Synoicum georgianum Sluiter, 1932

Turon, Xavier, Cañete, Juan I., Sellanes, Javier, Rocha, Rosana M. & López-Legentil, Susanna, 2016, Ascidian fauna (Tunicata, Ascidiacea) of subantarctic and temperate regions of Chile, Zootaxa 4093 (2), pp. 151-180 : 162

Synoicum georgianum Sluiter, 1932

Fig. 6

References and synonymy: Synoicum georgianum Millar (1960) p. 47; Monniot (1970) p. 22; Aplidiopsis georgianum Kott (1969) p. 73.

Localities: 1S, 4S.

Colonies of this species form aggregates of small capitate heads (5–10 mm in diameter, 15 mm in height). These heads are united basally and often coalesce forming bigger units. From one to several systems of zooids form each head. The colonies from Muelle Loreto (locality 4S) are externally coated with sand, which does not penetrate in the inner tunic, while those from Bahía Porvenir (locality 1S) are devoid of sand and whitish in colour. The internal tunic is soft and transparent.

The zooids measure up to 10 mm in length. The oral siphon has 6 lobes (sometimes subdivided). The atrial opening is narrow, forming a tube in front of the first row of stigmata, and is surmounted by a trifid languet. A couple of minute denticulations is sometimes visible in the posterior rim of the atrial aperture. The branchial sac has 10 to 12 stigmata rows with approx. 10 slits per half row. There are 7–9 longitudinal muscular bands at each side of the thorax. The abdomen has a quadrangular stomach, in which the oesophagus enters a bit sub-terminally. The stomach appears smooth-walled externally, except for the typhlosole, although a pentagonal shape is apparent in transverse sections. There is a post-stomach with a markedly thickened region, which connects to a slender midintestine in vertical position. The sharp transition between mid-intestine and posterior intestine lies at the bottom of the gut loop, and the posterior intestine ends about the level of the fourth posterior-most row of stigmata.

Some zooids have ovaries and testes (forming one or two rows) in the postabdomen, and only a few have brooding embryos (up to 4) posterolaterally (right side of the intestine) in the atrial cavity. No developed larvae could be seen.

Remarks. the fact that the incrustation with sand is a variable character had already been noted by Monniot & Gaill (1978). Although the number of stigmata rows is lower than reported by other authors (e.g., Millar 1960: 13– 15 rows), the structure of the colonies, shape of the atrial aperture and digestive system is in agreement with descriptions of this species, which has a circum-antarctic distribution but had not been previously reported from Chile.

Figures

FIGURE 6. Synoicum georgianum. A, colony; B, zooid; C, abdomen (stained material); D. transverse section at the stomach level. Scale bars: A, 1 cm, B, 1 mm, C, D, 0.5 mm.