Fiddler crabs

What are fiddler crabs? Fiddler crabs belong to the genus Uca. They are members of the ocypodid family of brachyuran crabs, the most recent marine animals to have invaded land. They spend the first part of their life as aquatic plankton and only settle in the intertidal zone after their last larval moult. Adults live in burrows on intertidal mudand sand-flats within dense, mixedage, mixed-sex and mixedspecies colonies (Figure 1). Each adult defends his or her own burrow and a small area around it. They are active on the surface during low tide, feeding on algae, bacteria and detritus in the topsoil. It is thought that fiddler crabs can live for up to seven years and adults of the largest species can reach a body size of about 5 cm. The crabs grow by moulting which, under favourable conditions, they do about every eight weeks. Fiddler crabs are highly social animals with a rich behavioural repertoire. They communicate by visual and vibratory signals; they have complex territorial interactions and flexible courtship and mating systems. Some species carry individually distinct colour patterns and some others even build mud or sand structures as homing aids and to enhance or limit social interactions. As their common name suggests, one of the most obvious behaviours in a fiddler crab colony is claw waving: males wave their one enlarged claw to attract females for mating and to repel intruders from their territory. The massive claw can weigh half a male’s body weight and is also used as a weapon. Interestingly, handedness differs among species: in most species there are equal numbers of leftand righthanded males, but in a few species virtually all the males are right-handed. We do not know yet what determines handedness nor what are its social consequences. Fiddler crabs have two distinct mating strategies, with some species exhibiting both forms. In one strategy, females leave their burrows and move through the colony visiting many males before choosing a mate. Males wave vigorously to attract these females to their burrow, where mating takes place underground and where the female will incubate her eggs. In the other strategy, mating takes place at the entrance to the females’ burrow, and it is the males that have to search for and locate the females. Little or no waving precedes surface mating. In some species that have both mating systems, the relative proportion of each type depends on the risks of wandering: females stop searching for suitable mates if predation pressure becomes too high, leaving the males to risk moving across the mudflat in search of receptive females.