Behaviour and population structure of soldier crabs Mictyris longicarpus (Latreille): Observations from a tidal flat in tropical North Queensland, Australia

Crabs of the family Mictyridae are common on sandflats of the Indo-West-Pacific. Activity patterns and population structure of Mictyris longicarpus (LATREILLE) were monitored in the Haughton River estuary in northeast Australia. The emergence of the crabs at low tide was intermittent and could not be related to lunar cycles, but the crabs were significantly more active on sunny than on rainy or overcast days. Of a total of 1494 crabs measured over a 14 months period, nearly 2/3 were males (951 crabs). No ovigerous females were found on the sandflat surface. Crabs active on the sediment surface ranged in size from 4.7 to 19.7 mm carapace length (CL). Distinct size-frequency distributions were identified for juveniles, females and males. The mean size of juveniles was 7.3 mm and of females 11.2 mm CL. Males showed a bimodal size-frequency distribution. The first mode (mean 8.5 mm) was clearly separated in size from the second mode (mean 15 mm). The population structure remained almost constant throughout the year. Juvenile and adult crabs showed different aggregation patterns and preferred different sediment types.