Halisotoma poseidonis (formerly Isotoma poseidonis) is a scarce inhabitant of the seashore, although it has probably been confused with Halisotoma maritima in the past and may be more common than records would indicate. Halisotoma poseidonis is greyish blue in colour and reaches a length of 2.5 mm. The mucro has only three teeth and the outer side of the tibiotarsus of the second leg bears a pair of distinctive fan setae (Fig. 1). Halisotoma poseidonis differs from Halisotoma maritima principally in the fine structure of the maxilla (Fig. 2), but also has a stouter claw with larger lateral spines, but a thinner seta A7 adjacent to the claw.