Didemnum lithostrotum Brewin 1956

Didemnum lithostrotum Brewin, 1956

(Figure 11F)

Didemnum lithostrotum Brewin, 1956a: p. 127 –129, figs 3A 1, A 2, A 3.

Didemnum lithostrotum: Millar 1982: 49, 51.

New record: Caswell Sound, Paua Bay (45° 01.01’S, 167° 08.40’E, 20 m, 3 February 2009, NIWA 49980, one colony).

Previously recorded: New Zealand: Chatham Islands (Brewin 1956a); Chatham Rise (Brewin 1956a); Stewart Island (Brewin 1958a).

The pink/orange (YR 7/6) colony from Caswell Sound is 1 mm thick, encrusting on the brachiopod Liothyrella neozealandica. The test is divided into irregular polygonal areas, each with a central common cloacal aperture (Figure 11F). Spicules occur throughout the colony, although more are concentrated in a layer near the surface and at the substratum. They have eight rounded rays in optical cross-section and measure 40–70 µm in diameter. The zooids are 1 mm long in fixed material, the branchial aperture is relatively tall with six low pointed lobes, and the atrial aperture is smooth rimmed with a short simple lappet. The stomach is small and globular. There are two testis follicles with six to seven coils of the vas deferens.

Possible synonymy of this species to Didemnum densum (Nott 1892) was mentioned by Millar (1982). We have placed the Fiordland colony in the species Didemnum lithostrotum based on the southern distribution of the holotype, and different morphology of the tunic (not in polygonal pattern in D. densum) and the branchial aperture lobes. The zooids in the current species are smaller than given by Brewin (1956a), but this is because they were contracted on collection and preservation. Didemnum densum in relaxed colonies has a branchial siphon with needle-like lobes whereas D. lithostrotum does not. An in-situ photograph by Stocker (1985) identified as D. densum shows polygonal pattern in the tunic; it could be a misidentification because this is an important character.