Didemnum fragum Kott, 2001

Kott, Patricia, 2007, New and little-known species of Didemnidae (Ascidiacea, Tunicata) from Australia (part 4), Journal of Natural History 41 (17 - 20), pp. 1163-1211 : 1179-1181

Didemnum fragum Kott, 2001

( Figures 2C, D , 8C )

Didemnum fragum Kott 2001, p 179 and synonymy; 2005a, p 2441.

Distribution

Previously recorded (see Kott 2005a): South Australia (Eyre Peninsula, Spencer Gulf, Gulf St. Vincent); Victoria (Bass Strait, Westernport, Port Phillip Bay); Tasmania (eastern coast, Port Davey); New South Wales (Coffs Harbour, Manning Bight). New record: Tasmania (Bass Strait, QM G308826; Tasmanian Canyons: Banks Strait, 168 m, QM G308884–5).

Description

The colony from Bass Strait is vertical and paddle-shaped, white in preservative and orange in life. The specimens from the Tasmanian Canyons are grey, smooth-surfaced cylinders that superficially look like the Polysyncraton chondrilla , each with a white, star-shaped terminal common cloacal aperture (see Michaelsen 1924, p 345, Figure 14 as Didemnum chondrilla ). The colony surface is always raspy, although the internal test is soft. A posterior abdominal common cloacal cavity with a terminal common cloacal aperture separates the zooid layer from the central test. In all colonies a thick opaque layer of crowded spicules is in the surface test and a thinner layer lines the common cloacal cavity. The soft, gelatinous central test has only sparse spicules and often it is aspiculate. Spicules are relatively small (to 0.07 mm and occasionally to 0.08 mm diameter) and stellate (with 9–13 relatively short, pointed, conical rays in optical transverse section).

Zooids are in clumps just beneath the surface layer of spicules. Each clump is surrounded by the deep primary common cloacal cavity which extends into a network of posterior abdominal canals that get larger toward the common cloacal chamber and its terminal aperture on the top of the colony. Zooids are relatively large with a long branchial siphon, a wide sessile atrial aperture and a fine retractor muscle. They have about eight stigmata in the anterior half row on each side of the body. Twelve coils of the vas deferens have been reported previously. Gonads were not detected in the present specimens. Nevertheless, those from the Tasmanian Canyons contained larvae in the central test. The larval trunk is 0.8 mm long and is deep, and almost spherical. An ocellus and otolith are present. Six lateral ampullae are along each side of the antero-median adhesive organs. A large horizontal ampulla lies along the left side of the larval trunk. The tail is wound the whole way around the trunk. Three rows of stigmata are in the larval pharynx. Other larval organs are not developed.

Remarks

The species has relatively small, regular, stellate spicules with sturdy, pointed conical rays. Although the gonads are not developed and the zooids are robust for this genus, its relatively few stigmata and the three rows of stigmata in the larval trunk distinguish it from the genus Polysyncraton . The colony, with its single terminal common cloacal aperture, is characteristic of D. fragum . The colour in life reported for the present specimen (QM G308826) falls within the wide range of colours (from beige to pink) recorded for the living specimens assigned to this species.

Polysyncraton chondrilla ( Michaelsen, 1924) from New Zealand has almost identical colony form to some of the present specimens, and the same spicule distribution, but its spicules are not larger than 0.035 mm diameter (see Millar 1982).

In addition to the present species, the following frequently recorded temperate Australasian species Didemnum lambitum (Sluiter, 1900) , Polysyncraton jugosum ( Herdman and Riddell, 1913) , and Polysyncraton pedunculatum Kott, 2001 also have large, robust, vertical colonies with a large terminal common cloacal aperture. These colonies usually comprise a single system and have a large posterior abdominal cavity separating the zooid layer from the central test mass. Didemnum lambitum has smaller spicules (to 0.05 mm diameter) and only 10 coils of the vas deferens and four pairs of larval lateral ampullae (see Kott 2001, p 181). The Australian species Polysyncraton jugosum has large (to 0.08 mm) diameter) spicules like the present species, but only seven to nine rays in optical transverse section and 10 pairs of larval lateral ampullae; and P. pedunculatum has a similar colony but is completely aspiculate and its larvae are not known. Polysyncraton chondrilla ( Michaelsen, 1924) , despite its colonies that superficially look identical to newly recorded colonies of D. fragum from the Tasmanian Canyons, can be distinguished by their generic characters as well as their small spicules to 0.03 mm diameter (see Millar 1982).

Some Trididemnum spp. also have colonies resembling the present species although rather than single-system colonies, they are more often vertical branches with terminal common cloacal apertures (e.g. Trididemnum amiculum Kott, 2001 ).

Michaelsen (1924) drew attention to the identical, albeit convergent, cloacal systems of the large western Indian Ocean specimens of Didemnum sycon Michaelsen, 1920 , a junior synonym of Didemnum molle ( Herdman, 1886) . These also are single-system colonies, although they are soft and have a very different consistency from the firm colonies of the present species and they are further distinguished by the symbiotic Prochloron in their common cloacal cavities.

Figures

Figure 2. (A, B) Didemnum abradatum (QM G308870, holotype): (A) zooid; (B) larva. (C, D) Didemnum fragum (QM G308826): (C) colony; (D) larva. (E) Didemnum jedanense (QM G308862): larva. Scale bars: 0.1 mm (A, B, D, E); 1 cm (C).

Figure 8. Scanning electron micrographs. (A) Didemnum coralliforme (QM G308861); (B) Didemnum crescente (QM G308815); (C) Didemnum fragum (QM G308826); (D) Didemnum grande (QM G308867); (E) Didemnum granulatum (QM G308843); (F) Didemnum jedanense (QM G308846); (G) Didemnum lissoclinum (QM G308814); (H) Didemnum mantile (QM G308887).