The population structure of the freshwater land crab Discoplax hirtipes (Gecarcinidae) was studied on Christmas Island, Indian Ocean, to which this species is endemic in its blue colour form. Despite previous overexploitation of this crab, this is the first such study to take place for this species on Christmas Island. Surveys were conducted in the dry (non-migration) season, at Ross Hill Gardens, which is one of several permanent ‘residential areas’ inhabited by D. hirtipes. The sex ratio was found to be skewed in favour of males 2∶1. Similarly to other gecarcinid species, male crabs were found to grow larger than females in terms of body weight, carapace width and chelae size (length and width). During the survey, few juveniles were found, which could suggest an ageing population. Despite being protected by law on Christmas Island, this is potentially worrying for a species that due to its restricted distribution continues to be especially vulnerable to outside threats, including that of aggressive invasive species and long term habitat destruction.
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Vol. 31 • No. 3