Local tidal regime dictates plasticity of expression of locomotor activity rhythms of American horseshoe crabs, Limulus polyphemus
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While horseshoe crabs Limulus polyphemus from regions with two daily tides express endogenous circatidal (~12.4 h) activity rhythms, much less is known about locomotor rhythm expression in horseshoe crabs from other tidal regimes. This study investigated whether horseshoe crabs (1) always express activity rhythms consistent with their natural tides and (2) can alter activity rhythm expression in response to novel tide cycles. Activity rhythms of L. polyphemus from environments with two daily tides (Gulf of Maine, 43°6′N/70°52′W, and Massachusetts, 41°32′N/70°40′W), one dominant daily tide (Apalachee Bay, Florida, 29°58′N/84°20′W), and microtides (Indian River Lagoon, Florida, 28°5′N/80°35′W) were recorded in 2011–2013 during three artificial tide conditions: no tides, a 12.4-h tidal cycle, and a 24.8-h tidal cycle. Interestingly, L. polyphemus from the microtidal site (n = 7) appeared “plastic” in their responses; they were able to express both bimodal and unimodal rhythms in response to different tide cycles. In contrast, the other two populations exhibited more fixed responses: regardless of the tides, they were exposed to, horseshoe crabs from areas with one dominant daily tide (n = 18) consistently expressed unimodal rhythms, while those from areas with two daily tides (n = 28) generally expressed bimodal rhythms. Rhythms expressed by L. polyphemus thus appear to be a function of endogenous clocks, the tidal cues to which individuals are exposed, and tidal cues that individuals experience throughout ontogeny.
KeywordsAmerican horseshoe crabs Limulus polyphemus Circadian Circalunidian Circatidal Activity
Indian River Lagoon
Gulf of Maine
This project could not have been completed without the assistance of Steven Simpson, Katherine Fondo, Alexandria Santry, Kyle Kenyon, Tyler Remillard, Jonathan Reider, and Albert Lamonda, who helped with animal care. Special thanks to Stephanie Kronstadt for collection of IRL animals, to Christopher Freeman for construction of the experimental apparatus, to Meghan Owings for feedback on earlier drafts, and to the reviewers, who provided constructive insight and suggestions. Financial support for this project was provided by NSF (IOS 0920342 to CCC and WHW III), Plymouth State University College of Graduate Studies, and the New Hampshire IDeA Network of Biological Research Excellence with Grants from the National Center for Research Resources (5P20RR030360-03) and the National Institute of General Medical Sciences (8P20GM103506-03), National Institutes of Health.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflicts of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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