Orientation of Limulus polyphemus in the Vicinity of Breeding Beaches

Abstract

1) Horseshoe crabs, Limulus polyphemus, emerge in mating pairs on sandy beaches to lay eggs then return offshore. Rapid offshore locomotion is exhibited during escape responses. A variety of potentially suitable orientational guideposts exist in the near‐shore environment, including visual cues, bottom slope, wave surge and currents to direct such movements.

2) A series of experiments was performed in which horseshoe crabs were tagged with styrofoam floats and released in the vicinity of known breeding beaches under varying conditions of surge, current, bottom slope and turbidity. The direction of movement (interpreted as an escape movement) was recorded at one minute intervals for ten minutes and the last five bearings of each crab were averaged and plotted.

3) The direction of movement correlated most closely with the direction of wave surge even in the presence of bottom slope and currents. Horseshoe crabs distributed randomly in the absence of wave surge although they typically maintained relatively straight paths.