Didemnum nocturnum Monniot & Monniot, 1999

Monniot, Françoise, 2010, Some new data on tropical western Pacific Ascidians, Zootaxa 2561 (1), pp. 1-29 : 7-8

Didemnum nocturnum Monniot & Monniot, 1999

( Figure 7A , 9 )

Didemnum nocturnum Monniot F. & Monniot C., 1999: 7 Figs 6 , 7A , pl. 1C.

Material. coll. Coral Reef Research Foundation, Palau, Southwest Isls, Helen Reef channel, 02°52.38’N – 131°44.14’E, 30m, 24/IX/2008, 0CDN 9889-J ( MNHN A2 DID. C 602 ) .

The colonies are dark blue in soft crusts 3–4mm thick ( Fig. 7A ). The zooids are in double rows along cloacal channels. In formalin the colour is lighter; the tunic is grey with black round pigment cells. These black cells are sparse at the colony surface, more abundant at the thorax level and form pellets in the basal layer of the tunic. There is no sediment encrusted and no spicules. The zooids are colorless, held in pillars of tunic crossing the wide common cloacal cavity. The cylindrical oral siphon has 6 small apical teeth, the atrial aperture is wide without a languet. No lateral organs were detected. Ten to 12 stigmata per side were counted in the first branchial row, in poorly preserved zooids. The muscular appendage is long ( Fig. 9A ), extending far below the abdomen level. The waist is short. The abdomen is smaller than the thorax. A single testis vesicle is surrounded by 5 turns of the sperm duct ( Fig.9B ). No larvae were found in this material and in their absence the species identification is difficult. Nevertheless the colour and structure of the colonies, the absence of spicules, the black round pigment cells, the numerous stigmata and the very long muscular appendage ascertain the identification of this material compared to the re-examined type.

Didemnum nocturnum was previously recorded only from the Indian Ocean: its presence in Palau widely increases its distribution but is no surprising as many ascidians are common to the Pacific and Indian oceans.

Figures

FIGURE 6. Didemnum biglutinum: A: abdomen, B: thorax, C: larva. Scale bars: A=0.2mm, B–C=0.5mm.

FIGURE 7. A: Didemnum nocturnum. B: Didemnum parau. C: Didemnum rodriguesi. D: Lissoclinum bistratum.

FIGURE 9. Didemnum nocturnum: A: thorax, B: abdomen. Scale bar 0.5mm.