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

Abstract

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. Fishermen break off the male major claw and throw the crab back into the mudflat to regenerate a new one. Approximately 38 % of the males sampled had a missing or a regenerating claw. Although individuals are not removed from the population the operational sex ratio is biased towards females since other males and females behave towards clawless males as if they were females. Moreover, removing the major claw from males prevents them from signalling (waving display) to females to attract them to their breeding burrows and it also places them at a disadvantage if they have to defend their burrows from males with intact claws. Thus, the harvesting of male fiddler crab claws has potential consequences at the population level. In this paper we investigated these potential consequences by comparing an exploited population of fiddler crabs at Ria Formosa with a remote population at the Mira estuary that is not under human exploitation. The unexploited population has significantly larger males and a significantly higher density of burrows. The operational sex ratio is also significantly different between the two populations with a female‐biased sex ratio in the exploited population. These preliminary 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 population structure is potentially affected need further investigation.

Effects of Autotomy Compared to Manual Declawing on Contests between Males for Females in the Edible Crab Cancer pagurus: Implications for Fishery Practice and Animal Welfare

“…Manual declawing of crabs is practiced in many fisheries, including the Southern Florida stone crab, Menippe mercenari, (Ehrhardt 1990), the North East Atlantic deepwater red crab, Chaceon affinis, the Southern Iberian fiddler crab, Uca tangeri, (Oliveira et al 2000) and in Northern Europe, the edible crab, Cancer pagurus (Patterson et al 2009). After declawing, the animal is released and the practice of manual declawing is defended because crabs may naturally autotomize a claw or walking leg, for example when grasped by a potential predator, and then regenerate the lost limb (Juanes and Smith 1995).…”

“…This dimorphism is even greater in fiddler crabs, Uca tangeri, in which the major claws of males are used for signalling to females to attract them to their breeding burrows and to defend their burrows from other males. The removal of this vital appendage biases the operational sex ratio towards females, as clawless males are treated as females by other males and females (Oliveira et al 2000). Thus, removing the major claw of male fiddler crabs has potential consequences at the population level.…”

Effect of harvesting, vegetation structure and composition on the abundance and demography of the land crab Cardisoma guanhumi in Puerto Rico

“…Physical factors such as temperature, availability of water and ions (Wolcott 1988), sediments (Frusher et al 1994) and topography (Blankensteyn et al 1997;Sherman 2003); plus biotic ones such as leaf litter (Capistrán-Barrada et al 2003), forest complexity and harvesting (Oliveira et al 2000;Pedersen et al 2003) have been identified as important determinants of the distribution and abundance of land crabs. Coastal ecosystems are increasingly being impacted and modified by humans.…”

Comparison of non-invasive methods for quantifying population density of the fiddler crab Uca tangeri

“…Ens et al, 1993;Mouton & Felder, 1996) and visual counts of surface-active individuals (e.g. Zucker, 1978; Wolfrath, 1993;Oliveira et al, 2000).Uca tangeri is the only ¢ddler crab to occur in Europe and is found on almost all mud£ats and salt marshes at Ria Formosa. Fieldwork took place from May to July 1999 in two periods between full moon and waning moon, at Cacela Velha and Fuzeta (Ria Formosa Natural Park, Algarve, Portugal).…”