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

Juergen Freund / Horsehoe crab (Tachypleus gigas) swimming on its back with gills creating the propulsion, Palawan, Philippines. Did you know? Horseshoe crabs are more closely related to spiders than crabs.

Juergen Freund / Horsehoe crab (Tachypleus gigas) underside, Palawan, Philippines

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

Anup Shah / Crab eating / Long tailed macaque (Macaca fascicularis) feeding on Horseshoe crab, Sumatran Coast Indonesia

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