Phorbas glaberrimus

Phorbas glaberrimus (Topsent, 1917)

(Figure 12)

Synonymy: Clathrissa glaberrima Topsent, 1917.

Anchinoe glaberrima (Topsent, 1917).

Material: Samples in 95% ethanol, tissue section and spicule preparation on slides; BELUM Mc 7651. Jagged Point, Possession Bay, South Georgia (54°04.514’S, 37° 07.188’W); depth 10.4m; collected by C. Goodwin, D. Poncet and P. Brewin, 23 rd November 2010. BELUM Mc 7671. Husvik, South Georgia (54°10.285’S, 36° 40.412’W); depth 18m; collected by C. Goodwin, D. Poncet and P. Brewin, 26 th November 2010. BELUM Mc 7675. Green Island, Stromness, Site 1, South Georgia (54°09.448’S, 36° 39.752’W); depth 17.4m; collected by C. Goodwin, P. Brickle and S. Cartwright, 27 th November 2010. BELUM Mc 7687 and BELUM Mc 7689. Green Island, Stromness, Site 2, South Georgia (54°09.381’S, 36° 39.852’W); depth 17.4m; collected by C. Goodwin, J. Brown, and S. Brown, 28 th November 2010.

Comparative material examined: NMHN DT734 Phorbas glaberrimus (Topsent, 1916) Holotype. From near Terre Alexandre. Spicule preparation and tissue section on slides.

External morphology: In situ appearance: Massively thickly encrusting sponge with the surface rising up into one or more mounds. Specimens very variable in size but can be quite large with diameters of greater than 30cm. Sponge encrusting on bedrock and are frequently overgrown by ascidians and bryozoans. Surface covered with large pore sieves (up to 1.5cm in diameter) and large, irregularly scattered, oscules, which can reach 2cm in diameter, are present. Sponge white to peach in colour but with a patchily dark chocolate brown surface layer present in some specimens, possibly formed of algae. The pore sieves often have a slightly lighter rim than the main sponge body. (Fig. 12a).

Preserved appearance: Firm but compressible grey sponge with pore sieves clearly visible on the tough ectosomal layer.

Skeleton: Choanosomal skeleton composed of a basal layer of acanthostyles from which thick ascending columns of up to 20 oxea ascend. Acanthostyles echinate the lower parts of these columns. In the ectosome the ends of the columns of oxea fan out to form a continuous surface layer. Chelae are very abundant and scattered throughout tissue (Fig. 12b).

Spicules: Measurements from Mc7689.

Acanthostyles: 216(264)313 by 9.5(14.1)19.7µm. Parallel sided with an abrupt point. Head not tylote. Strongly spined along whole length (Fig. 12c).

Ectosomal oxea: 365(428)476 by 9.9(13.0)17.9µm. Fusiform with abrupt points (Fig. 12d).

Chelae: 18.9(22.4)25.4µm (Fig. 12e).

Remarks: Phorbas glaberrimus (Topsent, 1917) was originally described from 297m in the Antarctic and appears to be a good match for these specimens, although Topsent reports slightly larger oxea (530–600 by 20–22µm). Rios (2006) reports smaller oxea of 420–590µm, more similar to the range in our specimens, and Koltun (1964) also gives a wider range (382–600µm). Rios (2006) reports two categories of acanthostyles (200–260 and 300–350µm), the acanthostyles of Mc7689 could be sub-divided into two length categories 216–233µm and 271–313µm but this division is not apparent in the other specimens and not obvious when viewed down the microscope.

Distribution: This species is widely distributed in the Antarctic: Alexander I land (Topsent 1917), Wilheim II coast, Banzare coast, Wilkes Land, Victoria Land, Princess Astrid Coast (Koltun 1964), MacRobertson Coast (Koltun 1976), Weddell Sea (Barthel et al. 1990; Gutt and Koltun 1995), Ross Sea (Pansini et al. 1994), South Trinidad Island and Bransfield Strait (Rios 2006) from depths 90–1370m. Burton (1929) also erroneously reported it as a synonym of Pyloderma latrunculioides (Ridley and Dendy, 1886) from the Antarctic but it is not clear which of his specimens are this species. There are no previous published records from South Georgia.