The Western Australian soldier crab, Mictyris Occidentalis Unno 2008 (Brachyura: Decapoda:Mictyridae): The importance of behaviour in design of sampling methods

Document Type

Journal Article


Royal Society of Western Australia

Place of Publication

Perth, Australia


Faculty of Computing, Health and Science


School of Natural Sciences / Centre for Ecosystem Management




Unno, J. (2008). The Western Australian soldier crab, Mictyrisoccidentalis Unno 2008 (Brachyura: Decapoda: Mictyridae): The importance of behaviour in design of sampling methods. Journal of the Royal Society of Western Australia, 91, 243-263.

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Soldier crabs are challenging marine organisms to sample as they do not fit neatly into established faunal behavioural categories. Information on the soldier crab life cycle, ichnological products, and behaviour is a prerequisite to designing appropriate sampling, involving the consideration of the study objective (what to sample), temporal factors (when to sample), spatial factors (where to sample) and sampling method (how to sample). Largely infaunal for most of their life cycle, soldier crabs may be relatively sessile or quite vagile within the substrate, and epifaunal (emergent) for late stages of their life cycle. They also exhibit population partitioning, based on sex and size classes (when there are mixed cohorts); for example, it is mostly adult males that emerge in swarms, and mainly females and juveniles that remain in the subsurface. This behaviour has implications for studies of population dynamics that involve abundance, population structure, and sex ratios. To date, there has been emphasis on sampling while the crabs are emergent (forming so called “armies”, or surface swarms), with the assumption that the whole population emerges. When infaunal and mobile, soldier crabs also present challenges to sampling in that they can vary in abundance laterally and vertically in a short time, and they can respond to prolonged sampling, or to researcher-induced perturbations during sampling, by burrowing downwards to avoid being collected (resulting in potentially spurious data with respect to their depth of occurrence and abundance). The complexity in behaviour of soldier crabs, the partitioning of populations, and the response to sampling have not been addressed to date by researchers elsewhere whereas it is an important component in the design of sampling methods.

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