Horseshoe crabs save lives, but their numbers are dwindling

Living on Earth
Horseshoe crab

An Atlantic horseshoe crab (Limulus polyphemus) on Sanibel Island, Florida. The Atlantic horseshoe crab's range extends from the Gulf of Mexico to the Northeastern United States.

A startling fact about horseshoe crabs: Their blood is worth about $60,000 a gallon.

Why? Horseshoe crab blood contains a chemical that makes its blood clot in the presence of even the most minute trace of bacteria. So biomedical companies use the crab’s blood to make sure vaccines and medical implants aren't contaminated.

The test is called the limulus amebocyte lysate test, or LAL. According to a PBS documentary, the “LAL has become the worldwide standard screening test for bacterial contamination. Every drug certified by the FDA must be tested using LAL, as do surgical implants such as pacemakers and prosthetic devices.”

Biomedical companies don’t kill the horseshoe crabs. They draw about 30 percent of the animals’ blood and then return them to the ocean. Evidence suggests, however, that this technique, while sparing their lives, has a negative effect on the animals’ ability to spawn and reproduce.

In addition to being harvested for medical purposes, eel and conch fishermen use horseshoe crabs as bait. As a result, horseshoe crab numbers are declining. These helmet-shaped arthropods have been on earth for more than 300 million years, but, as so often seems to be the case, humans have reduced their population to a dangerous level in a spectacularly short time.

Horseshoe crab underside

Despite their name, horseshoe crabs are not crabs; they are more related to spiders, scorpions and other Chelicerates.

A Massachusetts Audubon Society tagging program is hoping to find ways to reverse this. The program, largely made up of volunteers, aims to provide an accurate census to understand how many horseshoe crabs can be safely harvested from Massachusetts beaches.

Closing horseshoe crab fisheries in New Jersey, New York and other, neighboring states is expected to increase pressure on dwindling populations of Massachusetts horseshoe crabs.

So, on Cape Cod, teams of volunteers wade into the water to count and tag the animals; they also count them when the females come ashore to spawn. Special labels help them keep track of the crabs.

The standard process for tagging horseshoe crabs seems sort of barbaric, but research shows it doesn’t hurt the crab, says Mark Faherty, the science coordinator at Audubon’s Wellfleet Bay Sanctuary: “You drill a hole through the corner of their shell and put this tag on,” Flaherty says. “The tag uniquely identifies that crab. That data is used by researchers to estimate crab movements, crab survival, how they’re using the spawning beaches, that kind of thing.”

New technology may make this process easier on everybody, especially the crab. The Massachusetts Audubon Society recently recruited graduate student Michael Long to lead their newest horseshoe crab study. With researchers from the University of Massachusetts, Long will be tagging the crabs this summer with a telemetry label glued onto the crab’s shell.

“My study is going to be putting acoustic receivers out in the bay, and acoustic markers on the crabs,” Long explains. “The receivers have about a 600-meter detection radius, so anytime a crab that’s marked with an acoustic receiver comes within 600 meters of that receiver, it will mark where it is. Based on where each crab pings, you can track its movements around the bay.”

Long organizes new volunteers to help him count horseshoe crabs on the beach, and Faherty trains them in the survey procedures. Volunteers are trained to divide the beach into small sections, count the horseshoe crabs, and record all of their information. The volunteers do the survey when female crabs come onto the beach to lay their eggs in the sand. Males follow to fertilize the eggs after they’ve been laid.

Volunteers quickly learn to recognize the male, mainly because the males are alone and are unusually eager to spawn. “If you make a model of a horseshoe crab, the males will congregate around it and they’ll spawn. They’ll spawn with your boot,” Flaherty says. “These are just hormonally-charged animals that are ready to mate with anything. Females are not lonely for long in the horseshoe crab world.”

With help from their volunteers, scientists hope to figure out how many horseshoe crabs can be harvested sustainably, and from where. The research will help state wildlife regulators set limits on harvest and ensure that the horseshoe crabs are around for generations to come.

This article is based on a story that aired on PRI’s Living on Earth with Steve Curwood.