"Horseshoe crabs have a strong tidal rhythm. They know when high tide is coming, and they move to the edge of the water. Watson tested this several years ago with a colleague, by building a version of a hamster wheel .... They then placed it inside the buckets and found it would run every 12.4 hours, about the same cycle as the tides."

http://www.popularmechanics.com/science/health/a26038/the-blood-of-the-crab/