Horseshoe Crab
Limulus polyphemus

Horseshoe crabs are among the oldest animals that inhabit the earth. They have been in our oceans for more than 350 million years. (Compare with the dinosaurs that became extinct approximately 65 million years ago.) The horseshoe crab is not a crustacean (crabs, lobsters and crayfish) as its name may imply. Instead it is classified as a chelicerate along with arachnids (spiders mites and ticks). Horseshoe crabs spend most of their time along the bottom of the ocean from Maine to Mexico at depths up to 1,000 feet but can also be found in intertidal zones each spring when they come ashore to breed and to lay their eggs in the sand. They feed on shellfish and worms for the most part. Horseshoe crabs have no known enemies except for humans, who harvest them for use as fertilizers on northeastern farms or for use as bait in whelk and eel traps. Horseshoe crabs have been studied extensively in medical sciences for their applications in cancer research and retinal studies. An enzyme isolated from the crab's blood called limulus amoebocyte lysate (LAL) is used to detect bacterial contamination in pharmaceutical products and by NASA as well as to diagnose bacterial diseases such as spinal meningitis. The depletion of their populations has led to laws that protect the animal in many states along our northeast coast.

The horseshoe crab has a total of ten eyes consisting of two complex compound eyes (similar to fly or bee eyes) and eight simple eyes, some of which are microscopic. The horseshoe crab tail, or telson, is not a tool of defense but rather a means of flipping over when it gets turned by a wave. The blood of a horseshoe crab is blue due to an oxygen transporting protein called hemocyanin, which contains copper thus causing the blue color. In humans, the oxygen transporting protein is hemoglobin, which contains iron thus causing the red color.

Adult males can be distinguished from females by their "boxing glove" claspers on the first set of walking legs. These "boxing gloves" are used to attach to the female during mating. When a horseshoe crab gets older it outgrows its shell, which is an exoskeleton, the shell is released and a new larger shell replaces it. This process, called molting, occurs throughout the crab's life.

During mating, the male (which is smaller than the female) hangs on a female's back using the specialized "boxing glove" claspers. The female digs a hole in the sand at the intertidal zone and deposits the eggs, which are fertilized by the male as they are deposited. This typically occurs in late spring or early summer, usually on the full moon. The eggs hatch approximately 5 weeks after being deposited and the trilobite larvae remain in the sand for several weeks, eventually exiting the sand looking similar to adults but with a lighter, more translucent shell.

They reach adulthood at about 9 or 10 years, and can live for 15 - 20 years, reaching length of up to 24 inches and weighing as much as 10 pounds. The horseshoe crab is considered to be a "living fossil". Its ancestors roamed the Earth 350 million years ago and yet have changed very little since then.

For more information visit http://web.gc.cuny.edu/ashp/nml/artsci/crab.html

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