Aplidium, 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 : 1164-1165

Aplidium ? sp. 2

Distribution

Record: Western Australia CSIRO SS10/05 (Bald I., Stn 35, 157 m, 22.11.05, QM G328006.)

Remarks

The colony, encrusting rubble, has a tough cloudy test containing granular particles but without embedded sand. Zooids are mutilated, although they are seen to be long and threadlike with a long posterior abdomen and about 20 longitudinal folds in the wall of a small stomach. The atrial aperture is obscured and the presence of an atrial lip and its relationship to the aperture is not apparent. The specimen appears to be unidentifiable.

Family DIDEMNIDAE Giard, 1872

In this family, generic and species determinations are based on aspects of the morphology of the small, simplified and convergent zooids embedded in common test, often amongst crowded calcareous spicules. Spicule distribution is known to be adjusted to shade the colonies of a group of autotrophic species containing plant cell symbionts (see Parry 1987; Kott 2001) and Kott (2001) suggested that the white reflecting surfaces of the calcareous spicules may lower the temperature in shallowwater habitats. It appears from species in the present collection that spicules are crowded together in the test of large branching and encrusting colonies to form a firm hard self-supporting colonies or a supporting skeletal framework through the central axis of the colony and its branches.

Genus definition depends primarily on the structure of the male gonads, the opening of the atrial aperture, the number of rows of stigmata in the branchial sac, the larvae and, to some extent, the form of the colony and its common cloacal systems. Identification at species level relies on variations in those characters as well as the structure and size of the calcareous spicules (which are not affected by the various artefacts of collection and preservation that affect the soft parts of the colony).

Generally for identification of didemnid species and genera it is necessary to decalcify, stain and clear hand cut slices of the colony, zooids and larvae to detect most of the relevant characters, but difficulties in identifying genera are exacerbated when gonads are not developed, zooids are contracted and the zooids and colony mutilated. For instance, the pharyngeal walls of oesophageal buds and larval pharynges (both without well developed muscles) are not contracted and the number of rows of stigmata and the number in each row can often be readily detected in cleared preparations.

Additional characters to those traditionally thought to be reliable indicators of genera in the family are set out below:

1. Oesophageal buds with three rows of stigmata.......... Trididemnum Oesophageal buds with four rows of stigmata............................................... all other genera of Didemnidae

2. Larval oozooids with three rows of stigmata... Didemnum and Trididemnum Larval oozooids with four rows of stigmata................................................ all other genera of Didemnidae

3. Retractor muscle never present..................... Leptoclinides Retractor muscle usually present..... all other genera of Didemnidae

4. Atrial aperture on a posteriorly directed siphon......................................... Atriolum , Leptoclinides and Trididemnum Atrial opening not on a posteriorly directed siphon........................................... all other genera of Didemnidae

5. Larval lateral ampullae with primary four on each side usually subdivided........................................ Polysyncraton Larval lateral ampullae not usually subdivided.............................................. all other genera of Didemnidae

By detecting some or all of these characters it often is possible to assign specimens to the correct genus, and then to use the spicules to indicate a likely species, even when adult zooids are immature and their structure obscured by contraction and other artefacts.