Changing patterns of population structure and gene flow at different spatial scales in Birgus latro (the coconut crab)

The population structure of the coconut crab ( Birgus latro ) was studied by examining genetic variation at seven polymorphic enzyme loci. Individuals were collected from 10 locations (grouped in seven major populations) throughout the Indo-Pacific distribution of the species. Significant population differentiation was found among all seven major populations ( F ST = 0.078, P < 0.001) and among the six Pacific Ocean populations ( F ST =0.026, P < 0.01). There were no significant differences in allele frequencies among adjacent Vanuatu islands separated by up to 200 km. At any one location there were no significant changes in allele frequencies over time (up to 3 years). Estimates of gene flow varied considerably, depending on the method of calculation, but all supported the same interpretations of population subdivision. The pattern of population structure varied with the spatial scale under consideration. The Indian Ocean population (from Christmas Island) was clearly divergent from all Pacific populations, in the fashion of an island model. However, within the Pacific, the relation between genetic and geographical distance showed that the pattern of genetic variation closely resembled an isolation by distance model. Populations from adjacent Vanuatu islands exhibited panmixia.