The role of visual and chemical cues in the mating decisions of satellite male horseshoe crabs, Limulus polyphemus

Satellite male horseshoe crabs form spawning groups around some nesting pairs (female plus attached male) while leaving others to nest alone. Furthermore, females of pairs that attract satellites are larger and lay more eggs than females of pairs that do not attract satellites. We experimentally tested whether satellite males use visual cues to discriminate among large and small females and if satellites use chemical cues from eggs to distinguish among females. We conducted experiments in the field in the northern Gulf of Mexico at Seahorse Key, FL, U.S.A. in which we observed the behaviour of satellite males as they approached cement models of horseshoe crab pairs. Satellites were significantly more likely to approach and join model pairs with large females than with small females. However, there was no difference in the length of time satellites remained joined to the models. When chemical cues were examined, there was no difference in the number of males that approached or joined identical female models with and without eggs. However, once a satellite joined, it remained attached significantly longer to the model pair with eggs underneath. This study shows that satellite male horseshoe crabs use multiple cues, first visual to locate nesting pairs, then chemical to decide whether to remain with a nesting female. Our results show that males that are approaching from the water use visual cues from the amount of exposed carapace to distinguish the size of females nesting on land, even when those females are well buried in the sand.