Didemnum, Savigny, 1816

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 : 1179-1180

Didemnum ? candidum Savigny, 1816

( Figure 17D )

Didemnum candidum Savigny 1816, p. 194 ; Kott 2001, p. 157 and synonymy.

Distribution

Previously recorded (see Kott 2001): Western Australia (Kimberley); Queensland (Great Barrier Reef); Western Pacific, Indian Ocean (Gulf of Suez, Gulf of Arabia, West Indian Ocean, Mozambique, Malagasy, Mauritius, Tanzania). New record: Western Australia, CSIRO SS10/05 (Zuytdorp, Stn 104, 97 m, 05.12.05, QM G328457).

Didemnum candidum is a tropical species with a wide range in the Indo-West Pacific region.

Description

The colony is part of a small encrusting sheet growing on a sponge. It is whitish pink in preservative, the colour resulting from the orange zooids showing through the white spicules which are crowded throughout. Spicules are to 0.056 mm diameter with five to nine almost cylindrical round-tipped rays in optical transverse section. A shallow, horizontal common cloacal cavity is at thorax level. Zooids are tightly contracted and few features can be detected. A retractor muscle is contracted to a short triangular projection at the posterior end of the thorax. The vas deferens coils eight times around a flattened, lens-shaped to almost spherical, undivided testis.

Remarks

The only characters available to distinguish the present specimen are the short, contracted retractor muscle, eight vas deferens coils and stellate spicules with five to nine short, cylindrical round-tipped rays. Although the undivided testis, the presence of a retractor muscle and the coiled vas deferens suggest that this specimen could belong to either Trididemnum or Didemnum , the distinctive spicules are not known in the former genus. The spicules most resemble those of D. candidum Savigny, 1816 . That species is conservative in its morphology and many specimens have been erroneously assigned to it (see Kott 2001). The present specimen differs in having eight (rather than seven) coils of the vas deferens and the thorax could be larger than that of D. candidum but its spicules are very similar and the specimen is tentatively assigned to that species. Didemnum madeleinae F. and C. Monniot, 2001, from Papua New Guinea, has seven coils of the vas deferens but only a few spicules similar to the present ones are scattered amongst a majority with conical pointed rays. Didemnum delectum Kott, 2001 has similar spicules but with longer rays and D. plebeium Kott, 2005 has conical rather than rod-like rays.

Figures

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.