Locomotory Activity Rhythms in Some LittoralOrchestia[Crustacea: Amphipoda]

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Abstract
The cosmopolitan genusOrchestiais divisible into two ecological groups: littoral and terrestrial, as denned by Hurley (1959). Only littoral species are known from Britain and three which occur in the Medway estuary have been studied.O. gammarella(Pallas) andO. mediterraneaA. Costa are marine or estuarine, the former being found above the recent high water mark under wrack or stones and the latter below this level under stones.O. cavimanaHeller is a freshwater species, extending into the oligohalinicum of estuaries (Den Hartog, 1963), which is found near the wrack line. During a study of the biology of these species possible mechanisms which control the tidal levels occupied on the shore were investigated, including the temporal characteristics of locomotory activity.