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Abstract
Juvenile American horseshoe crabs, Limulus polyphemus, express both daily and tidal rhythms. To determine if, and how, tidal cues influence the expression of these rhythms, we exposed 25 animals to artificial tides, and 17 to artificial tides with inundation, both with a 12:12 LD cycle. In the first experiment, 24% expressed daily rhythms of activity, 24% tidal rhythms, 12% a combination of the two, and the rest were arrhythmic. Under subsequent atidal conditions some expressed daily rhythms, but more were circatidal. In the second experiment, 6% expressed daily rhythms, 71% tidal, 12% a combination, and 12% were arrhythmic. Those expressing tidal rhythms were more active during flood/high tide, while daily animals tended to be nocturnal. Under subsequent constant conditions, the majority exhibited circatidal activity, with some expressing one activity bout per day. We conclude that juvenile horseshoe crabs entrain to artificial tides, with inundation cycles providing stronger cues than water depth changes.
Acknowledgements
We are very grateful to Dr Daniel Gibson (Worcester Polytechnic Institute) for helping us find, and collect, juvenile horseshoe crabs on Cape Cod, MA. He also provided many excellent suggestions that helped our study. We also thank Jason Goldstein, Tom Langley, and Tracy Pugh for many helpful discussions and assistance with the experimental design and data analyses. Finally, we are grateful for the help of many undergraduates with many aspects of the project, including Haley White, Suzanne Johnson, Suzanne LaChance, Alysia Campbell, Michael Lemmen, Mallory Chaput and Kyle Jenks. Finally, thank you to the three anonymous referees of the manuscript.