Free-running activity rhythm in the natural environment

Abstract

RHYTHMICAL activities of plants and animals in their natural habitats have, so far, always been found to be synchronised with cyclic environmental changes^1–3. In the absence of appropriate environmental timing cues (usually light–dark or tidal cycles) the rhythms free-run with endogenously controlled periods other than those of the environmental cycle. These may be longer or shorter than those observed in the presence of the entraining environmental cycles. We describe here an apparently unique example of an invertebrate which seems to free-run in its natural habitat. The animal is a predatory prostigmatid mite (Bdella interrupta Evans) which inhabits marine salt marshes and is exposed to regular tidal inundation during periods of spring tides. During periods of neap tides the salt marsh is not covered by the sea. The mites showed day and night peaks of locomotory activity on the soil surface at creek edges (Fig. 1a) and, also, in isolated experimental conditions (Fig. 1b). The narrow peaks of activity imply that the individual members of the population are closely synchronised with one another.