Leach's Spider Crab

Overview

Leach's spider crab (Inachus phalangium) is a small, cryptic crustacean widely recognized for its commensal relationship with the snakelocks anemone (Anemonia sulcata). By sheltering among the anemone's stinging tentacles, the crab gains protection from predators while feeding on mucus and food scraps captured by its host.

Appearance

This species features a small, triangular carapace that is typically brownish-yellow or reddish-orange. Its long, slender legs and the upper surface of its body are covered in hook-like hairs, which the crab often uses to attach algae or sponges for camouflage. Males are distinguished by their significantly larger, sturdier claws compared to females. The carapace is marked by three tubercles arranged in a triangle, though these features—along with the crab's natural color—are frequently obscured by the epibionts it collects.

Distribution & habitat

Leach's spider crab is found throughout the northeastern Atlantic Ocean, ranging from Norway down to West Africa and the Cape Verde islands, as well as across the Mediterranean Sea. It typically inhabits sublittoral zones, often residing on mixed coarse or sandy substrates where its host anemones are anchored.

Biology & behavior

These crabs are known for their site-constant behavior, with females often remaining with a single anemone or group. Males are more mobile, patrolling areas containing multiple anemones to locate and guard females for mating. While they are highly adapted to their host, they may occasionally leave the anemone to forage for algae or other food sources. They are capable of acclimatizing to the anemone's sting through a specific behavioral pattern, allowing them to move safely among the tentacles.