Population structure of two galatheid species (Galathea mauritiana and Coralliogalathea humilis) inhabiting interstices of coral rubble

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Galatheid crabs are one of the dominant macro-invertebrates in interstices of coral rubble. Population characteristics of two species of galatheid crab, Galathea mauritiana Bouvier, 1915 and Coralliogalathea humilis (Nobili, 1905), were examined by using traps filled with coral rubble in a coral lagoon of Ishigaki Island, Japan. The number of C.humilis in the traps did not vary between three areas (back reef, seagrass bed and nearshore) or two seasons (winter and spring), while G.mauritiana was very rare in the seagrass bed. For both G.mauritiana and C.humilis, allometric regression analyses showed that the carapace width against the carapace length of females showed a steeper slope than those of males and that the cheliped length of males showed a steeper slope than those of females. Size frequency histograms showed that the size range expanded and that larger G.mauritiana and C.humilis were found in spring. The proportion of ovigerous females was 36.3% in G.mauritiana and 44.9% in C.humilis. Externae of rhizocephalan parasites were observed in 7.41% of G.mauritiana but none were observed in C.humilis.