Pseudodistoma digitata, Monniot & Monniot, 2001

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 225-227

Pseudodistoma digitata n. sp.

( Figs 18 ; 114D )

TYPE MATERIAL. — Papua New Guinea. Louisiade Archipelago, Deboyne Islands, Panaete Island, 10°41.72’S, 152°20.13’E, 12 m, 30. V.1998 ( MNHN A1 PSE 56).

ETYMOLOGY. — From the Latin digitus: finger.

OTHER MATERIAL EXAMINED. — Papua New Guinea. Louisiade Archipelago, N side Tagula Island, Ima Channel, wall, 11°25.92’S, 153°26.78’E, 7 m, 3.VI.1998 (MNHN A1 PSE 51).

A B C

DESCRIPTION

Yellow in life, this species becomes brown in formalin. The colonies are bushes of lobes ( Fig. 114D ), single or branched, with stiff long peduncles ending in soft enlarged heads. Each lobe may reach 25 mm in length with a head 6 to 7 mm in diameter. The peduncle is coated with sand and many epibionts but the head is naked. The internal tunic contains some sand. Zooids are not in systems. The siphons open at the top of the head. The abdomens and post-abdomens extend into the peduncle, each prolonged there by a thin vascular appendage.

The zooids have two short siphons with six lobes each. The oral aperture is terminal ( Fig. 18 A-C). The cloacal siphon is often displaced posteriorly by contraction. The body wall has a strong musculature along the whole body length, grouped in bundles on each side of the post-abdomen.

The thorax and abdomen have about the same length when expanded. The branchial sac contains about 25 long, narrow stigmata on each side in the first row and 20 in the two last rows.

The stomach lies in the lower third or quarter of the gut loop, at the base of a long cylindrical oesophagus ( Fig. 18 A-C). The stomach wall has four low ridges ( Fig. 18C ). The mid-intestine is very short and the rectum begins with caeca ( Fig. 18C ).

The post-abdomen has a variable length but is always longer than the thorax and abdomen together ( Fig. 18A, B ). The gonads lie in its posterior half or even farther down its length. Ovary and testis were found in separate zooids ( Fig. 18A, B ), they mature successively. The ovary is anterior to the testis lobes which form a double row down to the heart. A single larva is incubated at a time in the cloacal cavity ( Fig. 18B ). The tail makes a half circle around a trunk that is 550 µm long. There are three adhesive papillae that are well-separated on long stems, and four pairs of round ampullae. An odd vesicle appears ventrally. Ocellus and otolith are present ( Fig. 18D ).

REMARKS

The shape of the colony, with its ramified lobes, is exactly the same as in Pseudodistoma arborescens Millar, 1967 which differs in its blue colour. The characters of the zooids are not very different in both species, but several species of the genus have a similar organisation. P. arborescens has a stomach with deeper folds and its larva is more elongated with more anterior ampullae. The distinction between these two species is almost impossible in the absence of larvae, and so previous identifications of P. arborescens in the Pacific Ocean now appear doubtful (Monniot F. 1987, New Caledonia).

Pseudodistoma poculum Monniot F. & Monniot C., 1996 has also pedunculate lobes, but they are deep red and have a central hollow at the top of each head in which the cloacal siphons open. Zooids and larvae are similar to P. digitata n. sp.

Pseudodistoma aureum ( Brewin, 1957) and P. coronatum Monniot F. & Monniot C., 1996 have pillow-like colonies without embedded sand, and they incubate several larvae at once.

Pseudodistoma fragile Tokioka, 1958 ( Figs 19 ; 114E )

Pseudodistoma fragilis Tokioka, 1958: 316 , fig. 3. Type locality: Japan. — Monniot F. 1987: 508, fig. 5, New Caledonia.

Pseudodistoma gracilum Kott, 1992a: 433 , fig. 19, pl. 4a-e, Australia.

MATERIAL EXAMINED. — Philippines. Bohol Sea, Camiguin Island, 9°15.38’N, 124°39.12’E, 8 m, 19.IV.1997 ( MNHN A1 PSE 39).

Papua New Guinea. Coral Sea, Eastern fields, 10°01.02’S, 145°38.66’E, 7 m, 14. VI.1998 ( MNHN A1 PSE 52). — Coral Sea, Eastern fields, 10°09.35’S, 145°33.92’E, 20 m, 15. VI.1998 ( MNHN A1 PSE 55).

Mozambique. Ibo, 1995, coll. Monniot ( MNHN A1 PSE 37).

DESCRIPTION

The colonies are the same in all locations; gelatinous balls of vitreous tunic containing zooids arranged in circular systems ( Fig. 114E ). The colonies are white or for some of the colonies in Papua New Guinea show light brown pigment. The zooids everywhere have the same anatomy as the type specimen: a wide thorax, an appended incubation pouch, a wide oesophagus with a thickened ring, and a stomach with a square section and sometimes two longitudinal crests. The gonads are in a cluster at the posterior part of the postabdomen. The large larvae have three bulky adhesive papillae, one dorsal and one ventral ampulla, and two pairs of intermediate ampullae ( Fig. 19 ).

REMARKS

In the West Pacific stations, the zooids contain generally one fully developed larva and one younger embryo, as the specimens described from New Caledonia (Monniot F. 1987). In the Japanese type specimens that we examined, contrary to Tokioka’s (1958) description, the zooids have only one fully developed larva, 1.75 to 1.90 mm in length, and one or two young embryos. Consequently Pseudodistoma gracilum Kott, 1992 is synonym of P. fragile .

The collection of P. fragile in Mozambique extends the distribution of the species to the Indian Ocean.

Figures

FIG. 18. — Pseudodistoma digitata n. sp.; A, B, zooids; C, thorax and abdomen of another zooid; D, larva.Scale bars: A, B, 1 mm; C, 0.5 mm; D, 0.25 mm.

FIG. 114. — A, Monniotus papillosus n. sp.; B, Polyclinum psammiferum Hartmeyer, 1911; C, Pseudodistoma aureum (Brewin, 1957); D, Pseudodistoma digitata n. sp.; E, Pseudodistoma fragile Tokioka, 1958; F, Pseudodistoma novaezelandiae (Brewin, 1950).

FIG. 19. — Pseudodistoma fragile Tokioka, 1958, larva. Scale bar: 1 mm.