Horseshoe crabs are known for their large nesting aggregations, or groups, on beaches particularly in mid-Atlantic states such as Delaware, New Jersey and Maryland in the spring and summer.
Horseshoe crabs can nest year-round in Florida, with peak spawning occurring in the spring and fall.
When mating, male horseshoe crabs move parallel to the shoreline on sandy flats and intercept females as they pass by.
A male attaches himself to the top of a female’s shell by using his specialized front claws, in a position known as amplexus, and together they crawl to the beach.
The male fertilizes the eggs as the female lays them in a nest in the sand.
Some males (called satellite males) do not attach to females but still have success to fertilizing the female's eggs as they swarm around the amplexed pair.
Most of this nesting activity takes place during high tides in the three days before and after a new or full moon.
Florida Fish and Wildlife Conservation Commission ~ http://myfwc.com/research/saltwater/crustaceans/horseshoe-crabs/facts/

Fortescue Beach, NJ