Didemnum algasedens, Monniot & Monniot, 2001

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 265-266

Didemnum algasedens n. sp.

( Figs 51B ; 52 ; 120A )

TYPE MATERIAL. — Federated States of Micronesia. Chuuk Lagoon, sediment bottom surrounding

A C D

Fujikawa wreck, 7°20.92’N, 151°52.70’E, 36 m on Halimeda , 22.I.1993 ( MNHN A2 DID.C 417).

ETYMOLOGY. — From the Latin alga: seaweed and sedens: settled.

DESCRIPTION

The white colonies ( Fig. 120A ), 1 mm thick, do not exceed 2 cm across and are generally less. The oral openings are hardly visible on the smooth colony surface. The spicules are dense throughout the colony. The common cloacal cavity forms channels surrounding grouped thoraces. The oral siphons have six pointed lobes. The cloacal aperture is narrow ( Fig. 52A ), round or cut in a slit in the thick body wall. There is no languet. The lateral thoracic organs protrude at the level of the third stigmata row. The oesophageal peduncle ( Fig. 52A ) is particularly long in such a thin colony. The retractor muscle is as long as the thorax.

The abdomen ( Fig. 52B ) which is the same length as the thorax, has a wide gut loop. A poststomach and mid-intestine follow the spherical stomach: at its beginning the wide posterior intestine, red in life, is thicker than the remaider of the gut. The rectal extremity is narrow. The whole gut loop is folded on itself ( Fig. 52B ).

The single testis follicle is located on the posterior intestine and protrudes to the side ( Fig. 52B ), its development accentuating the abdominal fold. The sperm duct is coiled in seven turns. The ovary is on the side of the testis ( Fig. 52B ).

The general outline of the larva is round. The larvae ( Fig. 52C ) are incubated in the abdominal layer of the colony. The trunk measures 430 µm and is encircled by the tail in three quarter of a turn. Beside the three adhesive papillae are four pairs of elongated ampullae. A light brown pigment extends anteriorly on the base of the ampullae.

Ocellus, otolith, three rows of stigmata, and the outline of the gut are clearly visible; there is no trace of budding.

The spicules ( Fig. 51B ) are stellate with pointed rays. They measure 40 µm in diameter.

REMARKS

This species differs from other white Didemnum in the addition of several characters which individually are not distinctive but in combination are. They are a rather long oral siphon, a small cloacal aperture, a spherical larva with a brown pigment, stellate spicules with sharp rays.

D. algasedens n. sp. closely resembles the Caribbean D. halimedae F. Monniot, 1983 , in having the same zooid and larva but different spicules with shorter and more numerous rays.

Figures

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

FIG. 52. — Didemnum algasedens n. sp.; A, thorax; B, abdomen; C, larva. Scale bars: 0.25 mm.

FIG. 120. — A, Didemnum algasedens n. sp.; B, Didemnum cuculliferum (Sluiter, 1909); C, Didemnum lacertosum Monniot F., 1995; D, Didemnum madeleinae n. sp.; E, Didemnum obscurum Monniot F., 1969; F, Didemnum rodriguesi Rocha & Monniot F., 1993.