Knowing when to stop: Rhythms of locomotor activity in the high-shore limpet, Cellana grata Gould

The high shore limpet, Cellana grata, forages whilst awash, moving upshore with the rising tide and retreating downshore on the ebbing tide to become inactive in refuges. Spraying inactive, emersed individuals with seawater at low tide invokes a locomotory response, with limpets moving up the shore. Controlled laboratory experiments under continuous white or red light (to simulate light or dark periods respectively) and continuous emersion, immersion or seawater spray showed that C. grata possesses a free-running endogenous rhythm of locomotor activity. This rhythm was maintained over 30 days in continuous seawater spray and white light. Maximum entropy spectral analysis (MESA) revealed two major components to this rhythm, at 7.2 h and 12.4 h. The 12.4 h component is of a circatidal nature and appears to initiate activity, allowing individuals to anticipate immersion by the incoming tide, although this clock can be over-ridden by strong wave splash or spraying vigorously with seawater. The 7.2 h period, however, was the most significant component and is suggested to act as a stopwatch enabling the limpet to assess the duration of each foraging excursion in order to prevent being stranded at the wrong height on the shore. The environmental stimulus for both components of the endogenous rhythm in C. grata appears to be the time of first exposure to wave wash from the incoming tide. C. grata, therefore, has behavioural rhythms entrained to initiate and also terminate activity, which play a role in the limpet maintaining a fixed vertical level on the shore when inactive.