Calcinus tubularis

The genus Calcinus has its centre of diversity in the central Pacific Ocean,[2] and only two species occur in the north-eastern Atlantic – Calcinus paradoxus and Calcinus tubularis.[3] C. tubularis is a chiefly Mediterranean species; its range extends from Madeira in the west to Lebanon in the east,[3] with outlying records from Madeira, the Canary Islands, Cape Verde and Ascension Island.[4] 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.[4]

The carapace of Calcinus tubularis is bluish, with many red spots, and extends forwards as a short, triangular rostrum.[4] 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.[4] The colour scheme exists in a dark form and a light form, which appears to be linked to camouflage, particularly for females.[5] C. tubularis is of "normal size" for a Calcinus species, frequently exceeding a carapace width of 3 millimetres (0.12 in).[6]

Calcinus tubularis is a rare species, found below the intertidal zone.[7] 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.[7][8] Males inhabit gastropod shells, chiefly those of Pisania maculosa or Cerithium vulgatum,[5] which they can move freely; females occupy the fixed tubes made by the vermetid snail Lementina arenaria.[7]

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.[9]

Calcinus tubularis was first described by Carl Linnaeus in the 1767 12th edition of his Systema Naturae, under the name Cancer tubularis.[1] It was later described by Polydore Roux as Pagurus ornatus, and that species was transferred to the genus Calcinus in 1892 by Édouard Chevreux and Eugène Louis Bouvier.[1] Lipke Holthuis recognised that C. ornatus was a junior synonym of Cancer tubularis, and transferred Linnaeus' species to the genus Calcinus, at which point it reached its current scientific name.[1] The specific epithet tubularis refers to the animals habit of living in the tubes of vermetid molluscs.[7] Linnaeus' description is imprecise, and could refer to "practically any hermit crab",[10] but the type locality (the Mediterranean Sea), together with the statement that it lives in worm tubes, restricts the possibilities to this one species.[10]

The sister species of Calcinus tubularis is C. verrilli, which is endemic to Bermuda.[2] The two species show the same sexual dimorphism in shell choice, making this an example of niche conservatism.[2]