Didemnum dolium, Kott, 2008

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 : 1181-1182

Didemnum dolium sp. nov.

( Figure 17F )

Distribution

Type locality: Western Australia CSIRO SS10 / 05 (Bald I., Stn 35, 118.645E 35.1907S, 157 m, 24 November 2005, holotype Z27521 , QM G328008 ) .

The species may be from a deeper-water southern ocean fauna not previously sampled.

Description

The colony is vertical, its upper half divided into two conical lobes up to 1 cm high, each with a terminal common cloacal aperture. The solid central test mass is separated from the outer zooid-bearing layer by a series of posterior abdominal cavities. Spicules are in a crowded layer beneath an aspicular superficial layer of test but they are sparse in the remainder of the colony. The spicules (to 0.04 mm diameter) are either stellate with crowded conical and pointed rays, 13–15 in optical transverse section, or they are smaller and almost globular, with fewer flat-tipped, rod-like rays than the stellate spicules.

Zooids are small and contracted with a relatively long, tulip-shaped branchial siphon and large sessile atrial apertures exposing a large part of the branchial sac to the common cloacal cavity. There appear to be four rows of stigmata but the branchial sac is obscured by contraction. A long tapering retractor muscle projects from the oesophageal neck. Gonads were not detected in this specimen.

Remarks

Zooids lack the posteriorly oriented atrial siphon of Trididemnum and Leptoclinides and they have a retractor muscle (which also excludes Leptoclinides ). Therefore this species could be either in the genus Polysyncraton or Didemnum . The small zooids without an atrial lip are generally characteristic of Didemnum and have determined the assignation of this species.

Although its spicules are smaller, this species resembles D. sucosum Kott, 2001 in its branching colony, with posterior abdominal cavities and spicules largely confined to a layer on the base of the colony and a surface layer beneath an aspicular bladder cell layer. Didemnum species known with three-dimensional colonies similar to the present one but lacking the superficial layer of bladder cells are the tropical D. roberti Michaelsen, 1930 , D. spongioide Sluiter, 1909 (both with larger spicules with fewer rays) and D. fragum Kott, 2001 (with larger, to 0.07 mm diameter, exclusively stellate spicules with conical pointed rays).

Didemnum velum sp. nov. also has a similar arrangement of its spicules with a layer at thorax level beneath a superficial aspicular layer of bladder cells and another layer of spicules on the base of the colony. However, it lacks the globular spicules with rod-shaped rays found in the present species and its colony is a two-dimensional sheet without the posterior abdominal common cloacal cavities of the present species. Didemnum dealbatum Sluiter, 1909 (see Kott 2001, p. 145) has similar spicules but with fewer rays and the superficial aspicular layer is not present. Like the present species, the spicules in both the latter species are crowded in two layers, one on the surface and another on the base of the colony.

A mixture of stellate spicules with crowded conical rays and globular ones with flat-tipped rays similar to the present species occurs in a number of Didemnum spp. such as D. moseleyi and D. ossium (with two-dimensional cloacal systems) and D. elongatum (with three-dimensional cloacal systems), although their spicules are larger than those of the present species and they are present throughout the colony. The temperate species D. ternerratum Kott, 2001 has similar spicules but with more rays

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.