Didemnum plebeium Kott, 2005

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 : 1188

Didemnum plebeium Kott, 2005

( Figure 18D )

Didemnum plebeium Kott, 2005a, p. 245 .

Distribution

Previously recorded (see Kott 2005a): Western Australia (Dampier Archipelago. New record: Western Australia CSIRO SS10 /05 (Point Hillier, Stn 20, 419 m, 22.11.05, QM G328451 ).

Description

The newly recorded specimen is a solid, white, irregular sheet about 5 cm in maximum dimension. Ripple marks are on the base of the colony probably as a result of the tightly packed spicules. Zooids are small, the thoraces comma-shaped, being reduced in diameter posteriorly. A fine, tapering retractor muscle projects from the oesophageal neck just posterior to the end of the thorax. Stellate spicules, crowded throughout, are to 0.05 mm diameter with 9–11 conical pointed or blunttipped rays in optical transverse section.

Remarks

The newly recorded specimen is larger than the syntypes, which are small colonies to 0.5 cm in maximum dimension with spicule-filled surface papillae associated with each aperture. Other characters conform to the description of the types. Possibly the differences are the result of age or intraspecific variation over the extensive range represented by the two existing records of this species.

The comma-shaped thoraces are not unusual in this genus (see Kott 2001), being reported for D. membranaceum Sluiter, 1909 (which differs from the present species in having characteristic giant spicules), D. granulatum Tokioka, 1954 and D. perplexum Kott, 2001 (both with larger spicules with fewer rays) and D. delectum Kott, 2001 (with same-sized spicules but with fewer and more rod-like rays than the conical rays of the present species). The spicule rays of the present species are shorter than the long, pointed rays of D. multiampullae and D. ossium and they have fewer rays. Kott (2005) recorded the diameter of the spicules of this species as 0.06 cm, although spicules of that diameter were not detected in the present specimen. The spicules are otherwise characteristic of this species.

Figures

Figure 18. Scanning electron micrographs of didemnid spicules: A, Didemnum multiampullae, QM G328057: B, Didemnum ossium, QM G328055; C, Didemnum perplexum, QM G328034; D, Didemnum plebeium, QM G328451; E, Didemnum tapetum, QM G328016; F, Didemnum velum, QM G328120; G, Didemnum sp 1, QM G328160; H, Didemnum sp. 2, QM G328166.