Horseshoe crabs have long been important to humans as well. Their relatively simple biology has made them a popular organism for basic biomedical research. Important early advances in understanding human vision and neurobiology were made through studies of the horseshoe crab's primitive eyes and long optic nerve.
Today a multi-million dollar industry exists around the collection of horseshoe crab blood. A clotting agent called Limulus Amoebocyte Lysate (LAL) is extracted from the blood, and used to test for the presence of gram-negative bacteria. Human blood and all commercially produced intravenous drugs are tested for bacterial contamination using LAL.
Blood extraction from horseshoe crabs is usually non-lethal, and thus the pharmaceutical industry is not thought to be a major factor affecting crab populations. Of much greater concern to researchers and managers is the increased harvesting of horseshoe crabs for use as bait in American eel and whelk fisheries.
University of Delaware biologist Dr. Bill Hall says the high density of horseshoe crabs in the Delaware Bay region makes harvesting them for bait cost-effective. "This is one of the few places where it's economical to fish for them," he says. Concern over the possible effects of this harvest prompted Dr. Hall to join with Benjie Swan of Limuli Laboratories and Dr. Carl Shuster of the College of William and Mary to organize a volunteer-based survey of horseshoe crabs spawning on Delaware Bay beaches in 1990.
"There is much concern about the harvest," says Dr. Smith. "There's enough data to suggest that the number of crabs spawning in the bay has dropped recently, enough that shorebirds may be threatened." But though the volunteer surveys provided some indication of a decline, the project was not initially designed to provide a valid statistical sample of the entire Delaware Bay population.