Horseshoe crabs are long-lived and slow to mature in comparison to most other invertebrate groups. The adult size of Limulus polyphemus shows a distinct latitudinal gradient, with larger animals found toward the center of the range, and smaller animals found at the extremes of the range, north of Cape Cod, along the Florida coast, and in the Gulf of Mexico. Adult females in the Indian River Lagoon have an average prosomal width of 189 mm, while the average adult male has a prosomal width of 136 mm. Limulus polyphemus is generally dispersed sub-littorally, but spawns on sandy beaches. In spring, Limulus males, which often outnumber females many times over, patrol along the foot of the beach awaiting females.
Females move from deeper water directly to the beach where they nest.
Horseshoe crabs typically locate mates, achieve amplexus , then migrate to the high tide mark in the intertidal zone to deposit and fertilize eggs before returning to deeper water after the spawning season. When waters are calm, many males may cluster around individual females, and large spawning assemblages can occur. Under rough conditions, only one or two males are able to grasp onto a female while also avoiding being washed away. The reproductive cycle of horseshoe crabs has been found to be related to lunar activity in some areas. However, in St. Joseph Bay, FL spawning peaks occurred at the first and last quarter moons rather than around the new and full moons.