sanamyan and Sanamyan, 1999 .

At our disposal is a ®gure of another specimen of Octacnemus , drawn by Dr N. G. Vinogradova, which has not been published previously (®gure 12D). The caption under the ®gure is:`Octacnemus, empty test (the separated body exists), st. 5612, 8200±8050 m’. This station is from the Kurile-Kamchatka Trench ( Situla pelliculosa Vinogradova, 1969 was described from this station). This specimen resembles O. kottae in the presence of pinnules on the oral lobes, shape of the peduncle and position of the sessile atrial opening. Unfortunately we failed to ®nd this specimen in the collection of the Institute of Oceanology.

Etymology. The species is named after Dr Patricia Kott.