Hoseshoe Crabs

My husband and I recently moved from Missouri to North Carolina, which has recently been hit by hurricanes Dennis and Floyd. Between the two hurricanes, we took my parents (who were visiting from Texas) to the North Carolina coast to see the sights.

Part of our excursion was a visit to the beaches near Nags Head and Currituck. As we walked along the beaches, we looked at the debris that had been washed up by Hurricane Dennis. In addition to the usual shells, we also found egg sacks of skates (see my previous article "The Fish, Part II" for more information about cartilaginous fish), and the remains of horseshoe crabs.

My husband and I were very excited to find the shells of several horseshoe crabs, as we had never seen them in the wild before, only in aquariums. We would have preferred to see them alive, rather than after they had been fed on by the many sea gulls and other shore birds.

Horseshoe crabs get their name from the fact that, if you look at them from above, their general shape resembles the print that a horse's hoof would leave. Despite their name, they are not actually related to the true crabs, which are Crustaceans.

Horseshoe crabs are invertebrates, they are members of the Arthropod phylum. The Arthropods also include the insects (Class Insecta) (see my article "The Insects", the spiders and mites (Class Arachnida) (see my article "Spiders: Our Eight Legged Friends" for more information about spiders), as well as aquatic animals with hard shells, such as lobsters, crayfish, shrimp, and crabs (the Crustaceans). (See my article "Biological Nomenclature" for more information about how biologists divide animals into groups.)

Horseshoe crabs are marine (they live in salt water), and breathe using gills. Their bodies are divided into two main parts, the cephalothorax ("head thorax") and abdomen, which has a long telson or tail. They have two compound eyes (the same sort of eyes that humans and other vertebrate animals have), and they also have two ocelli, or primitive eyes. These ocelli allow them to sense differences in levels of light, and general shapes, but they do not see clearly with them.

Horseshoe crabs have five pairs of legs that they can use for walking on substrates, or for swimming. However, they mostly use these legs for burrowing near the sandy surface in search of prey such as worms and mollusks. In addition to the walking legs, horseshoe crabs also have a small pair of legs near their mouths called chelicerae (pronounced "kil lis er ree"). These chelicerae are small legs that have been modified for biting or piercing prey, and for maneuvering the food items to the crab's mouth. They are similar to the chelicerae that spiders have.

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