Satellite male groups in horseshoe crabs, Limulus polyphemus

Horseshoe crabs arrive on the beach in pairs and spawn in the high intertidal during the springtime, new and full moon high rides. Unattached males also come to the beach, crowd around the nesting couples and compete with attached males for fertilizations. Satellite males form large groups around some couples while ignoring others, resulting in a nonrandom distribution that cannot be explained by local environmental conditions or habitat selection. In experimental manipulations, pairs that had satellites regained them after they had been removed whereas pairs with no satellites continued nesting alone, which means that satellites were not simply accumulating around the pairs that had been on the beach the longest. Manipulations also revealed that satellites were not just copying the behaviour of other males. Based on the evidence from observations and experiments, the most likely explanation for the nonrandom distribution of satellite males among nearing pairs is that unattached males are preferentially attracted to some females over others. Females with many satellites were larger and in better condition, but did not lay more eggs, than females with few or no satellites.