The horseshoe crab lives in shallow, coastal waters, usually partially covered by mud or sand. It covers itself like this by driving the front of its round dorsal plate forward and downward into the earth. This crab is a sturdy creature, tolerating wide swings in salinity and temperature. As a scavenger, it spends much of its life feeding on all types of marine animals, including small fish, crustaceans, and worms. Interestingly, it swims through the water with its dorsal plate facing the bottom (on its back) by flapping its tail spine into its abdomen.
Horseshoe crabs mature sexually when they are between nine and 12 years old. Typically, when they breed, horseshoe crabs congregate in large numbers in shallow coastal waters. At such times, the male climbs onto the female's back, holding the sides of her dorsal plate. (The male is significantly smaller than the female.) She carries him around, sometimes for days, until spawning takes place. When ready to lay her eggs, she digs holes about 5.9 in (15 cm) deep in a tidal area and lays up to 1,000 eggs in each hole. While she lays these eggs, the male fertilizes them. In approximately six weeks, the eggs hatch into free-swimming larvae that look a lot like their parents, but their tail spines are missing. Because of the inflexibility of their dorsal plates, it is difficult for these animals to grow within their shells; thus they molt several times before their growth stops at sexual maturity.