The hermit crab Dardanus arrosor is a species belonging to the order Decapoda and the family Diogenidae. Its distribution is very extensive, and it can be found in the waters of the Atlantic Ocean, the Mediterranean Sea, the Red Sea, and the Indo-Pacific. It inhabits the continental shelf, in soft or not very compacted substrates such as the area between sand and rocks, and in a wide range of depths, from shallow waters to 750 meters deep.
The body of Dardanus arrosor is a deep red or orange color, with thick and robust eye stalks on the rostrum with white and red ringed coloration, at the end of which we find the prominent eyes. Being a species of the order Decapods, Dardanus arrosor has a body with ten legs, two of which are modified and take the shape of a clamp. The left claw, also called the left cheliped, is larger than the right one. It is one of the largest crab species in the Mediterranean, reaching up to 8 cm in length. Both legs and claws are covered with a kind of hair. Their body is made up of a spiral-shaped abdominal region with a soft consistency, and another area where the appendages and head are located. The soft consistency of the abdomen forces them to seek refuge in gastropod shells, such as those of the species Charonia lampas or Galeodea rugosa among others. Inside the shells they introduce their abdominal region, leaving only the extremities and the head region on the outside, although both can be retracted to the inside of the shell in case they feel in danger. The fact that they use shells that they do not produce themselves, but that they find on the seabed, means that as the specimens grow, they need to change shells to adapt to their body size.
It is common for specimens of Dardanus arrosor to carry in their shells either stinging anemones or sponges. In both cases a mutually beneficial relationship is established, in which the crab obtains extra protection, while the anemones and sponges obtain a capacity of movement that allows them to increase their chances of capturing their prey. Among the species that Dardanus arrosor usually carries on its back, we find the actinia Calliactis parasitica. As for the sponge species, it is usual that these are of the species Suberites domuncula. In this case, the sponge is attached to the shell and grows as the crab grows, thus creating a spiral duct where the crab lodges without the need to change shells.
Regarding the habits of Dardanus arrosor, it is a species that is extremely active during the night, when it moves in search of carrion to feed on. Its scavenger diet makes this species play an important role in the marine ecosystem, as it is responsible for cleaning the seabed of organic debris. Reproduction occurs during the summer months of July and August.Photos: