Polysyncraton trivolutum Millar 1960

Polysyncraton trivolutum (Millar, 1960)

Fig. 3 C,D,G

References and synonymy: Polysyncraton trivolutum Millar (1960) p. 58; Monniot & Monniot (1983) p. 43; Monniot et al. (2011) p. 10.

Localities: 5S.

This species forms crusts several cm across and 4–5 mm thick. The colour is white with a brownish tinge. The spicules are dense, in general less than 25 µm in diameter (occasionally reaching 40). They are stellate with short rays, up to ten in optical section. The general cavity of the colony is at the thoracic level, but it extends between groups of abdomina.

The thoraces reach 2 mm in length. They have an atrial languet that ranges from very small and simple to long and with several points. Cup-like thoracic organs are prominent between the second and third row of stigmata. No retractor muscle is present. The abdomina are smaller than the thoraces, and are in general in male phase. They have three testicular lobes surrounded by a sperm duct describing four coils. Some zooids also have a few developing oocytes adjacent to the testes.

Remarks. the presence of atrial languet, absence of retractor muscle, structure of thoracic organs and gonads allows the assignment of our specimens to this species. However, P. trivolutum is highly variable and may comprise a group of closely related species (Monniot & Monniot 1983). This taxon is common in Antarctic and subantarctic regions (Monniot & Monniot 1983, Monniot et al. 2011, Schories et al. 2015).