Polysyncraton galaxum, Kott, 2010

Kott, Patricia, 2010, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (part 2), Journal of Natural History 38 (26), pp. 2455-2526 : 2478-2479

Polysyncraton galaxum sp. nov.

( figures 4 , 17A )

Distribution. Type localities: South Australia (Kangaroo I. W of Western River Cave, Pissy Boy Rock, The Amphitheatre on rock wall 14–17 m, coll. K. Gowlett Holmes, 28 November 2001, holotype SAM E3200), Tasmania (Tasman Peninsula, Fortescue Bay, Cape Havy, 18–20 m, coll. K. Gowlett Holmes, 2 July 1998, paratype SAM E2927).

Description. Both colonies are firm, flat, encrusting sheets, white with a tinge of pinkish orange in preservative. Small common cloacal apertures in the centre of zooid-free areas about 1 cm apart have frilled margins or are contracted into a cross. Each opening is surrounded by thick spicule-filled radial ribs in the roof of the common cloacal cavity beneath it. Orange pigment particles, especially crowded, mixed with the spicules around each common cloacal aperture in the holotype colony, fade to pale pinkish orange further away from the openings. On the remain- der of the upper surface, away from the common cloacal apertures, are evenly spaced branchial apertures with their margins outlined with spicules. The surface layer of test is relatively thick with the anterior end of each zooid in a concavity in its under surface. The common cloacal cavity is a shallow, horizontal space at thoracic level, its shallowness contributing to the firmness of the colony. A short test strip is associated with the ventrum of each thorax as it crosses the common cloacal cavity. Abdomina are embedded in the basal test. Spicules are stellate, with five to seven conical pointed rays in optical transverse section and are crowded throughout. They are of two sizes, the larger ones to 0.06 mm diameter and the smaller and more numerous ones only to about 0.02 mm.

Zooids are relatively small. An atrial lip from the anterior rim of the opening is especially long on those zooids in the vicinity of the common cloacal apertures. Large, circular saucer-shaped lateral organs are on each side of the thorax. Eight stigmata are in the anterior row in the branchial sac, reducing to six in the posterior row. Five coils of the vas deferens surround four or five testis follicles in the holotype colony, although gonads are not present in the Tasmanian specimen. Also, robust larvae are in the basal test of the holotype. The larval trunk is 0.9 mm long, the tail is wound almost the whole way around it, 12 lateral ampullae are along each side of the three antero-median adhesive organs and a large, thick, external horizontal ampulla is on the left side of the trunk.

Remarks. The species has spicules with fewer rays than any of the few species in this genus (e.g. P. tenuicutis Kott, 2001 and P. rubitapum Kott, 2001 ) that have stellate spicules in two conspicuously different size ranges. Also, it is distinguished from P. rubitapum by its more numerous larval lateral ampullae. Polysyncraton tasmanense Kott, 2001 has similar but larger spicules (to 0.114 mm diameter) with more rays. Didemnum microthoracicum has large and small spicules, but although they have the same number of rays as the present species, they reach nearly twice the size (the larger ones to 0.09 mm diameter).

The common cloacal apertures in the present species are surrounded by zooids, and do not occur at the junctions of circular canals, as they do in so many species in the Didemnidae . Polysyncraton glaucum Kott, 2001 , P. multiforme Kott, 2001 , P. pavimentum Monniot, 1993 and the New Zealand species P. lithostrotum Brewin, 1956 have similar common cloacal systems, however, all of these species have more spicule rays than the present one.

Figures

FIG. 4. Polysyncraton galaxum [(A) SAM E3200; (B–D) SAM E2927]. (A, B) Thoraces; (C) abdomen; (D) larva. Scale bars: 0.1 mm (A–C); 0.5 mm (D).

FIG. 17. (A) Polysyncraton galaxum (SAM E2927); (B) Polysyncraton longitubis (SAM E2929); (C) Polysyncraton montanum (SAM E3224); (D) Polysyncraton peristroma (QM G308608); (E) Polysyncraton pseudomagnetae (QM G308612); (F) Polysyncraton pseudorugosum (QM G308600); (G) Polysyncraton reticulum (SAM E2917); (H) Polysyncraton rica (SAM E3216).