D. ROBERTS / SCIENCE PHOTO LIBRARY D. ROBERTS / SCIENCE PHOTO LIBRARY
Horseshoe crab. Coloured X-ray of the carapace of a horseshoe crab (Limulus polyphemus). Despite its name this animal is not closely related to true crabs, instead being closer to the spiders. It is a bottom-dwelling marine animal that inhabits the cool waters of the western Atlantic Ocean. It has remained essentially unchanged for the last 500 million years, and is considered to be a "living fossil". It is known for its mass spawning, in which thousands of horseshoe crabs mount the beaches of north-eastern North America to spawn and lay their eggs.
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