Migration of Limulus for Mating: Relation to Lunar Phase, Tide Height, and Sunlight

In the spring, horseshoe crabs (Limulus polyphemus) along the eastern coast of North America migrate toward shore to build nests close to the water's edge. In 1984 the mating season near Woods Hole, Massachusetts, extended from 14 May to 7 July. Mating activity during this period fluctuated with the phase of the moon, the height of the tide, and diurnal changes in daylight. As the moon approached new and full phases, large numbers of animals migrated into the intertidal zone to mate and build nests. They appeared 1-2 h before high tide, and returned to deep water about 2 h after high tide. No mating activity occurred during low tides. The two daily high tides in this region are unequal in height. The inequality is greatest during new and full moons. At these times most animals migrated toward shore on the higher tide, which occurred in the late afternoon and throughout the night. As the moon passed through quadrature the tidal inequality diminished and reversed. Mating activity changed accordingly: shore migration diminished, becoming nearly equal on the two equally high tides, and in several days shifted to the higher high tide. The cue for shifting appears to be the first light of dawn and not the height of the tide itself. We suggest that the migration of Limulus involves several sensory modalities including vision.