Calcinus elegans, also known as the blue line hermit crab, is a small, tropical hermit crab.
Description
It features the typical body plan of a member of the phylum Arthropoda: a segmented head, a thorax, and an abdomen. As a member of order Decapoda, this organism has 5 pairs of legs, with one pair having developed into sharper claws, or chela. This hermit crab supports the heavy shell of a gastropod with its four pairs of ambulatory legs, shielding its soft abdomen inside. It features a unique and exotic color pattern, sporting alternating bright blue and black stripes on its legs and olive green chela with white speckles on the ends. Two bright blue eyes peer out of the shell alongside two orange antennae and two orange antennules. The organism also has maxillae to help guide particles of food into its mouth. Some morphological differences arise based upon the geologic habitat the organism resides in. For instance, some individuals of Calcinus elegans found in Hawaii display orange bands on their ambulatory legs, differing from the pure blue bands found in individuals of the Indo-Pacific. Calcinus elegans is distinct from all other species in the Calcinus genus as it is the only one to be covered with small hairs known as seta.
Habitat
Calcinus elegans primarily makes its home in shallow tidal and subtidal regions found within the Indo-West Pacific, stretching from Eastern Africa to island chains just south of Hawaii. Some of them can also be found living in the tropics of the U.S. Virgin Islands near Puerto Rico. Some sightings have even been reported at various locations in Japan including the Ryukyu Islands, Izu, Ogasawara, Kochi, Boso, and the Kii Peninsula. Its distribution primarily follows the path of the equator; these places are known to have tropical conditions such as higher ocean temperatures, clear waters, and little primary productivity. These habitats have a depth range of , a sea surface salinity range of 30-35 ppt, and a sea surface temperature range of .
Behavior
Shell selection
Each member of the Calcinus elegans species will select a shell for itself, occasionally switching shells with other species members or empty shells it finds during its lifetime. Shell exchanges could be performed due to the current shell being structurally unstable, too heavy, or perhaps too large or small. Shell exchange was also found to occur within the Calcinus genus as a result of interspecific competition, as individuals of a certain species battle for a perfect shell. Some studies show that the sex of the individual may also play a role in the competition; this behavior is observed in individuals of Calcinus verrilli. These shells are primarily used to provide shelter and protection from predators as well as the surrounding environment. Individuals of Calcinus elegans were found to show a preference for the shells of dog whelks, but were also observed wearing shells of the snakehead cowrie and variable worm. This shows that the species exhibits a variety of morphology.
Shell shape was also found to influence the distribution of Calcinus elegans within their habitat. Those with cowrie or variable worm shells often remained in shallow waters, refraining from entering the subtidal areas out of fear of predation. When tested, cowrie shells and variable worm shells proved to be a weak defense system against the predators of the subtidal. Calcinus elegans that wear the shells of dog whelks prefer to live within the subtidal region, as their shell shape allows them to effectively deter predators.
Shell shape shows no significant impact upon the travel rate to obtain food for individuals of Calcinus elegans. Therefore, there is no select locomotive advantage of wearing a certain species’ shell.
Studies conducted using simulated surges show that shell shape influences the ability of Calcinus elegans to withstand wave action. Individuals wearing shells of dog whelks were proven to resist surge much more effectively than those with snakehead cowrie or variable worm shells.