Pseudodistoma Michaelsen, 1924

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 224-225

Genus Pseudodistoma Michaelsen, 1924

Pseudodistoma aureum ( Brewin, 1957) ( Figs 17 ; 114C )

Sigillinaria aurea Brewin, 1957: 580 . Type locality: New Zealand.

Synonymy: see Pseudodistoma aureum – Kott 1992a: 426. MATERIAL EXAMINED. — Tonga. Vava’u Fotula, small rock off west coast of Vava’u, 18°38.92’S, 174°04.12’W, 30 m, 13.XI.1997 (MNHN A1 PSE 50).

DESCRIPTION

The colonies are bulbous and pale yellow. The surface is slippery and without sand ( Fig. 114C ). A colony with a diameter of 25 mm has a length of 20 mm. There are no systems. The superficial layer of the tunic is very soft, the internal tunic tougher. When preserved, a white pigment makes the tunic opaque.

Both siphons are of equal length, with six lobes. The thorax is shorter than the abdomen ( Fig. 17A ). The gut loop is long. The stomach, with four folds ( Fig. 17A, C ) lies well posteriorly, followed by a short conical poststomach. The mid-intestine is short and transverse, and the rectum begins with caeca. The post-abdomen ( Fig. 17B ) has a variable length; when the testis and ovary are fully mature, it is about twice as long as the abdomen. The ovary is far from the gut and is followed by a string of testis follicles that reaches the heart.

The larvae ( Fig. 17D ) are incubated in the abdomen, alongside the oesophagus. The trunk measures 0.5 mm. The larva’s three rows of stigmata are already differentiated. The three long, diverging adhesive papillae alternate with four pairs of digitiform ampullae.

REMARKS

This species is characterised mainly by the balllike shape of its sand-free, yellow colonies, as well as by their translucent tunic and the zooid’s long oesophagus. It is distributed through the whole tropical western Pacific Ocean.

Figures

FIG. 17. — Pseudodistoma aureum Brewin, 1957; A, B, two parts of a zooid; C, abdomen; D, larva. Scale bars: A-C, 1 mm; D, 0.25 mm.

FIG. 114. — A, Monniotus papillosus n. sp.; B, Polyclinum psammiferum Hartmeyer, 1911; C, Pseudodistoma aureum (Brewin, 1957); D, Pseudodistoma digitata n. sp.; E, Pseudodistoma fragile Tokioka, 1958; F, Pseudodistoma novaezelandiae (Brewin, 1950).