Polysyncraton gratum, Kott, 2007

Kott, Patricia, 2007, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (part 4), Journal of Natural History 41 (17 - 20), pp. 1163-1211 : 1171-1172

Polysyncraton gratum sp. nov.

( Figures 1 D–F, 6H)

Distribution

Type locality: Tasmania (Tasmanian Canyons: Banks Strait, 168 m, Sherman Sled, 25 April 2004, holotype QM G308891 ) .

Description

The colony is a fleshy, robust encrusting sheet. It is a cream colour in preservative but may have been orange in life. The surface test has a loose or inflated appearance, with a superficial layer of spicules that make it raspy. Spicules are present throughout the colony, although a thin layer of test beneath the thoracic common cloacal cavity is aspiculate. Spicules are stellate, to 0.075 mm diameter, with seven to nine conical pointed rays in optical transverse section. Stellate branchial apertures are evenly spaced and the margins of the openings are lined with spicules. The anterior rim of the large, sessile atrial opening is produced out into a pointed lip exposing most of the branchial sac to the common cloacal cavity. Zooids are whitish or translucent. Four or five immature testis follicles were detected although a vas deferens was not. Larvae are being incubated in the basal test. The larval trunk is 0.8 mm long and the tail is wound the whole way around it. Up to 12 pairs of lateral ampullae, created by subdivision from four primary pairs, encircle the three antero-median adhesive organs. A large conspicuous horizontal lateral ampulla is on the left side of the trunk, extending back from the waist between the adhesive array and the oozooid. Four rows of stigmata are in the larval pharynx. Blastozooids were not detected. The ocellus is large.

Remarks

Although gonads are not conspicuous in this specimen, it is assigned to Polysyncraton on the basis of its sessile atrial aperture with a long anterior lip and larvae with four rows of stigmata, a long horizontal external lateral ampulla and numerous finger-like lateral ampullae surrounding the adhesive organs. Although Lissoclinum spp. have larvae with four rows of stigmata and sometimes numerous lateral ampullae, the latter are only rarely thick finger-like lobes like the present species and although some have an anterior atrial lip it is not long and pointed as it is in most Polysyncraton spp. Some Didemnum spp. also have larvae with numerous lateral ampullae and stellate spicules with relatively few conical rays. However, only three rows of stigmata are in the larval pharynx in species of that genus.

The southern Australian Polysyncraton infundibulum Kott, 2001 and P. montanum Kott, 2004c have similar-sized spicules (but with more rays than the present species); and in P. papyrus Kott, 2001 the spicules are significantly smaller than in the present species. Polysyncraton jugosum ( Herdman and Riddell, 1913) , known only from the central coast of New South Wales, has similar spicules and larval ampullae to the present species, although their larvae are about twice the size of the present species.

Polysyncraton galaxum Kott, 2004b (also 2005a) has similar-sized larvae with 12 pairs of lateral ampullae, a large horizontal external ampulla and similar-sized spicules but they have only five to seven rays in optical transverse section and a large number of small-sized (to 0.02 mm diameter) spicules. Also, separate common cloacal systems are a conspicuous feature of P. galaxum , P. polysystema Kott, 2005a and related species (see Kott 2005a), but they were not detected in the present species.

Characteristics of the present new species that distinguish it from others in this genus are its large horizontal common cloacal cavity without separate systems, stellate spicules to 0.75 mm diameter with seven to nine conical rays in optical transverse section found throughout the colony, a large atrial tongue, a large larva with up to 12 pairs of lateral ampullae, but without blastozooids and with a large ocellus.

Figures

Figure 1. (A) Atriolum irregulare (QM G323245 holotype): larva. (B, C) Leptoclinides fluxus (QM G308817 holotype): (B) colony surface; (C) zooid. (D–F) Polysyncraton gratum (QM G308891): (D) thorax; (E) abdomen; (F) larva. (G) Polysyncraton reticulum (QM G323330): larva. Scale bars: 0.1 mm (A, C–G); 1 mm (B).