Didemnum astrum Kott, 2001

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 : 1177

Didemnum astrum Kott, 2001

( Figure 7G )

Didemnum astrum Kott 2001, p 151 ; 2005a, p 2437.

Distribution

Previously recorded (see Kott 2005a): Western Australia (Cape Preston, near Green I. off Grey); Queensland (S. Great Barrier Reef); Indian Ocean (Cocos Keeling). New records: Queensland (Great Barrier Reef: 17.298 ° S, 146.205 ° E, 31 m, QM G308853 ; 16.705 ° S, 146.125 ° E, 34 m, QM G308857 ) .

Description

The colonies are thin, encrusting sheets with delicate zooids, each crossing the common cloacal cavity in a separate sheath of test that sometimes encases the whole zooid but at least the thorax, abdomina being embedded in the basal test. Spicules are characteristic, to 0.06 mm diameter, with numerous long, predominantly pointed, and crowded rays. Branchial apertures are on short siphons or funnels. The atrial aperture is wide open, with the upper rim sometimes extended out into a rounded lip. A fine retractor muscle projects from halfway down the relatively short oesophageal neck. About six long stigmata are in each row. Stolonic vessels are long. Ten coils of the vas deferens surround each undivided testis.

Remarks

The species is distinguished by its spicules, circular common cloacal canals lined on each side by zooids, 10 coils of the vas deferens, and small larvae with long tails, four pairs of ectodermal ampullae and lacking blastozooids.

Didemnum paa C. and F. Monniot, 1987 from New Caledonia has similar larvae, 10 coils of the vas deferens and similar-sized spicules. However, the present specimens have more and longer spicule rays and appear to represent a distinct species. The small rose-coloured colonies assigned erroneously to Didemnum ligulum by Monniot and Monniot (1987) also have spicules to 0.06 mm diameter (see Monniot and Monniot 1987, Plate IIc) but they have larval blastozooids, numerous larval ectodermal ampullae, and different cloacal systems that distinguish the colonies from the present species.

Figures

Figure 7. Scanning electron micrographs. (A) Polysyncraton magnetae (QM G308818); (B) Polysyncraton meandratum (QM G308849); (C) Polysyncraton otuetue (QM G308851); (D) Polysyncraton reticulum (QM G323330); (E) Polysyncraton scorteum (QM G323343); (F) Didemnum abradatum (QM G308870); (G) Didemnum astrum (QM G308857); (H) Didemnum candidum (QM G308765).