Drum Favourites: the Southern Sponge Crab

Andrew Hosie's blog | Created 5 years ago

Austrodromidia australis belongs to the Dromiidae family, commonly known as the sponge crabs. These unique creatures actually carry a well-fitting camouflage cap of living sponge or ascidian (sea squirt) over their back which they trim and keep tidy. As well as camouflaging the crab, the sponge can often taste bad, making potential predators reluctant to eat it. The sponge or other encrusting animal on the back of the Dromiid can often be larger than the crab itself. The morphology of these species is similar to other crabs however the last pair of legs is modified to hold the sponge, bending up and over the carapace or shell and often ending in a small pincer.

A mounted specimen of The Southern Sponge Crab, Austrodromidia australis

The Southern Sponge Crab, Austrodromidia australis.
Image copyright WA Museum

Most species are cryptic and rarely seen intertidally. They spend most of their day in caves and crevices within the reef and do their scavenging at night. They are omnivores and feed on animals and plants. Sponge crabs usually range from 0.5 – 10 cm in diameter but the specimen we found in our collection is about 15 cm.

A mounted specimen of The Southern Sponge Crab, Austrodromidia australis

The Southern Sponge Crab, Austrodromidia australis
Image copyright WA Museum

This specimen was found without its sponge and unfortunately no data was recorded for this specimen, so we don’t know where it was collected or when. You can see in the photos that this particular specimen has had the right side of the carapace shaved. This was done as part of the identification process, to make important features such as the small spines and cleft in the edge of the carapace visible.

A living sponge crab, complete with camouflage hat

A living sponge crab, complete with camouflage hat.
Image copyright WA Museum