Appearance
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.Distribution
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.Habitat
"Calcinus tubularis" is a rare species, found below the intertidal zone. It is one of only two species of hermit crab 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.
Evolution
The sister species of "Calcinus tubularis" is "C. verrilli", which is endemic to Bermuda. The two species show the same sexual dimorphism in shell choice, making this an example of niche conservatism.References:
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