The eyes are used for locating mates. Horseshoe crabs feed mostly at night and burrow for worms and mollusks. They will, however, feed at any time. Horseshoe crabs grow by molting and emerge 25 percent larger with each molt. Horseshoe crab eggs are important food for migratory shore birds that pass over the Delaware Bay during the spring mating season.
Fish also eat the juveniles or recent molts.
In the 1900s, horseshoe crabs were dried for use as fertilizer and poultry food supplements before the advent of artificial fertilizers. The medical profession uses an extract from the horseshoe crab's blue, copper-based blood called lysate to test the purity of medicines. Certain properties of the shell have also been used to speed blood clotting and to make absorbable sutures.