Ingo Arndt / Rear view of Horseshoe crab (Limulus polyphemus) with trail in sand, Delaware Bay, Delaware, USA, May

Doug Wechsler / Horseshoe crab (Limulus polyphemus) embryos, Delaware Bay, New Jersey, USA, June.

Sean Crane / Horseshoe Crab (Limulus polyphemus) group spawning at high tide at sunset, Cape May, New Jersey

Sean Crane / Horseshoe Crab (Limulus polyphemus) group spawning at high tide at sunset, Cape May, New Jersey

Steve Nicholls / Horseshoe crabs (Limulus polyphemus) spawning at low tide, Slaughter Beach, Delaware, USA, May

Fred Bavendam / Horseshoe Crab (Limulus polyphemus) group crawling ashore on the high tides of spring to lay their eggs in Delaware Bay's sandy beaches, New Jersey

Fred Bavendam / Horseshoe Crab (Limulus polyphemus) group crawling ashore on the high tides of spring to lay their eggs in Delaware Bay's sandy beaches, New Jersey

Fred Bavendam / Horseshoe Crab (Limulus polyphemus) eggs almost ready to hatch after two weeks incubation in the warm sands of Cape Anne, Massachusetts

Piotr Naskrecki / Horseshoe Crab (Limulus polyphemus) gathering on beach to lay eggs, seen under UV light, Delaware National Estuarine Research Reserve, Delaware

Alex Mustard / Toddler watching a pair of Horseshoe crabs (Limulus polyphemus) spawning in shallow water. Larger female and smaller male. Indian River, North Atlantic Ocean. Titusville, Merrit Island, Florida, USA....

Alex Mustard / Horseshoe crab (Limulus polyphemus) pair, larger female and smaller male, spawning in shallow water. Indian River, North Atlantic Ocean. Titusville, Merrit Island, Florida, USA. February.

Doug Wechsler / Horseshoe crabs (Limulus polyphemus) on beach mating, with Ruddy turnstone (Arenaria interpres) and Red knot (Calidris canuta) on beach, Delaware Bay, New Jersey, May.