Behavioural Rhythms in Littoral Prawns

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Abstract
Palaemon elegansRathke on South Wales coasts, unlikeP. serratus(Pennant), does not migrate offshore in winter but stays in intertidal pools throughout the year. Both species occur in rock pools in summer, withP. elegansextending farthest up the shore and often occurring in pools above M.H.W.N. Each shows a spontaneous but short-lived tidal rhythm of locomotor activity in constant laboratory conditions, the rhythm persisting longer in isolated prawns than in groups of animals recorded together. Peak activity occurred on the ‘expected’ ebb, and light/dark changes, including those associated with tidal rise and fall, appear to be important in synchronizing the rhythm. The tidal rhythm ofPalaemon elegansabove M.H.W.N. quickly rephases to a semi-diurnal dusk/dark rhythm during the period of neap tides, the rhythm again becoming tidal when spring tides return. The possible significance of ebb activity is discussed.