Horseshoe crabs date back to the Ordovician Period, and haven’t really changed in the intervening 450 million years; these ancient origins have earned them living fossil status. With their hard, dome-shaped shells, horseshoe crabs look like fossils; they molt this armor about 18 times over their life to keep growing. Under the shell, they have five pairs of clawed legs that converge at a central mouth. Instead of teeth, their mouths sport stiff bristles for eating mussels and worms.
These four species make up the order Xiphosurida, meaning “sword tail.” Despite this ominous name, their pointed tails (or teslons) aren’t weapons; they use them as rudders or to right themselves if overturned. Horseshoe crabs aren’t at all dangerous to humans; many people fear them simply because they look like giant bugs.
In the spring, horseshoe crabs breed on beaches during full tide, on nights with a new or full moon – thousands flock to Delaware Bay every May and June. Once on shore, a male clings onto a female with his front claws. The female pulls the male along the sand, digging holes to lay her eggs in; the male fertilizes as he’s towed along.
Horseshoe crabs have a total of ten eyes, used mainly to aid reproduction. The front two compound eyes let them see about three feet –good enough to locate a nearby mate. Five simple eyes on the back help determine when the moon’s phase signals breeding; there’s an additional eye of the tail used in the same way. Lastly, a final pair of front-facing eyes don’t seem to do anything at all.
Though horseshoe crab eyes are primitive, it was through studying them that scientists came to understand the human eye. Though this research was hugely important, it’s these critters’ blood that really sets them apart. While human blood uses iron to bind oxygen, horseshoe crab blood uses copper, making it blue instead of red. Their blood also contains amebocytes – primitive, large blood cells. These amebocytes are the only known way to produce Limulus amebocyte lysate – or LAL – a compound commonly used to test sterility of pharmaceuticals, prosthetics, and medical devices.
Their copper-based blood, moon-determined mating, and back-mounted eyes make horseshoe crabs pretty bizarre. Then again, these creatures have been around several hundred million years longer than humans, so who’s to say we aren’t the odd ones? I guess the horseshoe crab only proves the point: strangeness is in the eye(s) of the beholder.
