The horseshoe crab is not actually a crab, it is a marine arthropod more closely related to land spiders and scorpions. Slow moving, they are harmless creatures, their sharp tail is only used to right themselves in the surf.
Growing to a length of 2 feet or more they are normally found grubbing along sandy bottoms in shallow water for worms and crustaceans.
In the late spring during a flood tide, hundreds of horseshoe crabs will crawl onto a beach where the females will deposit a number of greenish eggs in a shallow depression excavated in the sand above the water line. After two weeks another flood tide will hopefully expose the nest and the moving sand particles will cut open the egg membrane allowing the larvae to escape into the sea.
Horseshoe crabs will often be host to a dozens of slipper shells clinging to the underside of their bodies.