Long-term circatidal rhythms of heart rate, gill ventilation, and locomotion in the American horseshoe crab, Limulus polyphemus Linnaeus, 1758 (Arthropoda: Chelicerata: Merostomata: Limulidae)

While several marine species exhibit biological rhythms of heart rate, gill ventilation, or locomotion, the relationship between these three measures in any species remains unexplored. The American horseshoe crab, Limulus polyphemus, expresses circalunidian locomotor rhythms and circadian eye sensitivity rhythms, but it is not clear if either heart and ventilation rates are controlled on a circadian, or circatidal basis or the nature of the relationship between these three measures. We aimed to determine the extent to which the heart and ventilation rates of L. polyphemus are coordinated with its endogenous rhythms of locomotion. We recorded rhythmic beating of the heart and movements of the gill plates in freely moving horseshoe crabs. Most individuals exhibited robust circatidal rhythms of locomotion, heart rate, and ventilation that were highly correlated with each other over three weeks of continuous recording. These results are the first showing long-term rhythms of all three measures in any marine species and suggest that heart rate and ventilation rhythms are coordinated in L. polyphemus, both with each other, and with locomotion, and thus are all modulated on a tidal basis.