Habitat rather than phylogeny may explain exoskeleton composition of three common land crabs from Christmas Island

Christmas Island in the Indian Ocean hosts large populations of land crabs, including the gecarcinids Gecarcoidea natalis and Tuerkayana celeste and the grapsid Geograpsis grayi. All three have transitioned into terrestrial habitats via marine environments, but through different evolutionary routes, and with different life history traits. These species provide an opportunity to investigate the incorporation of chitin, protein and minerals (particularly calcium and magnesium carbonates) into the arthropod exoskeleton given their shared terrestrial transition, but different evolutionary histories and contrasting habitat use. While these minerals are known to provide mechanical strength and resilience to external pressure, little is known of interspecific differences. If proportions are constrained by phylogeny we predict the gecarcinids would be more similar to one another than to the grapsid, but if there has been ecological specialisation this would not necessarily be the case. While there was more chitin present in the exoskeleton of the grapsid compared to the geocarcinids, no further significant differences in protein or mineral composition were detected. The [Ca²⁺]: [Mg²⁺] ratio differed significantly between all three species, but was lowest in the two most terrestrial species (G. natalis, T. celeste and G. grayi exoskeleton [Ca²⁺]: [Mg²⁺]: 9.25 (± 0.17), 19.9 (± 1.89), 15.6 (± 0.35), respectively) and not the two most closely related. It appears that higher [Mg²⁺] is associated with increased terrestriality, reflecting the chemical and physical properties associated with higher [Mg²⁺] in the mineral component of the crustacean exoskeleton, and provides limited support for any phylogenetically driven differences, perhaps with the exception of chitin.