Morphological diversification among island populations of intertidal mites (Acari, Oribatida, Fortuyniidae) from the Galápagos archipelago.

The intertidal oribatid mite species Alismobates galapagoensis and Litoribates caelestis occur on the archipelago of Galápagos. To test for morphological variation between populations of different islands, a comprehensive morphometric study was performed. Four A. galapagoensis populations from the islands Bartolomé, Isabela, Santa Cruz and San Cristobal, as well as two L. caelestis populations from Bartolomé and Santa Cruz were investigated. The L. caelestis populations did not show any significant differences whereas the A. galapagoensis populations exhibited clear divergences indicating speciation. Differences in overall size of A. galapagoensis apparently followed a gradient from East to West, with specimens from San Cristóbal being the largest and individuals from Bartolomé and Isabela being the smallest. Apart from size, significant shape differences were found in the epimeral region and females showed stronger variation among islands than males. The degree of morphological divergence seems to correlate with geographic distance, i.e. populations from islands located closer to each other showed fewer differences than populations from distant islands. Based on this correlation we suggest that transport between islands has happened mainly by drifting on ocean currents.