Soft Coral Spider Crab

Overview

Achaeus spinosus, commonly known as the soft coral spider crab, is a small decapod belonging to the spider crab family Inachidae. This species is widely recognized for its decorator crab behavior, often attaching pieces of sponge, hydroids, or soft coral polyps to its body for camouflage and protection. It is frequently encountered in tropical marine environments across the Indo-West Pacific region, including the Red Sea.

Appearance

The body of this small crab features an oval-shaped carapace with a whitish coloration and distinct red stalked eyes. Its entire body is thickly covered in fine setae or hairs, which function like Velcro to hold fragments of marine organisms in place. It has long, delicate, clawless walking legs, while its front pair of limbs is shorter and equipped with small pincers used for feeding and decoration.

Biology & behavior

This species exhibits a symbiotic or associative lifestyle, typically residing directly on soft corals, sponges, or hydroid colonies. By coating its shell in stinging hydropolypes, the crab gains an effective defense against predators while providing the polyps with mobility to reach new feeding grounds. It is primarily nocturnal, remaining hidden or active under the cover of darkness.