Polysyncraton millepore Vasseur, 1969

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 : 1176-1177

Polysyncraton millepore Vasseur, 1969

( Figures 13F–G ; 17A )

Polysyncraton millepore Vasseur 1969, p. 917 . Kott 2005, p. 2431 and synonymy.

Distribution

Previously recorded (see Kott 2005): Western Australia ( Dongara , Port Hedland ); Queensland (Whitsunday Is.), New Caledonia, West Indian Ocean ( Malagasy , Mozambique, Tanzania, Northern Natal ). New records: Western Australia CSIRO SS10 /05 (Point D’ Entrecasteaux, Stn 18, 100 m, 21.11.05, QM G328434 ; Albany, Stn 22, 99 m, 22.11.05, QM G328050 ; Kalbarri, Stn 102, 96 m, 05.12.05, QM G328056 ; Jurien Bay, Stn 82, 85– 92 m, 2.12.2005, QM G328033 , G328121 ; Jurien Bay, Stn 83, 113 m, 02.12.05, QM G328167 ) .

The species appears to be common on hard/soft substrates in continental shelf locations around the Indian Ocean and in the tropical West Pacific. Previously thought to be an Indo-West Pacific tropical species, the new record extends its known range into temperate waters off the southern coast of Western Australia .

Description

Colonies are thin, flat, brittle sheets with crowded spicules, curved outer margins and ripple marks on the otherwise flat, hard, basal surface parallel to the outer margin of the colony. One of the newly recorded colonies (QM G328033) is attached to the base of another thin, flat and brittle sheet ( Didemnum ossium, QM G 328055). These colonies appear to support one another, and may stand up vertically from the sea floor. Many colonies are extensive, one (QM G328050) is 20 cm in greatest dimension and divided into more or less equal segments like a three-leaved clover. This colony is symmetrical, fixed by the middle of the basal margin. The curved outer margins of each segment often roll back over the upper surface to expose the hard, rippled, basal surface where it probably was not attached to the substratum.

Evenly spaced common cloacal apertures are present on the upper surface, just inside the outer margin and have brown pigment in the test surrounding them. Zooids open along each side of the circular common cloacal canals that surround white slightly elevated zooid-free areas, giving a quilted appearance. Spicules are tightly packed throughout the colony. Most are stellate, to 0.06 mm in diameter with 9–11 conical rays in optical transverse section. Others have truncated, flat-tipped rays and some are almost globular.

Zooids are robust and are nearly 2 mm long, with thick, six-lobed, cylindrical to tulip-shaped branchial siphons. A long, narrow atrial tongue, with a spoon-shaped and slightly bifid tip, projects from the upper margin of the large sessile opening that exposes most of the branchial sac to the common cloacal canal. About eight stigmata are in each row. A strong retractor muscle projects from the top of the oesophageal neck. The posterior end of the gut loop is bent ventrally. A divided testis with three or four follicles is pressed behind (dorsal to) the ventral flexure of the gut loop and is surrounded by six coils of the vas deferens.

Remarks

The newly recorded specimens resemble those previously described in most characters (the zooids, branchial siphon and atrial lip, ventrally flexed gut loop, and the appearance of the colony with common cloacal apertures around the rounded outer margin, and a ripple-marked base). The pigmentation of the newly described colonies differs from the conspicuously two-toned colonies previously reported, in which brown pigment is scattered through the surface layer of test rather than being confined to the area around the common cloacal apertures (see Kott 2004a, 2005). However previously reported specimens of P. millepore have similar spicules to the present ones and six coils of the vas deferens. Although Kott (2001) found only five coils of the vas deferens (and only three zooids with mature testes), six coils were detected in the re-examined material (see Kott 2005). Amongst tropical species, Polysyncraton oceanium Kott, 2001 has a similar number of vas deferens coils but is distinguished by its superficial layer of bladder cells. Polysyncraton palliolum Kott, 2001 has nine coils of the vas deferens and its zooids lack a retractor muscle. Polysyncraton sideris Kott, 2001 has seven coils of the vas deferens but has differences from the present species in its colonies and zooids and its large spicules. Polysyncraton luteum Kott, 2004a has eight coils of the vas deferens but small globular spicules. Polysyncraton pseudorugosum Monniot, 1993 has a similar colony but only three or four vas deferens coils.

It should be noted that Kott (2005a) is clearly in error in reporting the diameter of spicules of this species to be 0.66 mm rather than 0.06 mm.

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 17. Scanning electron micrographs of didemnid spicules: A, Polysyncraton millepore, QM G328050; B, Polysyncraton?palliolum, QM G328161; C, Polysyncraton pavimentum, QM G328058; D, Didemnum?candidum, QM G328457; E, Didemnum cygnuus, QM 328456; F, Didemnum dolium, QM G328008; G, Didemnum jedanense, QM G328005; H, Didemnum moseleyi, QM G328455.