Figures 3e, f; 6 c-f
Psolidium tenue Mortensen, 1925: 8–9, text figs 4–5.
Psolidium incertum.— O’Loughlin et al., 1994: 552, 554 (non Psolidium incertum (Théel, 1886) = Psolidium poriferum (Studer, 1876)).
Material examined. Antarctic Peninsula, 90 m, USNM E40789 (1 specimen); 265 m, E40790 (1); 100–200 m, E40791 (2); 270 m, E40792 (6); 330 m, E40793 (2); 212 m, E40794 (1); 311 m, E40796 (1); 325 m, E40801 (1); 670 m, USNM 1005859 (2); Discovery stns 181, Palmer Archipelago, 160–335 m, BM(NH) 2008.3191 (1); 182, 278–500 m, BM(NH) 2008.3192 (1); 187, 0–195 m, BM(NH) 2008.3193 (1); 190, 0–250 m, BM(NH) 2008.3194-3196 (3); 599, W of Adelaide I., 0–150 m, BM(NH) 2008.3197-3198 (2).
Ross Sea, 923 m, USNM E40776 (1); NIWA Tangaroa 0802, 67- 74°S 167°E- 179°W, 420–866 m, NIWA 36003 (1); 36027 (20); 38102 (1); 38545 (1); 38815 (4); 38842 (1); 38872 (1); 38884 (18); 38956 (9); 45698 (3); 45727 (2); 45728 (1); Discovery stns 1644, 626 m, BM(NH) 2008.3199 (1); 1649, 695 m, BM(NH) 2008.3200-3201 (2); 1652, 0–500 m, BM(NH) 2008.3202-3205 (4); 1660, 0–351 m, BM(NH) 2008.3206-3208 (3); 2200, Balleny Is, 512–532 m, BM(NH) 2008.3209-3218 (11); NZOI Endeavour stns A461, 0–550 m, NIWA 43870 (many specimens); A 466, 555 m, NIWA 43871 (many); A 521, 569 m, NIWA 43872 (5); A531B, 348 m, NIWA 44705 (1); A 537, 546 m, NIWA 43873 (4); D 216, 463 m, NIWA 43874 (4); E 199, 278 m, NIWA 43875 (2); E 200, 646 m, NIWA 43876 (1); E 203, 187 m, NIWA 43877 (4); E220A, 371 m, NIWA 43883 (many); E220B, 371 m, NIWA 43878 (many); E212B, 91 m, NIWA 43879 (1).
Wilkes Land, 183 m, USNM E40799 (1); BANZARE stn 97, 474 m, SAM K2347 (1).
Prydz Bay, 320–768 m, NMV F68113 (2); F68116 (1); F68117 (2); F107415 (1); F107416 (1); F105417 (1); F107437 (1); F107438 (1); F107442 (1); F107443 (1); F107447 (1); SAM K2219 (1); BANZARE stn 103, SAM K2348 (4).
New Zealand, Antipodes I., 2100 m, USNM 1004885 (1).
Diagnosis. Psolidium species up to 50 mm long; body form elongate, narrow, high; dorsal and lateral scales conspicuous, thin, smooth, up to 3.0 mm wide; dorsal and lateral tube feet inconspicuous.
Sole: outer peripheral single series of smaller tube feet; inner peripheral single series of larger tube feet; lacking mid-ventral radial series of tube feet.
Dorsal ossicles: multi-layered perforated plates (scales), single-layered marginally, reticulate thickenings from near margin towards centre, multi-layered centrally; rarely any development of smooth white thickening with smaller perforations; rarely any development of radiating-to-margin thickenings; up to 3 tube foot canals per plate; rare tube foot support plates; endplates small, mesh-like.
Sole ossicles: throughout most of sole rare, small, smooth, thin, irregularly shaped perforated plates, 0–8 large perforations, up to 230 μ m long; sometimes as regular 4-holed plates with slightly knobbed margin, sometimes with surface knobs, plates 100–200 μ m long; near margin of sole and peripheral tube feet some thicker irregular perforated rod-plates, up to 570 μ m long, intergrading with irregularly oval thick perforated plates, with secondary layering, up to 480 μ m long.
Tentacles: largest tentacle trunk ossicles predominantly long, narrow, thick, rod-plates (rods joined to create perforations) and perforated plates with prominent rod thickenings; not predominantly smooth perforated plates; rod-plates variably straight or bent or curved, up to 800 μ m long.
Colour (preserved). Body variably dark brown to pale brown to off-white; coarse red-brown flecking dorso-laterally; scales with a light-coloured “haloed” marginal appearance.
Distribution. Antarctic Peninsula, Ross Sea, Wilkes Land, Prydz Bay, 90–923 m; New Zealand, Antipodes I., 2010–2100 m.
Remarks. We raise Psolidium tenue Mortensen, 1925, out of a synonymy by Gutt (1988) with Psolidium incertum (Théel, 1886), synonymised above with Psolidium poriferum (Studer, 1876). P. poriferum is an almost exclusively Sub-Antarctic species. The single type specimen for Psolidium tenue Mortensen, 1925, was taken north of Discovery Inlet in the Ross Sea from 640 m. There are no inconsistencies between the original description and the material studied here from the Ross Sea. The length (preserved length of 30 mm), paucity of ossicles in the sole, single small knob sometimes present on the ossicles in the sole, and size of ossicles in the sole are all consistent with the Ross Sea material. Psolidium tenue Mortensen, 1925, is distinguished diagnostically by the rodplate form of the largest tentacle ossicles.