Figures 4 a–f, 5.
Material examined. Holotype. Antarctica, Scotia Sea, South Orkney Is, 60 º 36.21 'S 44 º 19.67 'W, 211 m, US AMLR 2009 stn 19 – 13, S. Lockhart, 13 Feb 2009, NMV F 168861.
Paratypes. South Orkney Is, 60 º 26.52 'S 45 º 17.91 'W, 497 m, US AMLR 2009 stn 12 –7, 11 Feb 2009, NMV F 168857 (1); NMV F 168859 (5); NMV F 168860 (3).
Other material. South Orkney Is, 298–302 m, USNM E 43073 (3); 593–598 m, USNM E 43074 (2); 2355–2897 m, USNM E 43076 (63); 567–671 m, USNM E 43080 (3); 759–857 m, USNM E 43081 (7); South Shetland Is, 300 m, USNM E 43075 (3); 220–240 m, USNM E 43078 (1); South Georgia, 1968–2188 m, USNM E 43079 (3); Burdwood Bank, 339–357 m, USNM E 43077 (4).
Diagnosis. Psolus species up to 20 mm long, 10 mm high, 8 mm wide; form sub-cylindrical, smooth, rounded oral and anal ends, lacking oral and anal cone; scales up to 3 mm wide, imbricating, macroscopically smooth, microscopically finely beaded, mosaic arrangement of large and small, rounded and angular scales; lacking 5 discrete oral valves, oral scales numerous, digitiform or narrow pointed or triangular pointed; tentacles 10, 8 large, 2 small ventrally; calcareous ring with digitiform radial and interradial anterior projections, radial projections slightly swollen distally, concave radial and interradial posterior indentations; thin calcareous sole, single to zig-zag to double series of large inner peripheral tube feet, lacking outer peripheral series of smaller tube feet, rare mid-ventral tube feet present; single ventral polian vesicle; gonad tubules unbranched.
Dorsal and lateral multilayered ossicles only. Sole with crosses, thick, finely knobbed or bluntly-spinous, shallow concave or cupped, small and large, commonly 88–112 µ m long, rarely 168 µ m long, upturned branched arms of crosses may join to form cups with up to 8 perforations, marginal projections sometimes joined to create anastomosing branches over cups or transverse bridges across cups. Largest tentacle ossicles irregularly oval or triangular or narrow perforated plates, up to 480 µ m long, surface smooth, bluntly denticulate margin. Largest tube feet support ossicles up to 560 µ m long, thick smooth perforated plates, irregularly oval or triangular or narrow, bluntly denticulate margin.
Colour (preserved). White.
Habit. Lives with the lateral margins of the sole wrapped around Cidaridae echinoid spines of different species, with type specimens found on spines of Ctenocidaris speciosa Mortensen, 1910 and Rhynchocidaris triplopora Mortensen, 1909.
Distribution. South Atlantic Ocean, Burdwood Bank; Antarctic Ocean, Scotia Sea, South Georgia, South Shetland Is, South Orkney Is, 211–2897 m.
Etymology. Named for Susie Lockhart (US National Oceanic and Atmospheric Administration), with appreciation of her generous collaborative assistance in creating the opportunity for us to study US AMLR Antarctic holothuroid specimens.
Remarks. Psolus lockhartae sp. nov. is the only known species of Psolus that lives on echinoid spines. The psolids Echinopsolus acanthocola Gutt, 1990 and Echinopsolus parvipes Massin, 1992 have a similar life habit. In this study one specimen of Psolus lockhartae sp. nov. (NMV F 168857) was found attached to the same spine from the echinoid Ctenocidaris speciosa as a specimen of Echinopsolus acanthocola (NMV F 168858). In addition to a unique habit, the morphological characters that distinguish this species from all other Antarctic Psolus species are a combination of: smooth sub-cylindrical body form; absence of oral valves; absence of an outer peripheral series of smaller tube feet on the sole; distinctive thick knobbed and bluntly spinous concave crosses and cups in the sole; lacking ossicles other than scales dorsally and laterally.