Orange-Clawed Fiddler Crab

The carapace is up to 30 mm across, black-brown with blue patches to varying extent. Adult males have an enlarged claw with a grey or pink upper finger, and orange lower finger and hand. Like other fiddler crabs the claw is used to scare or fight other males, and in waving displays to attract females. The lower male claw has a large triangular protection in the most distant part from the body.

If caught, the colors of Gelasimus vomeris crabs will become less brilliant within minutes, and in response to the presence of predators in an area, the blue color of these fiddler crabs will change over a few days to a dull muddy brown.

The preferred habitat of these crabs is unshaded sandy mud. They are found in bays, estuaries and creeks, living in burrows at lower tide levels which they stay close to, typically <1 m while feeding. There is a predominance of right handed individuals in colonies, one study finding left handed individuals forming only 1.4% of the study population.