Horseshoe Crab (Limulus polyphemus)
This “living fossil” is actually not a crab, but a member of the spider family, and is more closely related to them and to scorpions and ticks than it is to crabs. The Atlantic horseshoe crab dwells on sandy and muddy bottoms along the eastern coast of North America. Millions emerge from the sea at the spring high tide to lay their eggs in the mud. Several species of shorebird time their migrations to coincide with this event, so as to take advantage of the rich food source provided by untold billions of eggs. Animal interest groups and natural history societies sponsor field trips to observe this amazing spectacle.
A chemical in the blood of the horseshoe crab is used to test for any impurities in drugs produced by companies in the United States. Millions yearly are collected and released after providing a blood sample for this purpose, especially in the Chesapeake Bay area.
A confirmed scavenger, this animal's constant foraging and digging about renders it a valuable addition to the aquarium. In doing so, it not only uncovers uneaten food but also helps prevent pockets of anaerobic bacteria from developing.
Small horseshoe crabs are extremely entertaining — always on the move and perpetually hungry. If flipped upon their backs they use the long spikelike tail to flip themselves back over. Although many people are afraid of this animal, assuming it to be able to sting, it is actually quite harmless.
Large specimens (they can grow to be 20 inches) are best observed in the wild, as they require quite a lot of space and also seem to lack the ability to go around any object in their path, preferring instead to knock it over.
Horseshoe crabs readily accept small bits of clam, prawn or fish, and also appreciate an occasional meal of live black worms or other such creatures. They require a fairly deep, soft substrate in which to burrow but are by no means shy about exposing themselves once adjusted to captivity.