Polysyncraton cerebellum, Monniot & Monniot, 2001

Monniot, Françoise & Monniot, Claude, 2001, Ascidians from the tropical western Pacific, Zoosystema 23 (2), pp. 201-383 : 273-275

Polysyncraton cerebellum n. sp.

( Figs 57 ; 58A ; 121B )

TYPE MATERIAL. — Palau. Southwest Islands, Helen Reef, overhang, 2°53.03’N, 131°44.94’E, 12 m, 10.XII.1996 ( MNHN A2 POL 97).

ETYMOLOGY. — From the Latin cerebellum : brain.

OTHER MATERIAL EXAMINED. — Papua New Guinea. Louisiade Archipelago, Calvados Island Chain, Brooker Channel, 11°03.09’S, 152°28.62’E, 12 m, 1.VI.1998 (MNHN A2 POL 96).

DESCRIPTION

In large crusts about 2-3 mm thick, these colonies are marbled with brown on a white ground. The pattern suggests a brain’s surface ( Fig. 121B ) (hence the species name). The zooids are irregularly placed on the sides of large channels delimiting oval or polygonal areas. The common cloacal apertures are numerous and stellate. The pigment cells are superficial. Dark brown inclusions lie in the basal test. The colonies are easily delaminated into a surface layer containing the oral siphons and the cloacal languets and a basal layer containing the abdomens and larvae. The thoraces are included in pillars of tunic uniting these layers.

The oral siphons are large, funnel-shaped, with six lobes ( Fig. 57A, B ). The cloacal aperture uncovers most of the branchial sac. The languet is either short with a round tip ( Fig. 57A ) or, in zooids near the common cloacal apertures, large and T-shaped ( Fig. 57B ). The branchial sac has eight to 10 stigmata per side in the first row. The lateral thoracic organs protrude from the rim of the cloacal aperture at the level of the first or second stigmata row. The retractor muscle, shorter than the thorax, projects at the base of the oesophageal peduncle ( Fig. 57A ). The gut loop is largely opened and only slightly twisted when the testis is fully mature. There are two testis follicles, tightly pressed against each other, covered by five turns of the sperm duct ( Fig. 57C ). The ovary develops against the testis with one large oocyte. The larval trunks are 1 mm long. Larvae have three adhesive papillae and at their base a comb-like crown of 24 pairs of ampullae.

The spicules ( Fig. 58A ) are stellate with numerous short, pointed rays. The largest spicules reach 65 µm in diameter. They are very dense throughout the tunic, making it stiff.

REMARKS

This species is very similar to P. thallomorpha Monniot F., 1993 . It differs in the colony’s pigment pattern and the arrangement of the zooids in meshes. Also, the oral siphon is less wide in this new species. The lateral thoracic organs are above the third row of stigmata in P. thallomorpha . The larvae are very similar in both species. The spicules have more rays in P. cerebellum n. sp.

Polysyncraton lithostrotum Brewin, 1956 from New Zealand and recorded again in New Caledonia (Monniot F. 1993: 4, fig. 1, pl. 1A) has also a mosaic pattern but a very different one; the zooids in that species are included in polygons in the centre of each of which opens a single common cloacal cavity. The anatomy of the zooids and larvae in that species are exactly the same as in P. cerebellum n. sp.

Polysyncraton thallomorpha Monniot F., 1993 ( Fig. 121C )

Polysyncraton thallomorpha Monniot F., 1993: 13 , fig. 7, pl. IIA. Type locality: New Caledonia.

MATERIAL EXAMINED. — Philippines. Bohol Sea, Camiguin Island, 9°12.89’N, 124°38.03’E, 7 m, 18.IV.1997 ( MNHN A2 POL 88).

DESCRIPTION

The colonies have the characteristic shape described in New Caledonia; hard, thin crusts partially raised above the sediment. The colour is darker here, almost black, but the margins of the lobes remain white ( Fig. 121C ). The undersurface of the leaf-like crust is extremely hard with ridges.

The pigment cells are dendritic, restricted to the colony’s upper surface.

The zooids have the characteristic long and wide oral siphon, and a large cloacal aperture with a languet varying in length according to the proximity of the zooids to the common cloacal cavity. The lateral thoracic organs protrude at the level of the third stigmata row. The two testis lobes overlap the gut loop and are covered by five turns of the sperm duct. The larvae have three adhesive papillae encircled by very numerous digitiform ampullae.

The spicules are irregular, as figured in Monniot F. (1993: pl. II A).

Figures

FIG. 57. — Polysyncraton cerebellum n. sp.; A, B, thoraces; C, abdomen; D, larva. Scale bars: A, C, D, 0.5 mm; B, 0.1 mm.

FIG. 58. — Spicules; A, Polysyncraton cerebellum n. sp.; B, Polysyncraton vestiens n. sp. Scale bars: 10 µm.

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.