Unpaired or satellite male horseshoe crabs, Limulus polyphemus, are attracted to and often form a group
around a pair (a female with an attached male) that is nesting in the high intertidal zone. These males are
engaged in sperm competition. We observed nesting pairs and their associated satellites in the wild,
collected and reared their eggs and used genetic markers to examine paternity. We found that the
unpaired, satellite males are highly successful at fertilizing eggs; two satellites can leave the attached male
with few fertilizations. Two satellites together are each as successful as one spawning with a pair. A
satellite’s location around the female greatly affects his success, and males compete for access to a
position over the dorsal canal between the prosoma and opisthosoma of the female and under the front
margin of the paired male where they are most likely to fertilize eggs. Although eggs and sperm retain
their viability for some time after spawning, nearly all eggs are fertilized by the satellites that are around
the nesting pair at the time of egg laying and by the attached male. A number of factors including beach
current, female size and male behaviour affect the outcome of sperm competition in this externally
fertilizing species.
Unpaired or satellite male horseshoe crabs, Limulus polyphemus, are attracted to and often form a group
around a pair (a female with an attached male) that is nesting in the high intertidal zone. These males are
engaged in sperm competition. We observed nesting pairs and their associated satellites in the wild,
collected and reared their eggs and used genetic markers to examine paternity. We found that the
unpaired, satellite males are highly successful at fertilizing eggs; two satellites can leave the attached male
with few fertilizations. Two satellites together are each as successful as one spawning with a pair. A
satellite’s location around the female greatly affects his success, and males compete for access to a
position over the dorsal canal between the prosoma and opisthosoma of the female and under the front
margin of the paired male where they are most likely to fertilize eggs. Although eggs and sperm retain
their viability for some time after spawning, nearly all eggs are fertilized by the satellites that are around
the nesting pair at the time of egg laying and by the attached male. A number of factors including beach
current, female size and male behaviour affect the outcome of sperm competition in this externally
fertilizing species.
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