Taxonomy and ecology of the Western Australian Soldier Crab, Mictyris Occidentalis (Crustacea: Decapoda: Brachyura: Mictyridae)

The taxonomy and autoecology of the Western Australian soldier crab, Mictyris occidentalis Unno, 2008 is presented including a new species description, a comprehensive ichnology, sampling methods, an extensive habitat description and a decadal population dynamics study. Such a wide ranging, holistic study has not been carried out for any mictyrid specious previously. In detail, a taxonomy component compares M. occidentalis with congeners, clearly establishing it as a separate species from M. longicarpus to which it was previously referred, and the issues affecting taxonomic classification in the Mictyris genus in general are discussed. A species identification key is provided for the genus. A full suite of ichnological products created by M. occidentalis including cavities, shafts, exit holes, pellets, rosettes, tunnels and pustular structures, is described and related to the behaviour and life stages of the crab from juvenile to adult. The principle of understanding the behaviour of a species before designing sampling strategies is illustrated, using the example of the rapid burrowing escape mechanism employed by M. occidentalis. The habitats of M. occidentalis are described across its entire geographic range encompassing thousands of kilometres of coastline. Factors characterising the soldier crab habitat are investigated from the regional, to large, to local, to micro-geomorphic habitat scale including the abiotic factors of wave energy, tidal level, submergence frequency, the sediment characteristics of grain size, composition, moisture content, salinity and also groundwater salinity. Also, the biotic factors of the densities of sea couch roots and mangrove pneumatophores are considered. A model of the soldier crab habitat is provided for prediction of the presence/absence of soldier crab populations in any particular coastal zone. The results of a 30 year study of the population dynamics of M. occidentalis in King Bay, Dampier Archipelago are presented in which the juvenile recruitment patterns, intra and interannual abundance and size classes of the population and gender composition and size classes of swarms and subsurface population components are determined. Periods of ovigery of M. occidentalis females are compared with those of other mictyrid species. The lifecyle of M. occidentalis is determined to consist of a cryptic infaunal phase for most of the crab’s life followed by an emergent adult stage. Swarms comprise predominantly adult males with most females and all juveniles remaining in the subsurface during a swarm event. The extended period of swarming adults on a tidal flat is explained by the extended period of juvenile recruitment resulting in a continuous series of cohorts reaching maturity and commencing the emergent phase. Environmental management recommendations to conserve populations of M. occidentalis are provided based on a synthesis of the findings of this ecological study