Cryptopygus clavatus (Schött 1893), formerly Proisotoma admaritima (Murphy, 1953), is a rare, apparently littoral species of up to 1.3 mm in length. It was described by Murphy (1953) from Millport, Great Cumbrae Island, and has not been reliably re-discovered since. Proisotoma admaritima is blue/black in colour, has 8+8 ocelli, 1+1 ventral manubrial setae, and a mucro with two teeth (Fig. 1). The tibiotarsus has a very prominent tenant seta (Fig. 2). Proisotoma admaritima was included in Bolger's (1986) checklist for Ireland. However, the Glenamoy specimens that Steve Hopkin checked were all Cryptopygus thermophilus and the records from central Ireland seem doubtful if Proisotoma admaritima is a seashore species. It should be looked for around the Scottish coast where it is likely to turn up in many more localities.
Arne Fjellberg reports that this species is fully wettable (unlike other aquatic Collembola), and kept some in sea water culture for some months, creeping about between stones while permanently submerged.