Horseshoe Crabs: Merostomata

Every spring and summer adults migrate (MY-grayt), or move, from deeper waters onto nearby beaches to mate and lay eggs, or spawn. Spawning occurs at high tide on the beaches of estuaries, bays, and coves. One species swims upstream into rivers to spawn.

A male grasps the edges of the female's shell with his clawed legs, while the female digs a hollow in the sand with her legs and the front section of her body. As the female lays the eggs into this hollow, the male fertilizes (FUR-teh-lye-sez) them. The pair moves to a nearby site and begins the process all over again, using the sand dug from the new site to cover the previously laid eggs. Horseshoe crabs may spawn more than once a year, laying up to several thousand eggs after each mating.

The eggs hatch into larvae. The larvae swim constantly but soon settle to the seafloor after their first molt. They rest by burying themselves in shallow burrows in the mud and silt. Although they usually crawl on the bottom, horseshoe crabs also swim upside down using their book gills to propel themselves through the water.

Horseshoe crabs molt six times in their first year and sixteen to seventeen times during their entire lives. Males reach adulthood between nine and eleven years of age. Females become adults when they are ten to twelve years old.