Horseshoe Crabs: True Blue Bloods

Some people are described as “blue bloods” when they come from a royal birth line. There are true “blue bloods”, but they’re not human-they’re animals. One of these animals is called a Horseshoe crab, also known as the King Crab or Horsefoot crab. The largest population of horseshoe crabs is found in the Delaware Bay area in the United States.

The horseshoe crab isn’t really a crab at all-it’s related to the arachnid (spider) family because it has pincer-like appendages, five pairs of walking legs, “book” gills, no jaw, and no antennae. The name horseshoe crab came about because of their shape- the shell of this animal resembles a horse’s hoof. The shell is hard and durable, with a tail that is thin, hard, and long. This tail is used mostly to right the horseshoe crab if it tips over. The tail also has a photoreceptor at the end of it. Scientists think that this cell keeps the animal’s biological clock on schedule. Females are larger, males are about two-thirds smaller. When a male wishes to mate with a female, he’ll hold onto her tail with his front pincers. (Only males have these-that’s one way to tell male from female.) She walks along the beach during a high tide, pausing every few feet to lay eggs in the sand. He fertilizes the eggs as he passes over them. The tide comes in and buries the eggs. Though it’s dark, the horseshoe crab can see a mate with a special set of eyes.

The main set of eyes on a horseshoe crab is located on the top of the shell. These are compound eyes. Each eye contains many parts. The optic nerve for these eyes is located right below the shell and can be four inches long. Scientists have studied the horseshoe crab’s compound eyes to understand vision and how people see contrast. Dr. H. Keffer Hartline and his colleagues recorded electrical impulses from the horseshoe crab’s optic nerve and uncovered the basic principles of how vision works. They won a Nobel Prize for their research.

Medical research owes much to horseshoe crabs. This awkward-looking creature has saved many lives. This is where that “blue blood” comes in. When a horseshoe crab’s blood is exposed to air, it turns blue. Its blood is copper-based, and when a crab gets wounded, its blood forms a clot. This can kill certain types of bacteria that can be harmful. In the early 1950’s, a scientist named Frederick Bang experimented with horseshoe crab blood. He isolated the chemical that caused the clots to form and named it LAL (Limulus Amoebocyte Lysate). LAL detects endotoxin, a type of bacteria. This test is used to diagnose urinary tract infections and spinal meningitis in humans, check for spoilage in milk, fish, and ground beef, check air and water quality, and test the purity of medicines. Currently, cancer researchers have been experimenting with LAL as a possible test for cancer.

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