Cacerolitas

Reference: Biol. Bull. 215: 34 – 45. (August 2008) © 2008 Marine Biological Laboratory

Rhythms of Locomotion Expressed by Limulus polyphemus, the American Horseshoe Crab: I. Synchronization by Artificial Tides
CHRISTOPHER C. CHABOT1*, STEPHEN J. SKINNER1, AND WINSOR H. WATSON III2
1

Department of Biological Sciences, Plymouth State University, Plymouth, New Hampshire 03264; and 2 ZoologyDepartment, University of New Hampshire, Durham, New Hampshire 03824

Abstract. Limulus polyphemus, the American horseshoe crab, has an endogenous clock that drives circatidal rhythms of locomotor activity. In this study, we examined the ability of artificial tides to entrain the locomotor rhythms of Limulus in the laboratory. In experiments one and two, the activity of 16 individuals of L.polyphemus was monitored with activity boxes and “running wheels.” When the crabs were exposed to artificial tides created by changes in water depth, circatidal rhythms were observed in animals exposed to 12.4-h “tidal” cycles of either water depth changes (8 of 8 animals) or inundation (7 of 8 animals). In experiment three, an additional 8 animals were exposed to water depth changes under cyclicconditions of light and dark and then monitored for 10 days with no imposed artificial tides. Most animals (5) clearly synchronized their activity to the imposed artificial tidal cycles, and 3 of these animals showed clear evidence of entrainment after the artificial tides were terminated. Overall, these results demonstrate that the endogenous tidal clock that influences locomotion in Limulus can beentrained by imposed artificial tides. In the laboratory, these tidal cues override the influence of light/dark cycles. In their natural habitat, where both tidal and photoperiod inputs are typically always present, their activity rhythms are likely to be much more complex. Introduction Adult American horseshoe crabs, Limulus polyphemus, migrate into the intertidal zone along the eastern coast of NorthAmerica in the late spring-early summer to attempt to
Received 13 September 2007; accepted 8 February 2008. * To whom correspondence should be addressed. E-mail: chrisc@ plymouth.edu 34

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) as well as by rising water levels (Ehlinger et al., 2003). During this time, breeding activity is synchronized to high tides, with horseshoe crabs moving into mating areas 1–2 h before high tide and returning to deeper waters about 2 h 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 thehighest 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, andin general, they are speciesspecific (DeCoursey, 1983; Naylor and Williams, 1984). Inundation cycles (Williams and Naylor, 1969), hydrostatic pressure changes (Naylor and Atkinson, 1972), as well as 12.4-h cycles of increased or decreased temperatures and salinities (Reid and Naylor, 1990) are sufficient to entrain the locomotor rhythms of Carcinus maenas, the green crab. Hydrostatic pressure changesalso synchronize the behavioral activity rhythms of the portunid crabs Liocarcinus holsatus and Liocarcinus depurator (Abello et al., 1991), as ´ well as at least two amphipod species—Nymphon gracile (Morgan et al., 1964) and Excirolana chiltoni (Enright, 1965). Salinity changes appear to be effective synchronizing agents for the crabs Rhithropanopeus harrissii (Forward et al., 1986) and C....