Factors influencing growth of giant terrestrial robber crab Birgus latro (Anomura: Coenobitidae) on Christmas Island

Abstract

Birgus latro has significant conservation and socio-economic importance throughout its range, yet very little is known about its growth in its natural habitat. An extensive mark-recapture program was undertaken on Christmas Island, Australia, in which over 1440 individual crabs were uniquely identified using passive integrated transponder (PIT) tags. Over 18% of the tagged population was recaptured at least once and remeasured, providing substantial information on growth of B. latro. Our results show that in its natural setting B. latro is a very slow-growing species. Individuals commonly experience conditions that result in zero or negative growth. Sex, size, and injury all strongly influence the crab’s moult increment. Males approached zero growth at around 65 mm in thoracic length (TL), while growth in females started to plateau at around 50 mm TL. Injury frequently led to negative growth, particularly in individuals that had lost a claw and/or multiple limbs—presumably due to a loss of foraging opportunities or access to other resources and a redistribution of available energy into limb regrowth. Our study indicates that previous growth curves do not adequately describe the growth of B. latro in its natural setting and that individual crabs may live over 30 yr longer than previously predicted, with large crabs likely to be over 100 years old. This may have serious implications for the long-term conservation of the species in the face of continuing exploitation.