Red-legged hermit crab

Geographical Origin and Habitat: Dardanus arrosor is widely distributed in the Mediterranean Sea, the eastern Atlantic Ocean (from Portugal to Senegal, including the Azores, Madeira, and the Canary Islands), and parts of the Indo-Pacific. It prefers sandy, muddy, or detrital seabeds at depths ranging from 10 to 150 meters. It is a benthic organism that actively crawls along the ocean floor searching for food and discarded gastropod shells.

Taxonomy: Belongs to the class Malacostraca, order Decapoda, and family Diogenidae. First described by Herbst in 1796, it is distinguished by its robust, striped legs and its anemone-carrying behavior. Taxonomy indicates that hermit crabs of the family Diogenidae typically feature a left claw that is larger than or equal to the right claw, a characteristic clearly visible in this species.

Social Behavior: Exhibits a fascinating mutualistic relationship with sea anemones, particularly Calliactis parasitica. The hermit crab actively transfers the anemone onto its shell to gain protection from predators (such as octopuses) via the anemone's stinging tentacles. In return, the anemone benefits from the crab's mobility and feeds on scraps from its meals.

Coloration and Sexual Dimorphism: The body and legs are colored a bright reddish-orange or reddish-brown, decorated with prominent white or yellowish transverse stripes that form an attractive ring pattern. The claws are granular and robust. There is no visible external sexual dimorphism without examining the pleopods on the soft abdomen, which is tucked inside its shell.

Tank Setup: Requires a marine or Mediterranean aquarium of at least 150 liters, with a large sandy area on the substrate to allow easy movement. It is essential to supply several empty gastropod shells of increasing sizes (such as Tonna or Murex shells) so the crab can change shells as it grows. Live rock is recommended but must be secure to prevent tipping.

Diet and Feeding: An opportunistic omnivore and highly active scavenger. It feeds on organic detritus, dead fish, pieces of mollusks, shrimp, and algae. In the aquarium, it enthusiastically accepts any food offered: fish scraps, defrosted shrimp, mussels, and bottom-dweller sinking pellets. It helps keep the sand clean by consuming left-over food.

Water Quality: If sourced from the Mediterranean, it prefers cooler water temperatures between 18°C and 22°C, though it can tolerate up to 24-25°C for short periods. Chemistry parameters should be stable: pH of 8.1 to 8.4, specific gravity of 1.024 to 1.026, and alkalinity between 8 and 12 dKH. Like all crustaceans, it is sensitive to copper and requires iodine for molting.

Compatibility and Tankmates: Can be territorial and aggressive toward other hermit crabs when competing for food or shells. It is not compatible with live, small gastropods, which it may kill to steal their shells. It is safe with medium-sized fish and hard corals, but its heavy shell and clumsy movements can knock over unattached corals.

Aquarium Breeding: Captive breeding is extremely difficult and does not occur in home aquaria. The fertilized female carries eggs under her soft abdomen inside her shell until they hatch. The released larvae are planctonic (zoea) and pass through multiple development stages that are difficult to rear in closed systems with mechanical filtration.

Risks and Diseases: The main risk is molting failure due to iodine deficiencies or poor water quality. During molting, the crab is highly vulnerable and will hide for several days until its new exoskeleton hardens. A lack of upgrade shells will cause it to attack live snails or other hermits.