Polysyncraton fistulum, Kott, 2008

Kott, Patricia, 2008, Ascidiacea (Tunicata) from deep waters of the continental shelf of Western Australia, Journal of Natural History 42 (15 - 16), pp. 1103-1217 : 1174-1175

Polysyncraton fistulum sp. nov.

( Figures 13B–E ; 16H )

Distribution

Type locality: Western Australia CSIRO SS10 / 05 (Two Rocks, Stn 02, 115.2440E 31.72243S, 102 m, 18 November 2005, holotype WAM Z27510 QM G328446 ) .

Description

The holotype is a large colony consisting of numerous (15 or more) vertical fingerlike cylindrical lobes, each about 1.5 cm diameter and up to 7 cm long, branching from common basal stalks of about the same length. The whole colony is about 15 cm high overall. Each of the lobes has a single terminal common cloacal aperture. Zooids are in the surface layer of test, which is separated from the central core by an extensive but shallow horizontal common cloacal cavity, traversed by strands of test connecting the central core to the surface zooid-bearing layer. Embryos and larvae are in the central core of test, the most advanced being in the layer of test adjacent to the common cloacal cavity.

Spicules are crowded throughout the colony but they are especially crowded in the central core where a hard three-dimensional network of rods of compacted spicules (suspended amongst less crowded ones) forms a supporting skeleton for the colony. The rods of spicules appear more crowded (the meshes of the network are smaller) where they form a central vertical axis, and also where they form an almost continuous calcareous wall that surrounds the colony lobe just beneath the common cloacal cavity. The central axis and the peripheral wall are connected by the lateral branches of the rods.

The moderately sized stellate spicules (to 0.06 mm diameter) have seven to nine relatively long but irregular rays in optical transverse section. The rays are of various lengths and the tips are often bidentate or even pectinate.

Stellate, six-lobed branchial openings are evenly distributed over the surface of the colony, their margins lined with spicules. The atrial apertures are large sessile openings exposing most of the branchial sac directly to the common cloacal cavity and an atrial lip was not detected. A large circular lateral organ is on each side of the thorax. The plug of spicules is attached to each side of the test sheath that envelops the thorax as it crosses the common cloacal cavity. About eight stigmata are in each of the four rows of stigmata on each side of the thorax. The gut loop is vertical and oesophageal and thoracic buds are present. Zooids have a circular clump of about seven testis follicle, although the vasa deferentia were not detected in these specimens.

Embryos have a trunk about 1.2 mm long and the tail winds all the way around it. Eight parallel lateral ampullae are on each side of the sturdy antero-median adhesive organs, each with a relatively thick stalk and a deep cone of adhesive cells in a deep epidermal cup. Blastozooids were not detected in the larvae.

Remarks

Without the evidence of its coiled vas deferens, this species could be mistaken for one of the few Lissoclinum spp known to have a number of testis follicles and a large thorax with a wide sessile atrial opening (see Kott 2001) like the present species. However the large larval trunk resembles many Polysyncraton spp , being rounded (rather than attenuating) posteriorly, having a large number of stumpy lateral ampullae anteriorly (rather than the tapering club-shaped ampullae of Lissoclinium ) and lacking the pronounced waist and the fine stalks of the adhesive organs of Lissoclinum larvae. Also, the gut loop is not folded up behind the thorax as it so often is in Lissoclinum .

The colonies of the present species are quite distinctive, lacking the surface sculpture and irregularity of Didemnum spongioides ( Sluiter, 1909) . Colonies of D. roberti Michaelsen, 1930 and D. ossium Kott, 2001 also resemble the present species to some extent, but in addition to their generic differences they are all distinguished from the present species by aspects of their spicules, zooids and larvae. Didemnum diversum Kott, 2004a has small stellate spicules with bifid and pectinate ray tips similar to, and about the same size as, those of the present species. Spicules of the present species also are smaller than, but otherwise similar to, the spicules of P. tasmanensis Kott, 2001 and Trididemnum cristatum Kott, 2001 . Polysyncraton pseudorugosum Monniot, 1993 has spicules crowded together to form a skeleton similar to that of the present species, but it has other characters distinguishing it from the present species.

Figures

Figure 13. Polysyncraton dealbatum sp. nov. (QM G328119). A, larva. Polysyncraton fistulum sp nov. (QM G328446): B, colony; C, crosssection through colony lobe; D, longitudinal section through part of colony lobe showing hard rods of crowded spicules forming skeletal support; E, larva. Polysyncraton millepore (F, QM G328050; G, QM G328033): F, portion of colony showing ripple marks on undersurface; G, zooid. Polysyncraton pavementum (QM G328058): H, surface of colony showing separate systems. Scales: A, E, G, 0.2 mm; B, 2.0 cm; C, H, 3.0 mm; D, 1.0 cm; F, 3.0 cm.

Figure 16. Scanning electron micrographs of didemnid spicules: A, Atriolum tubiporum, QM G328004; B, Leptoclinides durus, QM G328158; C, Leptoclinides fungiformis, QM G328444; D, Leptoclinides vesica, QM G328458; E, F, Polysyncraton calculum, QM G328140; G, Polysyncraton dealbatum, QM G328119; H, Polysyncraton fistulum, QM G328446.