Polydistoma oculeum, Monniot & Monniot, 2001

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 255-256

Polydistoma oculeum n. sp.

( Figs 43 ; 119A)

TYPE MATERIAL. — Philippines. Mindanao, Davao, SW side of Samal Island, wall, 6°56.31’N, 125°47.66’E, 12 m, 7.IV.1996 ( MNHN A3 POL.C 1).

ETYMOLOGY. — From the Latin oculeus: with eyes.

DESCRIPTION

The dark green colonies (Fig. 119A) are crusts 5 cm thick and up to 15 cm across with an irregular outline. The surface has numerous wide common cloacal apertures each on a short raised cylinder of tunic. Each aperture corresponds to a system’s independent common cloacal cavity. The oral openings of the zooids form irregular rings around these common clocal apertures, and after fixation in formalin they, too, protrude a little at the colony surface.

The tunic is opaque, very dark, and soft, but the surface layer is more elastic. When fixed, the colonies are very contracted and curled, the upper surface becoming concave. The tunic’s pigment diffuses in formalin but not in alcohol. The zooids ( Fig. 43A ) lie perpendicular to the colony surface and are almost black. Both siphons are tubular, narrow, with plain rims. Equal to or slightly longer than the oral siphon, the cloacal siphon is located anteriorly in the thorax and opens into the common cloacal cavity of the system. Sixteen long oral tentacles are distributed along two circles. The neural ganglion is very close to the tentacle ring. Muscles make a strong sphincter around each siphon. Some thin transverse fibres run between the siphons, and, starting from the oral siphon, six to eight thin, regularly spaced longitudinal bundles run down each side of the thorax toward the abdomen. These longitudinal fibres join to make two ribbons on the wall of the abdomen itself, converging toward the posterior part of the gut loop, where they make an anchoring point. A few muscle fibres extend into a postabdominal process.

The branchial sac has five rows of 12 to 14 elongated stigmata ( Fig. 43A ). The languets of the dorsal lamina are clearly displaced on the left side. There are no parastigmatic vessels.

The abdomen is not neatly separated from the thorax ( Fig. 43A ). The cylindrical oesophagus is short and narrow. The stomach is spherical and smooth-walled, with the oesophageal and intestinal connections close to each other ( Fig. 43B ). The first intestinal segment following the stomach is narrow with a slightly protruding ring at its middle. The mid-intestine is inflated between two narrowings. The posterior intestine begins with caeca. The rectum crosses the oesophagus and opens through a bilobed anus at the level of the third transverse vessel, to which it is attached by a strip of tissue.

The gonads ( Fig. 43B ) are located far posteriorly on the left side of the body, against the midintestine, and bulge around it beneath the gut loop. The testis is divided into two to five (more often three or four) joined lobes, with short sperm ducts fused into a straight common sperm duct that runs alongside the rectum. The ovary develops on the external side of the testis ( Fig. 43B ). Only one oocyte grows at a time, protruding below the abdomen.

There are one or two vascular processes ( Fig. 43A ) issuing from the right side of the abdomen, one of which always has some muscular fibres at its origin and ends in a clear, wide ampulla. In addition one or two cylindrical prolongations, more opaque and of variable length, extend from the zooid into the deeper part of the tunic.

No larvae were present in the colonies examined, and no trace of an incubatory pouch was detect- ed. Some rare and poorly developed buds lie isolated in the depths of the colony.

REMARKS

The characters leading us to include this species in the genus Polydistoma Kott, 1990 are the five rows of stigmata in the branchial sac, the oesophagus placed as it is partly inside the thorax, and the position of the gonads beside the gut loop. Polydistoma oculeum n. sp. differs from the two other species of the genus, P. fungiforme Kott, 1990 and P. longitube (Kott, 1957) , in several characters. It has common cloacal cavities that are absent in the others and a very different colony shape, no lobes on the siphonal rim, and a short cloacal siphon direct- ed dorsally. This last character results in the circular disposition of the zooids around a nearby common cloacal opening.

The genera Hypodistoma and Exostoma have common cloacal cavities, but only three rows of stigmata, and the abdomen in them is clearly separated from the thorax.

Figures

FIG. 43. — Polydistoma oculeum n. sp.; A, zooid; B, gonads and gut. Scale bar: 1 mm.