Polymastia invaginata Kirkpatrick, 1907

(Figure 23)

Specimens. BELUM. Mc 2015.598 Grotto Island, Verdansky Base (Site 1) (65°14.615’S, 64° 15.019’W), depth 14–24 m; collected by C. Goodwin and E. Priestley, 16/02/2015 ; BELUM. Mc 2015.645 Rocks near San Martin Is- lands (65°41.297’S, 65° 20.091’W), depth 6–21 m; collected by C. Goodwin and E. Priestley, 17/02/2015 . BELUM. Mc 2015.691 Rocks NW of Laktionov Island (65°45.536’S, 65° 47.319’W), depth 6–23 m; collected by C. Goodwin and E. Priestley, 22/02/2015 . BELUM. Mc 2015.696 Vieugue Island (65°38.758’S, 65° 12.540’W), depth 10–22 m; collected by C. Goodwin and E. Priestley, 23/02/2015 .

External morphology. In situ appearance (Figure 23A): Low brown hispid mound with large single pale yellow papillae with single terminal oscule.

Preserved appearance. Firm beige sponge. In cross-section choanosome with clearly visible ascending columns. Surface covered with a strongly hispid layer or projecting spicules (up to 5 mm long), giving a fur like texture. Beneath this there is a distinct ectosomal layer (1 mm in diameter).

Skeleton: Radiate skeleton of bundles of large styles, these penetrate the ectosome and form the thick surface pile (Figure 23D). Stellate groups of small tylostyles are present between the fibres. The ectosome is formed of a dense palisade of tylostyles, positioned vertically with their points towards the surface. This layer forms a fibrous cortex to the sponge, easily visible on slides.

Spicules: Tylostyles (Figure 23B): 114 (169) 275 by 5(8) 11 µm. Fusiform tylostyles with a neat swelling at their head. Some forming stellate clusters between the fibres but these do not seem to represent a separate size category.

Styles (Figure 23C): 1630(2512)3167 by 24(31) 45 µm. Fusiform long styles.

Remarks. Our specimens are a good match with the type description and specimens, and correspond to the external form and spiculation of other specimens assigned to this species (Hentschel 1914; Koltun 1964; Brueggeman 1998; Plotkin & Janussen 2008; Goodwin et al. 2012; Hajdu et al. 2016). However, like previous authors, we did not record the sceptre-like spicules noted by Plotkin & Janussen (2008) in the cortical palisade, and our styles are of a larger size than those noted by Boury-Esnault and van Beveren (1982). Polymastia invaginata can be distinguished from other Antarctic and Southern Atlantic species of Polymastia by its single inhalant papillae, densely hispid surface and single spicule layer in the cortex (Plotkin & Janussen, 2008). Kirkpatrick (1907) noted that the papillae in all of his specimens was ‘invaginated’, flush with the surface of the basal mound, this is presumably the origination of the species name. He was studying preserved material and this may have been an artefact of preservation, in all our living specimens the papillae stood proud.

Distribution. Originally recorded from Winter Quarters (18–55m depth) and from off Mount Erebus (914m depth) (Kirkpatrick 1907). Widespread in the Antarctic and sub-Antarctic: records from Kerguelen and Heard Islands (Boury-Esnault & Van Beveren 1982); Weddell Sea (Plotkin & Janussen 2008), McMurdo Sound (Burton 1929), South Georgia, South Orkneys and South Shetlands (Burton 1932, Hajdu et al. 2016) in depths of 18–1080m. Polymastia invaginata var. gaussi Hentschel, 1914 was regarded as a synonym by Burton (1932), but this is a much smaller sponge (maximum 8 mm high), and has smaller spicules (styles up to 1792μm, tylostyles 120–600 μm), so is a distinct species (Plotkin & Janussen 2008).