Symmetry isn’t everything: Tales of a fiddler crab

Fiddler crabs belong to the genus uca and have a wide distribution spanning Africa, the Atlantic, the pacific and indo pacific. Classed as semi-terrestrial, these crabs can be found on land and in water, with common habitats comprising sandy/muddy beaches, lagoons, mangroves and swamps. As in all species of crab, these crabs exhibit ecdysis, that is, the shedding of their exoskeleton to enable growth and regeneration of lost limbs. Once the exoskeleton (an external skeleton which is also called a shell) has been shed in a process called moulting, the crabs become reclusive for a period of time until their new shell has hardened and can provide adequate protection.

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The most striking feature of fiddler crabs is the lack of claw symmetry in males. Male crabs possess one very large claw termed the major claw and one regular sized claw termed the minor claw. As females possess two regular size claws, fiddler crabs represent an example of a sexually dimorphic animal, that is, an animal whose sexes display differences in characteristics beyond those relating to sexual organs. One function of the major claw is to attract mating partners, which is achieved through a waving display. Females will select males based on claw size and quality of the wave. More energetic wave displays are thought to be associated with healthier crabs which are likely to produce more viable offspring. It is also suspected that a bigger claw size is associated with a wider burrow, which may create more optimal temperatures for incubating eggs. In many species, once the female has selected her mate she will go into his burrow to lay her eggs. The female will normally stay within the burrow for a short gestation period before laying her eggs in the retreating tide, where offspring will undergo a planktonic stage before developing into juvenile crabs. Burrows are dug out by males and can often be recognised by the presence of small ‘sediment balls’ around the burrow. When feeding, crabs will pass sediment covered food (algae, fungus or detritus) to the mouth using their smaller claw where the sediment is filtered out and placed as a ball on the ground nearby. It is thought that the feeding habitats of crabs may help to preserve wetland environments through substrate aeration.

A second function of the major claw is to enable the males to fight with other males and defend their territories. As a longer, lighter claw is optimum for signalling to females but not for fighting there is an evolutionary trade off in these crabs between being a better fighter or a better signaller.  

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Jess Webster

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