Limulus polyphemus has existed for over 300 million years. Horseshoe crabs evolved together with trilobites (see left hand side of the picture below). While trilobites disappeared at the end of the Paleozoic era, horseshoe crabs survived several mass extinctions including the K-T event that killed off the dinosaurs 65 million years ago and the most recent ice age! Amazingly, they have kept the same body plan for millions of years (right hand side of picture below). For these reasons horseshoe crabs are commonly referred to as living fossils.
Peabody Museum collection, New Haven CT
Horseshoe crabs are more related to spiders, ticks, and mites, than they are to crabs. There are only four living species of horseshoe crabs today. Limulus polyphemus resides on the eastern coastline of North and Central America. The three remaining species inhabit the the coastlines of Japan, India, and Indonesia.
Ecological importance of horseshoe crabs
Probably because the horseshoe crabs have been around for so long, they are very tightly woven into their environment. Aside from playing a key role in the survival of a number of migratory shorebird species that consume their eggs to fuel their flights north, horseshoe crabs are themselves environments. Scuds, ghost anemones, Asteriids, snail furs, blue mussels, barnacles, sea strawberries, seal lettuce, red beard sponges, eastern oyster, northern rock barnacles, skeleton shrimps, sand builder worms, bushy bugulas, hard tube worms, flat worms, oyster drill eggs and Agardh’s red seaweed all make the carapace of Limulus polyphemus their home!
Importance of horseshoe crabs to humans
Horseshoe crabs are harvested by fishermen and used as bait in the eel and conch fisheries along the Atlantic Coast. In Connecticut, fishermen are legally allowed to hand harvest horseshoe crabs during certain times of the year. In 2007, fisherman were allowed to collect horseshoe crabs from May 22 through July 7 with the exception of weekends.
Extensive research has been conducted on horseshoe crabs with respect to their eyes and vision. This research has resulted in important findings pertaining to the manufacture of surgical sutures and development of dressings for burn patients. One might argue that the most important finding of horseshoe crab research is related to their blood. In the 1950's Frederick Bang discovered special cells in horseshoe crab blood called ameobocytes. These cells attach to bacteria forming a viscous gel that prevents the bacteria from invading the horseshoe crab body. Bang realized these cells could be used as a fast a efficient way to test pharmaceutical drugs for the presence of bacteria. Biomedical companies now harvest blood from horseshoe crabs to produce Limulus Amebocyte Lysate (LAL). NASA is now testing the use of LAL in space to assist in the diagnosis of astronauts.
Telling Males and Females Apart
If you capture a horseshoe crab, it is fairly easy to determine its sex by flipping them over and looking at the first pair of appendages. Horseshoe crabs have a total of six pairs of appendages.
Females are larger than males and their first pair of appendages are pincers. Take a look at the pincers above thumb in the picture below!
Males are generally smaller than females and their first pair of appendages are quite different from the females. Instead of normal pincers, males have a special set of mating claws. These are commonly referred to as "boxing gloves". In the picture below, look at the tip of the claw above the thumb. These claspers allow the male to hold onto the female during mating.