Microcosmus vesiculosus, Monniot & Monniot, 2001

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 351-353

Microcosmus vesiculosus n. sp.

( Figs 110 ; 111)

TYPE MATERIAL. — Palau. Koror, Malakal Island, 7°20.27’N, 134°27.40’E, 1 m, 8.IX.1995 ( MNHN S2 MIC 158).

ETYMOLOGY. — From the Latin vesiculosus : with many vesicles.

DESCRIPTION

The specimens are light yellowish, with white longitudinal stripes on the internal side of the siphons. The largest of the three specimens is 3.5 cm in diameter. The specimens were collected on an intake line, attached by their ventral side with other fouling species. The tunic is soft but thick. The siphons lie well apart. The siphonal spinules are long and conical ( Fig. 111). There are no epibionts and the tunic surface is smooth. The body wall is brown in formalin. The muscles are strong and form a regular network of crossed bundles issuing from each siphon. The internal side of the body wall is particularly thin and transparent. There are about 30 relatively small tentacles in three orders of size. The largest have two orders of ramifications. They are inserted at the base of the velum.

The prepharyngeal band has two equal blades. The dorsal tubercle in its V-shaped indentation is large and protruding with an opening that has horns rolled inward. The dorsal lamina is a low blade.

The branchial sac has seven folds on the right side but only six on the left side. The formula is:

R.E. 3 11 5 13 5 13 5 15 5 16 5 16 3 15 2 D.L. D.L. 3 16 3 17 5 15 5 16 5 15 5 14 4 E.L.

The folds are high, overlapping each other. There are six to seven stigmata in a mesh between the folds and four to five on the sides of the folds. One or two stigmata in each row at the top of the folds are curved. There are parastigmatic vessels.

The gut forms a closed loop ( Fig. 110A, C ) attached to the body wall along its whole length. The end of the rectum is attached to the dorsal lamina. The hepatic gland has two equal lobes, it is covered with thin papillae ( Fig. 110C ). The anus is lobed. On each side there is one gonad that forms a single mass there ( Fig. 110 ). The gonoducts are very short. The male and female papillae are joined. They open at some distance from the cloacal siphon.

The endocarps are particularly numerous for the genus. They are large on the left side and smaller on the right side. Their shape is papillated, the protrusions being more visible as they are filled with brown cells.

There is a large cloacal velum covered with numerous fine hairs.

REMARKS

This species is characterised by massive gonads in a single lobe, and by how the left gonad is much larger than the right one. The left gonad is placed more anteriorly than in other species of the genus. The large number of endocarps, especially on the left side, is exceptional (hence the species name). The gut loop occupies most of the left side of the body, which is much more developed than the right side.

This species is closely allied to Microcosmus anchylodeirus Traustedt, 1883 , a species known from the Caribbean area: Saint Thomas and Martinique. They mainly differ in the number of endocarps on the left side of the body. The geographical distance between the two species’ sites of occurrence persuades us against synonymizing them.

Figures

FIG. 110. — Microcosmus vesiculosus n. sp.; A, B, left and right sides of the body; C, body ventrally opened. Scale bars: 1 cm.