We happened to catch one of our male fiddler crabs "signaling" this morning! This claw waving is meant to impress females and intimidate other males. This is actually the primary function of the oversized claw, it is virtually useless for feeding and he feeds himself with his other, much smaller, claw. Females have two small claws, making it very easy to tell males from females.
Fiddler crabs are very common in pet stores, but most die from improper care. In order to thrive in captivity fiddler crabs must have both land and water areas. Also, while they may survive in fresh water for awhile, they need brackish water for long term health. Brackish water is a mix of fresh and salt water which we achieve by mixing one part saltwater from our marine tank with two parts freshwater.
Our brackish tank includes four fiddler crabs and three lyretail mollies, fish that can live in fresh water but prefer brackish. This tank is on display in the front window of our store.