Synoicum molle (Herdman, 1886)

Maggioni, Tamara, Taverna, Anabela & Tatian, Marcos, 2016, Redescription of the deep-sea colonial ascidian Synoicummolle (Herdman, 1886): first record since its original finding during the Challenger Expedition, Zoosystematics and Evolution 92 (2), pp. 181-185 : 181-183

Taxon classification Animalia Aplousobranchia Polyclinidae

Synoicum molle (Herdman, 1886)

Polyclinum molle Herdman, 1886: 194, pl. XXV, figs 7-9; non Rocha and Costa 2005: 59, fig 2-4.

Synoicum molle ; Van Name 1945: 84, fig. 20.

Material examined.

Station 2: 37°58'S, 55°12'W; 7 colonies; 308 m; Ago/17/2012.

Description.

The colonies are almost identical in shape: globular, nearly spherical (Fig. 2). The biggest colony measures 9.1 cm in diameter by 3.2 cm in height, while the smallest one reaches 2.5 cm in diameter. The tunic is grayish with variation in intensity, the smallest colonies being the darkest. The zooids, when alive, are white. When fixed in formalin, they turn pale yellow. The test is soft and free of foreign material. Only in two small specimens, some grains of sand and a few epibiotic foraminifera were detected. The tunic is consistent and rigid. The zooids, in variable numbers, are arranged in irregular rosette-like systems around common, although not visible to the naked eye, cloacal apertures. One colony shows zooids with no arrangement at all in the uppermost area, while the area nearest to the base maintains the irregular rosette-like configuration.

All colonies present zooids with a marked variation in size and also of sexual maturation. When fully developed, zooids are large, with an average length of 13 mm (Fig. 3). Zooids with empty thoraces and no digestive systems but fully developed gonads were also found in the biggest colony. These reach a maximum length of 29 mm. The oral siphon bears 6 distinct lobes. The atrial aperture is small, most of the times completely surrounded by an extension of the tunic that forms a small ring. It extends between the 4th and 8th rows of stigmata. The atrial languet is generally thin, long and simple, though it can also be bifid. It extends as long as the entire length of the thorax, or long enough to cover the atrial aperture. The margins are smooth or slightly serrated.

The thorax bears 8 to 10 thin longitudinal muscle bands on each side, running along the entire body and joining at the end of the post-abdomen. There are between 14 and 16 simple and stout oral tentacles, alternating in size and placed in a circle. The dorsal tubercle is small and rounded.

There are from 12 to 14 rows of stigmata. Rarely, zooids may bear only 9 or 10. Nonetheless, they never exceed 14 rows. Each row contains 10 longitudinal rectangular-shaped stigmata. These vary slightly in size, being thinner and longer towards the center of the thorax. Branchial papillae are not present.

The straight and thin-walled esophagus connects with the stomach vertically. The stomach wall is smooth, although some striations -never folds- might occur randomly (Fig. 4). The stomach shows two alternate shapes: almost spherical or dome-shaped with a straight base. The intestine turns to the dorsal and anterior end vertically. The anus shows two lobes and is located at the level of the 8th row of stigmata.

The gonads are situated in a long post-abdomen, either directly adjacent to the abdominal region or at some distance (0.3 to 4.7 mm) away from it. The ovary contains from one to 2-3 or 6-8 small oocytes. The male follicles are found just below or surrounding the oocytes, disposed in clusters or arranged in a straight line.

On average, a dozen of immature larvae are present in the atrial cavities of a few zooids of one colony. They are arranged in double rows along the entire length and half the width of the thorax. In that immature stage, larvae had only developed a small and stout tail but lacked sensory organs, papillae and vesicles.