Didemnum cygnuus Kott, 2001

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 : 1180-1181

Didemnum cygnuus Kott, 2001

( Figure 17E )

Didemnum cygnuus Kott, 2001, p. 169 .

Distribution

Previously recorded (see Kott 2001): Western Australia (Port Hedland, Swan River Estuary ). New record: Western Australia CSIRO SS10 / 05 (Lancelin, Stn 76, 100 m, 01.12.05, QM G328456 ) .

Description

The single colony is an encrusting sheet on a sea-grass blade. It has been damaged and may have been dried out. Branchial apertures are conspicuous on the surface where the six lobes of each aperture are filled with spicules and protrude slightly from the surface, looking like small daisies. Internally groups of zooids are slung across an extensive internal common cloacal cavity in branching test connectives that connect the thin surface and basal layers of test. Spicules are scattered evenly throughout the test but are relatively sparse. They are small (to 0.04 mm), stellate but often irregular sometimes assymmetrical and occasionally with rays of different lengths in the one spicule. Generally they have seven to nine conical rays in optical transverse section, but spicules with five or 11 rays also occur. Four rows of stigmata were detected in the branchial sac, but little else of the zooid can be detected.

Remarks

This specimen has the same habitat (on a sea-grass blade) and the protruding branchial lobes filled with spicules have the same daisy-like appearance as the previously recorded specimens, including the type material. Also the spicules appear to be identical. The open appearance of the common cloacal cavities in the newly recorded colony probably resulted from desiccation of the specimen, reducing the strands of test (in which the zooids are embedded) to relatively thin strands.

The form of this colony is similar to Trididemnum spumosum from the same habitat (on a blade of sea-grass) and Lissoclinum maculatum (see below). Their similar burr-like spicules (distinguished from one another by their size) are different from the present species.

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.