Didemnum astrum Kott, 2001

Kott, Patricia, 2005, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (Part 3), Journal of Natural History 39 (26), pp. 2409-2479 : 2437-2438

Didemnum astrum Kott, 2001

( Figure 15D )

Didemnum astrum Kott 2001, p 151 .

Distribution

Previously recorded (see Kott 2001): Western Australia (Cape Preston); Queensland (S. Great Barrier Reef); Indian Ocean (Cocos Keeling). New record: Western Australia (near Green I. off Grey, WAM 601.89 ) .

Description

The newly recorded colony is a thin irregular encrusting sheet. The under surface is hard, spicule-filled with some fine, sharply defined parallel ridges crossing it. Spicules are crowded throughout the remainder of the colony, including the smooth upper surface where they obscure the branchial apertures. Spicules mostly are stellate, to 0.06 mm diameter, with 13–15 short crowded rays in optical transverse section, although some have shorter, round-tipped rays and are almost globular. Sometimes the branchial siphon is about half the length of the remainder of the small thoraces in the newly recorded colony, which appear to be juveniles. Only six stigmata per side reducing to four in the posterior row were detected in the newly recorded colony, although Kott (2001) recorded eight in the anterior rows of the small zooids she examined. The abdomina are mature, each with a well-developed, undivided testis surrounded by 10 coils of the vas deferens.

Remarks

The hard, ridged, basal test of this species occurs also in Leptoclinides durus Kott, 2001 and Polysyncraton millepore Vasseur, 1969 (see above), L. uniorbis F. and C. Monniot, 1996 from Micronesia, and Didemnum parancium Kott, 2001 from Townsville. The present species is distinguished from these and others by its spicules and the large number of vas deferens coils, both of which are clearly recognizable in all the specimens so far examined. There are 12 separate records of this species from the Capricorn Group at the southern end of the Great Barrier Reef. Since it is recorded also from NW Australia and the Indian Ocean it can be expected to occur in coral reef locations in the Indian and western Pacific Oceans.

Figures

Figure 15. Scanning electron micrographs of the calcareous spicules of: (A) Polysyncraton millepore (WAM 122.93); (B) Polysyncraton orbiculum (WAM 128.90); (C) Polysyncraton papyrus (SAM E3253); (D) Didemnum astrum (WAM 601.89); (E) Didemnum cilicium sp. nov. (SAM E3251, holotype); (F) Didemnum corium sp. nov. (SAM E3249, holotype); (G) Didemnum cygnuus (QM G308716); (H) Didemnum fragum (SAM E3266).