ORIENTATION OF THE HORSESHOE CRAB,<i>LIMULUS POLYPHEMUS</i>, ON A SANDY BEACH

Abstract

Adult horseshoe crabs (Limulus polyphemus) spawn on sandy intertidal beaches and then return toward the water. Field experiments demonstrated that beach slope was more significant than vision in this orientation behavior. Both blinded and normally sighted crabs showed rapid seaward orientation on beaches with a seaward slope of approximately 6°. Orientation performance was poor on a flat beach, although sighted crabs slightly out-performed blinded crabs. The observed orientation behavior was correlated with the large numbers of horseshoe crabs which failed to return to the water after spawning on sand bars or similar habitats lacking a slope gradient.