Phase-Responsiveness of the Circatidal Locomotor Activity Rhythm of Hemigrapsus Edwardsi (Hilgendorf) to Simulated High Tide

Abstract

Phase-responsiveness to single pulses of simulated high tide has been tested for in the locomotor activity rhythm of Hemigrapsus edwardsi, a grapsid crab from a locality of equal semi-diurnal tides. The crab exhibits endogenous circatidal locomotor rhythmicity with only a weak circadian component. Exposure to a single 3 h ‘pulse’ of simulated high tide (immersion and low temperature, with or without light) in crabs kept otherwise in constant conditions (moist air; dim red light; 15°C), resulted in slight phase delays in the free-running rhythm when the tidal ‘pulse’ was applied in the first 2–3 h of the ‘expected’ high tide period. Tidal pulses initiated at and just after the expected time of high tide induce a phase advance. The results support the hypothesis of a phase-responsive curve on a tidal (12·4 h) rather than 24 h time-base. However, the phase advances and delays are small when compared with circadian phase response curves. It is suggested that the circatidal rhythms depend mainly upon repeated exposure to tidal variables for entrainment and that each individual tide is able to adjust the behavioural rhythm only slightly from its endogenous pattern. In this way, behaviour of populations of coastal animals is less susceptible to environmental perturbations associated, for example, with severe storms.