Human exploitation of male fiddler crab claws: behavioural consequences and implications for conservation

Male fiddler crabs have one of their feeding claws greatly enlarged, which may comprise up to 40% of their weight. In southern Portugal (Ria Formosa) the major claw of the fiddler crab Uca tangeri is a local delicacy. Fisherm en break off the male major claw and throw the crab back into the mudflat t o regenerate a new one. Approximately 38% of the males sampled had a missin g or a regenerating claw. Although individuals are not removed from the pop ulation the operational sex ratio is biased towards females since other mal es and females behave towards clawless males as if they were females. Moreo ver, removing the major claw from males prevents them from signalling (wavi ng display) to females to attract them to their breeding burrows and it als o places them at a disadvantage if they have to defend their burrows from m ales with intact claws. Thus, the harvesting of male fiddler crab claws has potential consequences at the population level. In this paper we investiga ted these potential consequences by comparing an exploited population of fi ddler crabs at Ria Formosa with a remote population at the Mira estuary tha t is not under human exploitation. The unexploited population has significa ntly larger males and a significantly higher density of burrows. The operat ional sex ratio is also significantly different between the two populations with a female-biased sex ratio in the exploited population. These prelimin ary results suggest that claw harvesting in fiddler crabs has a measurable effect at the population level. The consequences of this type of fishery in which the individuals are not removed from the population but the populati on structure is potentially affected need further investigation.