Didemnidae

Monniot, Françoise, Dettai, Agnès, Eleaume, Marc, Cruaud, Corinne & Ameziane, Nadia, 2011, Antarctic Ascidians (Tunicata) of the French-Australian survey CEAMARC in Terre Adélie, Zootaxa 2817, pp. 1-54 : 9-10

Didemnidae

Didemnum biglans ( Sluiter, 1906) ( Figure 2 )

Sluiter, 1906: 29 pl. 2 figs 27, 28. Millar 1960: 57 fig. 11. Kott 1969: 75 and synonymy. Monniot & Monniot 1983: 42 fig. 7 DE, pl.3 H, I and synonymy. Vasseur 1974: 617 figs. 4, 5, pl. 2 fig 1

Stations (events when several trawling operations per station): 2-5-8-9-13 A-21-40-54A-65(322).

The colonies settle in thin crusts on all kinds of substrates: rocks, pebbles or other invertebrates. The abundance of spicules varies in different parts of a same colony. The wide common cloacal cavity encircles pillars of tunic containing the zooids. The larvae are embedded in the basal layer of the colonies, close to the substrate; they are often missing in the damaged colonies of the CEAMARC cruise. The main characters of the species are a small atrial languet, well expanded lateral thoracic organs, no fixative appendage. The testis, with one or two closely pressed vesicles, is circled with 4 to 5 turns of the sperm duct. The larvae 1.6 to 1.8mm long have 3 parallel adhesive papillae and 6 pairs of lateral vesicles ( Fig. 2 A). The stellate spicules (40µm) have numerous short rays ( Fig. 2 B), and many of them are poorly calcified.

Two highly divergent (20.77%) COI sequences were obtained for specimens A2 DID.C 597a (BOLD: ASCAN001-10) and A2 DID.C 598a (BOLD: ASCAN002-10). No close hit in BOLD (best: 82%). Further data would be needed to be able to conclude on the wide molecular divergence observed for this species, as the colonies were damaged.

This species is common in Terre Adélie between 200 and 800 m deep and was previously recorded in the peri- Antarctic area down to 1000 m.

Polysyncraton trivolutum ( Millar, 1960) ( Figure 3 )

Didemnum trivolutum Millar, 1960: 58 . Polysyncraton trivolutum: Monniot & Monniot 1983: 43 fig. 8A–H, pl. 4A–D and synonymy. Tatian et al 2005: 209.

Stations (events when several trawling operations per station): 2-3-8-10 -21-27(45)-27(46)-79.

The colonies are arranged in thick cushions ( Fig. 3 B) with a convoluted surface and appear light brown in fixative liquids. They settle on diverse substrates, mainly on sponges. The oral openings appear in a meandriform design. The very resistant tunic contains few spicules, present only in the surface layer of the colony. The common cloacal cavity is spacious around the zooid thoraces and penetrates into the abdominal layer. The oral siphon has 6 short pointed lobes. The atrial aperture is wide with a small upper languet. The thorax is smaller than the abdomen. Ten stigmata were counted in the two first branchial half-rows. There is no muscular appendage. The testis has generally 3 or 4 lobes enclosed in 6 to 7 turns of the sperm duct, but sometimes an additional testis with its sperm duct can be found in the abdomen. The larvae ( Fig. 3 A) 1.6 to 2.1mm have 3 slender divergent adhesive papillae arising from a circle of numerous elongated vesicles. Ocellus and otolith are present. The spicules are burr-like with rays in needles.

Sequences were obtained for specimens A2 POL 122a (BOLD: ASCAN004-10) and A2 POL 120a (BOLD: ASCAN003-10) with 0.56% divergence between the two. No close hit in BOLD (best: 82%).

Polysyncraton trivolutum is eurybathe and widely distributed in the Antarctic and Sub-Antarctic regions.

Figures

FIGURE 2. Didemnum biglans. A, larva; B, spicule. Scale bars: A = 0,5 mm; B = 10 µm.

FIGURE 3. Polysyncraton trivolutum. A, larva, scale bar = 1 mm; B, colonies.