Tube Hermit Crab

The carapace of Calcinus tubularis is bluish, with many red spots, and extends forwards as a short, triangular rostrum. The eyestalks are white with similar red spotting, as is the last segment of each of the walking legs, and both the fixed and movable fingers of the claws. The colour scheme exists in a dark form and a light form, which appears to be linked to camouflage, particularly for females. C. tubularis is of "normal size" for a Calcinus species, frequently exceeding a carapace width of 3 millimetres (0.12 in).

The genus Calcinus has its centre of diversity in the central Pacific Ocean, and only two species occur in the north-eastern Atlantic – Calcinus paradoxus and Calcinus tubularis. C. tubularis is a chiefly Mediterranean species; its range extends from Madeira in the west to Lebanon in the east, with outlying records from Madeira, the Canary Islands, Cape Verde and Ascension Island. Although it is distinctively coloured, the sedentary behaviour of C. tubularis allows epibionts to colonise its shells, providing excellent camouflage, and it can easily go unnoticed; it was first reported on the coast of the Portuguese mainland in 2011, but is thought to have been living there for a long time.

Calcinus tubularis is a rare species, found below the intertidal zone. It is one of only two species of hermit crab (the other being the closely related C. verrilli) in which sexual dimorphism in shell use has been observed. Males inhabit gastropod shells, chiefly those of Pisania maculosa or Cerithium vulgatum, which they can move freely; females occupy the fixed tubes made by the vermetid snail Lementina arenaria.

The only parasite known to attack Calcinus tubularis is a rhizocephalan barnacle, probably of the genus Septosaccus, although it is also targeted by another barnacle, Trypetesa lampas, which is an egg predator of various hermit crabs.