Entrainment

Adult American horseshoe crabs, Limulus polyphemus, migrate into the intertidal zone along the Eastern coast of North America in the late spring-early summer in order to attempt to mate (Rudloe, 1979; Brockmann, 2003). Spawning appears to be triggered by both elevated temperatures and photoperiod (Cohen and Brockmann, 1983; Shuster and Botton, 1985; Barlow et al., 1986; Penn and Brockmann, 1994; Ehlinger et al., 2003). During this time, breeding activity is synchronized to high tides, with horseshoe crabs moving into mating areas an hour or two before high tide and returning to deeper waters about two hours after high tide (Barlow et al., 1986; Penn and Brockmann, 1994). This tidal pattern of activity appears to be further modulated by a general preference for the highest high tide (Barlow et al., 1986), which may explain why in some areas most mating occurs around the high tides associated with the new and full moons (Rudloe, 1980, Smith et al., 2002).

Many other animals also synchronize various behaviors with tidal cycles. In some cases, environmental factors that may serve to synchronize the animal's activity to the tidal cycle have been identified and, in general, they are species specific (DeCoursey, 1983; Naylor and Williams 1984). Inundation cycles (Williams and Naylor, 1969), hydrostatic pressure changes (Naylor and Atkinson, 1972), as well as 12.4 hr cycles of increased or decreased temperatures and salinities (Reid and Naylor 1990), are sufficient to entrain the locomotor rhythms of the shore crab. Hydrostatic pressure changes also synchronize the behavioral activity rhythms of the Portunid crab (Abelló et al., 1991), as well as at least two amphipod species (Nymphon gracile, Morgan et al., 1964; Excirolana chiltoni, Enright 1965). Salinity changes appear to be effective synchronizing agents for estuarine crabs (Forward et al., 1986; Reid and Naylor 1990), but not for green crabs (Palmer, 1995). Finally, periodic agitation is sufficient to entrain tidal rhythms in two species of isopods (Klapow, 1972; Hastings, 1981) and juvenile horseshoe crabs (Ehlinger et al., 2006). Our goal in the following studies was to investigate the role of the various tidal cues on the circatidal rhythms of activity in horseshoe crabs.