Horseshoe crabs are long-lived and mature later than other invertebrates. Males mature between 9 and 11 years of age and females, between 10 and 12 years. Horseshoe crabs spawn during the spring and summer. Spawning occurs at high tide on low-energy beaches of estuaries, bays, and coves. One species (Carcinoscorpius rotundicanda) moves upstream into rivers to spawn.
During mating, the male grasps the edge of the female's opisthosoma. The female uses her legs and prosoma to dig a nest, into which she deposits a cluster of eggs. The eggs are fertilized by the male, and the pair moves 4-8 in (10-20 cm) farther in the sand and repeats the process. As the female digs the second nest, the excavated sand is pushed backward to cover the previous nest. Individual horseshoe crabs are capable of spawning more than once per season. The eggs hatch into trilobite larvae; after molting into juveniles, horseshoe crabs settle to the seafloor.
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