Horseshoe Crab - Anatomy and Behaviour

Anatomy and Behaviour

The entire body of the horseshoe crab is protected by a hard shell. They have two primary compound eyes and seven secondary simple eyes, two of which are on the underside. Beneath the carapace they look quite similar to a large spider. They have five pairs of legs for walking, swimming and moving food into the mouth. The long, straight, rigid tails can be used to flip themselves over if they are turned upside down, so a horseshoe crab with a broken tail is more susceptible to desiccation or predation.

Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming. Horseshoe crabs normally swim upside down, inclined at about 30° to the horizontal and moving at about 10–15 centimetres per second (0.22–0.34 mph). Despite this, they usually are found on the ocean floor searching for worms and molluscs, which are their main food. They may also feed on crustaceans and even small fish.

Females are larger than males; C. rotundicauda is the size of a human hand, while L. polyphemus can be up to 60 centimetres (24 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size.