Oligotrema lyra

Oligotrema lyra (Monniot C. & Monniot F., 1973)

Figure 8

Gasterascidia lyra Monniot C. & Monniot F. 1973: 457, figs. 31E–F, 32, 33A; Monniot C. & Monniot F. 1974: 777; Monniot C. & Monniot F. 1977: 697; Monniot C. & Monniot F. 1984a: 209; Monniot C. & Monniot F. 1984b: 131; Monniot C. & Monniot F. 1985: 35; Monniot C. & Monniot F. 1990: 251, figs. 1B, 5A–C.

Oligotrema lyra: Kott 1989: 526, Sanamyan K. & Sanamyan N. 1999: 1873; Tatián et al. 2011: fig. 1.

Material examined: 62°9’S, 49°3’W, station 14, 3400 m, 18 March 2005, four specimens.

Description. The four specimens measure from 2.2 to 3.6 cm long (distances between the oral and atrial apertures). The tunic is transparent. It is completely covered by thin projections and detritus. Some individuals also had rests of sponges and polichaete tubes. The oral siphon, made up of six prominent oral lobes, occupies one third of the total length of the body (Fig. 8A). Its internal wall has seven longitudinal rows of approximately 16 papillae. The atrial siphon is located posteriorly and consists in a projection of variable size (although always smaller than the oral siphon). At the base of the oral siphon there are 14 small oral tentacles distributed in a circle (Fig. 8B). Below this row of tentacles, the reduced branchial sac has numerous circular perforations that connect with the atrial cavity. The mantle is thin and transparent. The musculature is strong: densely distributed thick muscle fibers cover most of the body. The oral siphon is completely innervated by longitudinal fibers. The oral lobes and the base of the oral siphon are covered by a series of circular fibers. On each side of the oral siphon, from its base until the middle of the body, a group of longitudinal fibers extend and cover both right and left gonads. In between both siphons, over the dorsal region, there is another group of transversal muscle fibers. The ventral region does not have any musculature. The esophagus is short and wide. It connects with a large and rounded stomach, which occupies more than half of the body. The stomach walls are thin, smooth and delicate. On top of the stomach, at the level of the esophagus, there is an oval vesicle with a concretion in its interior. The rectum is short and ends near the atrial siphon. There is one gonad on each side of the body. The tubular ovaries have a characteristic “S” shape and are full of ovocytes. On the distal ends of both ovaries there are two clumps of testicular follicles. The oviduct is short and wide and ends in the atrial cavity.

Stomach contents. We found zooplanktonic prey items in only one specimen: one amphipod (undet.); a fragment of an isopod; and fragments of what could belong to Isopoda or Tanaidacea crustaceans. The remaining specimens had: an empty stomach, a stone, and detritus.

Remarks. Although the species is cosmopolitan, its records are more frequent in the northern hemisphere. This is the second register of O. lyra in the Weddell Sea.

On the revision of the family Hexacrobylidae, Kott (1989) established that the species assigned to S orbera Monniot C. & Monniot F. 1974 and to Gasterascidia Monniot C. & Monniot F. 1968 presented all characters assigned to Oligotrema, and that the differences did not justify the maintenance of both genera. To this author, Sorbera and Gasterascidia are junior synonyms of Oligotrema.