Eudistoma sp.

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 248

Eudistoma sp.

( Figs 37 ; 118 )

MATERIAL EXAMINED. — Palau. Western barrier reef, Blue Hole, 7°08.10’N, 134°13.90’E, 12 m, 11. V.97 ( MNHN A3 EUD 231).

DESCRIPTION

The colonies are flat cushions, the largest 5 mm in thickness and 25 mm in diameter. The tunic is entirely embedded with sand: but a small area around each oral siphon remains clear. This gives an areolated appearance to the smooth colony surface ( Fig. 118A ). The tunic and zooids are colourless. Some white pigment spots are present in the mucous layer surrounding the zooids.

The zooids are very contracted. The most relaxed ones reach 5 mm, but they may be longer in life. The abdomen is much longer than the thorax ( Fig. 37A ). Both siphons are short. The number of stigmata could not be counted.

The abdomen is much narrower than the thorax. The oesophageal part is especially slender. The abdomen is enlarged only in its basal part, which contains a round stomach and the gonads ( Fig. 37C ). The gut loop is straight in the most relaxed zooids. The testis, at the bottom of the gut loop, has numerous lobes. The ovary, beside the testis, is poorly developed in this specimen. One or two larvae are incubated inside the cloacal cavity ( Fig. 37B ). Only one mature larva has been found ( Fig. 37D ). It has three adhesive papillae on a line, paired ampullae dorsally and ventrally, and three odd, alternating ampullae. The trunk measures 0.7 mm.

REMARKS

The strongly contracted state of the colonies does not allow one to ascertain the musculature, the branchial sac, or shape of the gut loop. The single larva, though, is enough to indicate that this specimen cannot be assigned to a known species. An arrangement of zooids into systems was not evident, and it was not possible to decide if the siphons were in circles or not.

By its colonial structure as low cushions, the embedded sand, and the clear spots around the oral apertures, this species looks like Eudistoma constrictum Kott, 1990 . But the larva, with only odd ampullae between its adhesive papillae, does not correspond to the description of this species.

Figures

FIG. 37. — Eudistoma sp.; A, B, two zooids; C, abdomen; D, larva. Scale bars: A, B, 1 mm; C, 0.5 mm; D, 0.25 mm.

FIG. 118. — A, Eudistoma sp.; B, Polycitor giganteus (Herdman, 1899); C, Polycitor translucidus Kott, 1957; D, Polycitorella coronaria Monniot F., 1988; E, Polycitorella mariae Michaelsen, 1924; F, Polycitorella stellifera n. sp.