Activity rhythms expressed by juvenile American horseshoe crabs, Limulus polyphemus

The goals of this study were to determine if juvenile horseshoe crabs, Limulus polyphemus, express daily or tidal patterns of activity and how light and tidal cycles influence these patterns. When exposed to a light:dark cycle (n=24), 63% of juveniles exhibit daily patterns of locomotion and 25% of juveniles express circatidal patterns. When subsequently exposed to constant darkness, 17% express circadian rhythms of activity, 25% express a combination of circadian and circatidal patterns, and 46% express circatidal patterns of activity. When exposed to tidal cycles (n=42), 55% of juveniles express tidal patterns of activity, while the remainder exhibit either a daily pattern (17%) or no pattern (28%) of activity. Of those synchronized to the tidal cycle, 43% show entrainment by expressing circatidal activity when subsequently held at constant water depth. Overall, these results demonstrate that juvenile horseshoe crabs possess endogenous clocks influencing circatidal and circadian patterns of locomotion