Eudistoma laysani ( Sluiter, 1900 )

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 242-243

Eudistoma laysani ( Sluiter, 1900)

Distoma laysani Sluiter, 1900: 9 . Type locality: Indonesia.

Eudistoma laysani – Monniot F. & Monniot C. 1996: 190, Papua New Guinea. — Kott 1990: 193, Australia.

MATERIAL EXAMINED. — Fiji. Somosomo Straits, 16°46.75’S, 179°55.81’E, 8 m ( MNHN A3 EUD 167).

Eudistoma ovatum ( Herdman, 1886) ( Figs 32 ; 117B)

Psammaplidium ovatum Herdman, 1886: 246 . Type locality: northern Australia.

Eudistoma ovatum – Hastings 1931: 82. — Kott 1972a: 43; 1990: 222, fig. 85a-f.

MATERIAL EXAMINED. — Palau. Ongael Marine Lake, marine lake on island just west of Ongael Island, 7°15.09’N, 134°22.53’E, 1 m, 14.III.1994 ( MNHN A3 EUD 123).

Papua New Guinea. Louisiade Archipelago, Deboyne Lagoon, Nivani Island, 10°47.46’S, 152°23.08’E, 12 m, 30. V.1998 ( MNHN A3 EUD 225).

DESCRIPTION

The colonies are up to 2 cm thick and 12 cm in length. They contain fine sand embedded in the tunic. The zooids’ apertures are arranged in regular, round systems (Fig. 117B). The colour in life was brown or pale yellow. In addition to the sand, there are abundant crystals inside the tunic. The zooids are elongated. Both siphons have six lobes. The number of stigmata could not be counted. The abdomen is much longer than the thorax and thin in the oesophageal region. The rectum is twisted below the stomach, as described by Hastings (1931). The gonads are in the gut loop, protruding on each side of the abdomen. One or two larvae are incubated in the distended cloacal cavity, which does not form a pouch. Larval trunks are about 1 mm long ( Fig. 32 ). They have three adhesive papillae that are not equidistant and numerous ampullae arranged in two irregular circles around the papillae. These ampullae become more numerous by division: they extend into the ventral region.

REMARKS

Abundant large vesicles of symbiotic algae are distributed through the entire thickness of the colonies, but more abundantly in the surface layer. The species is mostly characterised by its colony: a surface incrusted with fine sand, an annular pattern of siphons, and the presence of crystals in the tunic. Additionally, the twisted basal part of the rectum and the larva with numerous anterior ampullae help to distinguish this ascidian.

The species seems widely distributed in western, northern, and eastern Australia and in Palau. The synonymy with Eudistoma vulgare Monniot F., 1988 , suggested by Kott (1990), cannot be retained, as that species has a smaller and different larva with fewer ampullae.

Figures

FIG. 32. — Eudistoma ovatum (Herdman, 1886), larvae. Scale bar: 0.5 mm.