Horseshoe crabs are arthropods that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will, however, occasionally come on to the shore, e.g. for mating. They are commonly used as bait and in fertilizer, and in recent years there has been a decline in number of individuals, as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America.
Horseshoe Crabs, are not true crabs at all and of a much older order, more closely related to Trilobites and Sea Scorpions, of the Merostomata. Around for more than 400 Million Years, Horseshoe Crabs, in many forms, have outlasted many terrible extinctions including the Permian Mass Extinction and K-T Extinction. This survivability has been attributed to their relentlessly consistent reproductive cycle. In modern times, humans have threatened their huge success story.
Horseshoe Crabs were featured in the third episode, Cruel Sea, when at shore at night, some female Horseshoe Crabs lay their eggs. In the morning, those that are still on shore and their eggs become a banquet for Eustreptospondylus and Rhamphorynchus.