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 : 1191-1192

Didemnum ? sp. 1

( Figure 18G )

Distribution

Record: Western Australia CSIRO SS10/05 (Jurien Bay, Stn 083, 113 m, 02.12.05, QM G328464 on a specimen of Aplidium clivosum QM G 328160).

Description

These small, flat, white plates about 0.5 cm diameter, are epibionts on the surface of Aplidium clivosum . Each consists of a single system with a sessile common cloacal aperture in the centre of the upper surface. The common cloacal cavity is horizontal at thorax level and each thorax crosses it in its own independent spicule-filled test sheath. Spicules are crowded throughout. Spicules are stellate to 0.04 mm diameter with 11–13 conical rays in optical transverse section. The surface of the colony is white with conspicuous six-lobed branchial openings, each lobe is filled with spicules. Minute spicule-filled papillae are crowded on the surface between the branchial apertures. Zooids are small and contracted. Seven coils of the vas deferens were detected, although the testis is not entire and appears to have been damaged. Large yellow eggs are present. Oesophageal buds have four rows of stigmata with about six stigmata in each half row. A small larva, its tail wound all the way around the 0.34 mm long trunk, was found in the basal test of one of the colonies. It has an ocellus and otolith, and four large club-shaped ampullae on each side of the three antero-median ampullae.

Remarks

These small, single-system colonies are juveniles, and do not appear to have resulted from lobulation, being well separated from one another on the surface of the host Aplidium colony. Species of Didemnum with single systems are few. Didemnum minisculum Kott, 2001 is a single system, lobulating species but its spicules generally are only to 0.01 mm diameter and have only five to seven rays. Didemnum mobile Kott, 2001 has similar but less regularly pointed spicule rays and Didemnum etiolum Kott, 1982 has single system colonies but it has different spicules and is further distinguished by its plant cell symbionts. Didemnum mantile Kott, 2007 has similar colonies and spicules but its larvae have more lateral ampullae.

Spicules are similar to some of D. membranaceum which also has surface paillae. However, it differs from the present species in having characteristic giant spicules as well as the smaller stellate spicules. Didemnum delectum has similar small larvae with the tail coiled all the way around the trunk, but also has fewer (5–9) spicule rays than the present colonies, and has multisystem colonies. Didemnum lissoclinum Kott, 2001 has similar spicules to the present species, but they are larger (to 0.07 mm diameter); it has larger larvae and large branched colonies.

The lack of a Leptoclinides atrial siphon, the presence of four rows of stigmata in the oesophageal buds and the coiled vas deferens are evidence of Didemnum or Polysyncraton but the larva has a small trunk with undivided primary ampullae found in Didemnum rather than Polysyncraton and the specimens appear to be correctly assigned to the former genus.

There is some evidence that these zooids are sexually mature, but it is possible that the colonies are juveniles and characters that would support confident assignation of these specimens to a known species were not detected.

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.