Compact aggregation and life-history strategy in a continental Antarctic mite

The continental Antarctic oribatid mite, Maudheimia wilsoni Dalenius, is often found in compact multi-instar aggregations, under stones. These aggregations may persist for several years. Relative abundances of the four juvenile instars of M. wilsoni in the aggregations suggest an increase in instar duration with progressive development from protonymph to tritonymph, with larval and protonymphal stages apparently being of similar duration. Development from egg to protonymph may occur in a single summer season but, thereafter, at most a single moult is achieved each summer. This suggests that all juvenile instars are capable of overwintering. Minimum generation time (egg-egg) must therefore be three years, but the life cycle of many individuals is probably considerably longer. Development of individuals is more closely synchronised within aggregations than in the wider population, with synchronisation being particularly apparent in the larval and protonymphal stages. Aggregation behaviour is hypothesised to improve moult synchronisation and accelerate moulting, potentially reducing the number of winters spent as a juvenile, and hence juvenile mortality. There is evidence that adults actively leave or join aggregations, possibly allowing males to locate emerging females, and females to find suitable oviposition sites. Repeated overwintering and aggregation are not unique to M. wilsoni and other Antarctic oribatid mites, but are unusual for the Oribatida in general. We suggest that these features are essential for the survival of Antarctic oribatid mites which are faced with a limited annual growing period and phylogenetically-constrained long generation times

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