Trididemnum poma, Monniot & Monniot, 2001

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 264-265

Trididemnum poma n. sp. ( Figs 50 ; 51A ; 119E)

TYPE MATERIAL. — Commonwealth of the Northern Marianas. Saipan, 6 m, coll. Paulay( MNHN A2 TRI 152).

ETYMOLOGY. — From the Latin poma : lid.

DESCRIPTION

The brittle colony is encrusting, 1 mm thick when preserved in formalin, with a pattern of deep, curved grooves in which the cloacal siphons open. The colour in life is pure white (Fig. 119E). The spicules are dense and the zooids extremely small.

The zooids are distributed in double rows along the cloacal channels, the cloacal siphons opening directly into the grooves when the colony is contracted. The two rims of each channel are folded above the groove and shut it as would a lid (hence the species name poma = lid). There is no roof of tunic above the grooves.

The oral siphon is short. The cloacal aperture uncovers a large part of the branchial sac when relaxed ( Fig. 50A ) but becomes a transverse slit by contraction. There are only three stigmata in a row on each side ( Fig. 50A ). The retractor muscle is very short, as short as the oesophageal peduncle. The abdomen has the same size as the thorax ( Fig. 50 B-D). The gut is a flat open loop with the usual compartments ( Fig. 50D ).

The testis, in a single follicle ( Fig. 50B ), lies inside the intestinal loop covered by six to seven coils of the sperm duct. The ovary ( Fig. 50C ) is on the side of the testis.

The larvae ( Fig. 50E ) are small in the basal layer of the colony. The trunk is 0.25 mm in length and the tail makes one turn around it. The larvae have an ocellus and an otolith. There are three adhesive papillae, four pairs of ampullae, and no buds.

The spicules, 35 µm in diameter, have short conical rays ( Fig. 51A ).

REMARKS

This species has a very peculiar colonial anatomy, with the cloacal channels opened at the colony surface and only covered by a lid made by their folded rims when the colony is contracted. The zooids and larvae are particularly small. This species has been found only in a submarine cave.

Trididemnum strigosum Kott, 1980 (Fig. 119F)

Trididemnum strigosum Kott, 1980: 8 , figs 9; 10, pl. 3-3. Type locality: Philippines ; 1981: 188, Fiji. — Monniot C. & Monniot F. 1987: 22, fig. 4A-D, Polynesia .

MATERIAL EXAMINED. — Palau. KB Channel, Airai on sea grass, 7°19.35’N, 134°31.73’E, 0.5 m, 20. VI.1996 ( MNHN A2 TRI 134).

DESCRIPTION

This species has algal cells embedded in the tunic. The colonies are small round or oval patches, grouped in clusters. The colour is green to pale green (Fig. 119F), the intensity depending on the abundance of spicules in the superficial layer of the colonies. There is generally a single common cloacal aperture in the centre of each colony. There are accumulations of dark brown bodies in the abdominal layer.

The zooids are extremely small. They have six sharp oral lobes. The cloacal aperture is a wide transverse slit. The leaf-like lateral thoracic organs protrude at the level of the second stigmata row. There is a short retractor muscle.

The abdomen is the same size as the thorax and has a single testis follicle. The larvae contain algal cells but are not fully developed. The spicules are stellate with a few short rays.

REMARKS

This species, first collected in the Philippines, then in Fiji and Polynesia, is now recorded in Palau. The species probably has a wide distribution in the western Pacific region, but is rather inconspicuous with its small colonies.

Figures

FIG. 50. — Trididemnum poma n. sp.; A, thorax; B-D, abdomen; E, larva. Scale bar: 0.25 mm.

FIG. 51. — Spicules; A, Trididemnum poma n. sp.; B, Didemnum algasedens n. sp. Scale bars: 10 µm.