DENGER, B.C.*; TANKERSLEY, R.A.; Florida Institute of Technology, Melbourne; Florida Institute of Technology, Melbourne: Rhythmic activity of adult fiddler crabs Uca pugilator from areas with different tidal regimes

Intertidal species, such as the fiddler crab Uca pugilator, have long served as models for studying endogenous tidal rhythms in marine organisms. Previous studies indicate that U. pugilator from areas with semi-diurnal tides exhibit a circatidal activity rhythm in which the active phase occurs at low tide. However, few studies have compared the activity patterns of crabs from areas with different tidal patterns. To test the hypothesis that crabs possess circatidal rhythms that match their local tidal regime, the locomotory behavior of male U. pugilator from non-tidal areas and from beaches with semi-diurnal, diurnal, and mixed tides were analyzed under constant conditions. Crab activity was monitored for one week using an infrared actograph system and the resulting time series was analyzed using periodogram analysis. While all four groups of crabs exhibited endogenous rhythms in activity, there were significant differences in the dominant periodicities present in the time series. Crabs from semi-diurnal areas possessed rhythms with a dominant periodicity of 12.2 h, whereas those from diurnal areas exhibited rhythms with peaks in activity at 23.3 h. Crabs from areas with mixed tides possessed rhythms with dominant periodicities of 12.5 h and 24.4 h, corresponding to the main semi-diurnal (M2 and S2) and diurnal (K1 and O1) components of the tidal regime at the collection site. Actographs of crabs collected from non-tidal areas also contained two significant peaks in activity at 11.9 h and 23.3 h. Peaks in activity for crabs from tidal areas were synchronized with times of expected falling tide. Given the proximity of the collection sites, differences in the activity patterns of the four populations suggest that the circatidal behaviors of U. pugilator are phenotypically plastic.