Trididemnum poma, Monniot & Monniot, 2001

Trididemnum poma View in CoL n. sp.

( Figs 50 View FIG ; 51A View FIG ; 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 View FIG ) but becomes a transverse slit by contraction. There are only three stigmata in a row on each side ( Fig. 50A View FIG ). The retractor muscle is very short, as short as the oesophageal peduncle. The abdomen has the same size as the thorax ( Fig. 50 View FIG B-D). The gut is a flat open loop with the usual compartments ( Fig. 50D View FIG ).

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

The larvae ( Fig. 50E View FIG ) 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 View FIG ).

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.