Redspot Sponge Crab

The redspot sponge crab is a tiny, primitive reef crab that wears a living sponge, sea squirt or piece of soft coral on its back like a hat, a disguise that also makes it a nasty mouthful for predators.

It hides by day among coral rubble, algae and reef crevices, or shelters in the tops of sea anemones, sponges and tunicates, on shallow tropical Indo-Pacific reefs from the intertidal zone down to about 30m. Some larger members of the sponge crab family live as deep as 100 to 500m, but this one keeps to the warmer upper reef.

Appearance

The body and legs are covered in fine hair or fuzz. The shell is only 1 to 1.5cm across, but the crab can look several times bigger than that, because its overall size depends on how large its hat has become.

The last two pairs of legs are short and bend upwards over the back, where they grip the cover in place.

Behaviour

The crab is nocturnal and keeps out of sight during the day. Its hat does more than hide it. The cover stays alive and keeps growing, and among sponge crabs generally a sponge will in time mould itself to the shape of the shell. It often gives off unpleasant or toxic chemicals, and these put off predators such as fish and octopuses.

Feeding

It feeds on dead animals and plants.