They crowd up on the beach at high tide. The female scoops out a hole in the sand with the front part of her body and lays thousands of eggs and the male fertilizes them. After they spawn, they go back to the deep water. The eggs look like a very small green sphere. The embryp looks like the adult, but it has its tail folded beside it.
Birds fly up from South America, going north to nest.
They stop by the Delaware Bay to feast on the horseshoe crab eggs. They eat enough to triple their weight before they continue their journey. For many years the farmers used the horseshoe crabs for fertilizer. Fishermen used them for eel bait. A scientist at the University of Delaware found that fluid taken from the horseshoe crab will make blood coagulate.