Anatomy and Physiology
Horseshoe crabs have three main parts to the body: the head region, known as the 'prosoma', the abdominal region or 'opisthosoma', and the spine-like tail or 'telson'. The smooth shell or carapace is shaped like a horseshoe, and is greenish grey to dark brown in colour. The sexes are similar in appearance, but females are typically 25 to 30 percent larger than the male and can grow up to 60 centimetres (24 in) in length (including tail).
Horseshoe crabs possess the rare ability to regrow lost limbs, in a manner similar to sea stars.
A wide range of marine species become attached to the carapace, including algae, flat worms, mollusks, barnacles, and bryozoans, and horseshoe crabs have been described as 'living museums' due to the number of organisms that they can support. In areas where Limulus is common, the shells, exoskeletons or exuviae (molted shells) of horseshoe crabs frequently wash up on beaches, either as whole shells, or as disarticulated pieces.
The brain and the heart are located in the prosoma. On the underside of the prosoma there are six pairs of appendages, the first of which (the small pincers or chelicerae) are used to pass food into the mouth, which is located in the middle of the underside of the cephalothorax, between the chilicerae. Although most arthropods have mandibles, the horseshoe crab is jawless.
The second pair of appendages, the pedipalps, are used as walking legs; in males they are tipped with 'claspers' which are used during mating to hold onto the female's carapace. The remaining four pairs of appendages are the 'pusher legs', also used in locomotion. The first four pairs of legs have claws, the last pair has a leaflike structure used for pushing.
The opisthosoma bears a further six pairs of appendages; the first pair houses the genital pores, while the remaining five pairs are modified into flattened plates, known as book gills, that allow them to breathe underwater, and can also allow them to breathe on land for short periods of time, provided the gills remain moist.
The telson (i.e., tail or caudal spine) is used to steer in the water and also to flip itself over if stuck upside down.
Among other senses, they have a small chemoreceptor organ which senses smells on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.
Read more about this topic: Atlantic Horseshoe Crab
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