Polysyncraton adelon, Monniot & Monniot, 2001

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 272-273

Polysyncraton adelon n. sp.

( Figs 54B ; 56 )

TYPE MATERIAL. — Palau. Southwest Islands, Sonsoral, 5°20.88’N, 132°13.80’E, 15 m, 24.VII.1995 ( MNHN A2 POL 94).

ETYMOLOGY. — From the Greek adelon : hardly visible.

DESCRIPTION

Grey-green in formalin, the colonies form very soft, extensive sheets 1 to 2 mm thick. They are easily delaminated, as the common cloacal cavities are extensive at the thoracic level. The spicules are dense everywhere and mixed with dark pigment cells, which are more abundant in the upper layer and around the thoraces. The common cloacal cavity does not penetrate the abdominal layer of the colony.

The zooids are greenish in formalin. The oral siphon is short with very small lobes. The cloacal opening is very wide, without a languet, uncovering a large part of the branchial sac ( Fig. 56A ). There are eight or nine stigmata in the first and second half rows, seven in the posterior row. The lateral thoracic organ is round, placed above the third transverse vessel or the fourth stigmata row ( Fig. 56A ). The retractor muscle is of variable length, but not longer than the thorax, and arises mid-way down the oesophageal peduncle.

The abdomen ( Fig. 56B ) is slightly smaller than the thorax. The gut loop is wide open, with a spherical anterior stomach. The gonads are included in the digestive loop. There are three or more often four testis lobes, pressed into a rosette and covered by four to five turns of the sperm duct ( Fig. 56B ). The ovary ( Fig. 56C ) replaces the testis in the zooid’s reproductive cycle.

No larvae were present in the colonies studied.

The spicules reach 35 µm in diameter. They are spheres made of multiple thin blunt rays, not pointed ones ( Fig. 54B ).

REMARKS

This species is placed in the genus Polysyncraton instead of Didemnum according to the pragmatic understanding detailed earlier (Monniot F. 1993), as it has no cloacal languet but four testis lobes. Rather few species in this genus exist with many testis lobes but no cloacal languet. By its colour when preserved, the present species looks like P. hartmeyeri Michaelsen, 1923 from the Red Sea, which has five testis lobes, five to six turns of the sperm duct, more stigmata in a row, and – the most telling distinction – no retractor muscle.

P. magnetae Hastings, 1931 from the Great Barrier Reef has five testes but only two coils of the sperm duct, and its zooids are distributed in double rows. Polysyncraton victoriensis Kott, 1976 from southern Australia has three to four testis follicles, but the spicule shape and arrangement is different and there are only six stigmata per half row. She does not mention a cloacal languet in her description.

P. tubiporae Michaelsen, 1920 from Zanzibar has a spicule-free layer at the colony surface, lateral thoracic organs placed anteriorly, and six to eight testes encircled by two to three loose turns of the sperm duct.

Polysyncraton aspiculatum Tokioka, 1949 ( Fig. 121A )

Didemnum (Polysyncraton) aspiculatum Tokioka, 1949: 2 , pl. 2, fig. 1-3. Type locality: Japan. Synonymy: see Monniot C. et al. in press, South Africa.

MATERIAL EXAMINED. — Philippines. Luzon, NE side of Maricaban Island, Gamao Point, 13°38.39’N, 120°56.86’E, 16 m, 25.VII.1994 ( MNHN A2 POL 81).

DESCRIPTION

The colonies are thick light brown crusts ( Fig. 121A ) with a network of depressed lines above the cloacal channels. The cloacal apertures are not numerous. The colony surface is smooth and clean. Spicules are totally absent. The tunic is filled with vesicular cells and round, brown pigment cells.

The structure of the zooids is the same as in specimens from other regions and in the type specimen. The cloacal languet is large with two long lobes in zooids near the common cloacal apertures. The branchial sac has eight stigmata per row on each side and is as brown as the remaider of the body. The testis follicles are numerous, about eight in a rosette encircled by two and a half coils of the sperm duct.

The larvae are large compared to the zooid size. They have three adhesive papillae and eight pairs of ampullae grouped in pairs. The body wall has vesicular cells as figured by Tokioka (1953) for this species.

REMARKS

The distribution of P. aspiculatum in the western Indian Ocean, southern Australia, the Philippines, and Japan is particularly wide. The variability in density of spicules and pigment cells in different locations raises questions about the presence of just a single species, but the zooids and larvae are identical.

Figures

FIG. 54. — Spicules; A, Didemnum madeleinae n. sp.; B, Polysyncraton adelon n. sp. Scale bars: 10 µm.

FIG. 56. — Polysyncraton adelon n. sp.; A, thorax; B, abdomen; C, larva. Scale bar: 0.5 mm.

FIG. 121. — A, Polysyncraton aspiculatum Tokioka, 1949; B, Polysyncraton cerebellum n. sp.; C, Polysyncraton thallomorpha Monniot F., 1993;D, Diplosoma marsupiale n. sp.; E, Diplosoma pannosum n. sp.; F, Diplosoma pavonia Monniot C. & Monniot F., 1987.