Polysyncraton millepore Vasseur, 1969

Kott, Patricia, 2004, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (part I), Journal of Natural History 38 (19), pp. 731-774 : 741-743

Polysyncraton millepore Vasseur, 1969

( figure 6A )

Polysyncraton millepore Vasseur, 1969 : figure 917; Kott, 2001: 113 and synonymy. Polysyncraton thallomorpha Monniot and Monniot, 2001: 275 .

? Polysyncraton cerebellum Monniot and Monniot, 2001: 273 .

Distribution. New record: Western Australia (Dongara, 44 m, WAM 207.93). Previously recorded (see Kott, 2001; Monniot and Monniot, 2001): Queensland (Whitsunday Is), New Caledonia, Philippines, West Indian Ocean.

Description. The newly recorded specimen is flat and hard like a brittle wafer. It appears to be a portion (about half) of a disc about 6 cm diameter. In preservative, flecks of brown pigment are in the surface test and siphon linings. The stellate branchial apertures are along each side of depressions over the primary common cloacal canals that surround white circular elevations (about 5 mm diameter) and form a network in the surface. Regularly positioned common cloacal apertures with brown frilled rims are just inside the rounded outer margin of the colony about 1 cm apart. The base of the colony is rippled, white and hard with spicules. Spicules are crowded throughout and a conspicuous plug of spicules is in each branchial siphon. Both surface and basal test are relatively thin, although the abdomina are embedded in the basal test. The common cloacal cavity penetrates in around the thoraces which cross the common cloacal cavity separately, each with a ventral strip of spicule-filled test. Spicules are to 0.06 mm with 11–13 conical rays in optical transverse section, the ray-tips pointed or truncated. Zooids have the characteristic tulip-shaped branchial siphon, about half the length of the thorax with a small false siphon formed by a velum projecting up into its base. The atrial tongue varies in size but is seldom very large and usually is not divided at the tip (see Kott, 2001: figure 54). A tapering retractor muscle of moderate length is present. The gut forms a double loop. Neither gonads nor embryos are in the newly recorded specimen (collected in February).

Remarks. The thin, hard, wafer-like colonies of this species, black on the upper surface and white below, or mottled like the preserved material (see Kott, 2001) are remarkably consistent despite a wide geographic range (see Vasseur, 1969 from Madagascar; Kott, 2001 from NE Australia; Monniot and Monniot, 1997 from Tanzania; and Monniot and Monniot, 2001 from the Philippines). Further, the zooids (with their large branchial siphons and tongue-shaped atrial languets), the larvae (each with a corolla of 24 ectodermal ampullae) and the spicules (some with truncated and others with pointed rays) are the same from all locations. Other species with a similar zooid and atrial tongue are in the genus Didemnum , namely D. bicolor Kott, 2001 , D. bisectatum Kott, 2001 , D. elongatum Sluiter, 1909 and D. inveteratum Kott, 2001 . Of these species D. bisectatum has two male follicles, but in addition to larger spicules with more rays than the present species it has more vas deferens coils and fewer larval ectodermal ampullae. Other species with a wellformed branchial velum are Lissoclinum patella and species in the dubius group of Leptoclinides .

Polysyncraton cerebellum Monniot and Monniot, 2001 , from the Palau Islands and Papua New Guinea, is said to have more spicule rays than the present species, but the only evidence for this is a scanning electron micrograph ( Monniot and Monniot, 2001: figure 58A) showing one small spicule (0.03 mm diameter) with 19 rays in optical transverse section. However, others with fewer rays are in the same micrograph, the species has a mottled pattern like P. millepore: Monniot and Monniot, 1997 from Tanzania and other specimens of the present species (see Kott, 2001) and it has the same cloacal systems. It does not appear to be significantly different from the present species.

Figures

FIG. 6. Polysyncraton millepore (WAM 207.93): (A) colony. Polysyncraton miniastrum (QM G308537): (B) zooid; (C) spicules. Scales: (A) 1.0 cm; (B) 0.1 mm.