Horseshoe Crab Monitoring
Horseshoe crabs have been crawling ashore in Massachusetts for about 350 million years, and they look the same now as they did when living side-by-side with dinosaurs.
In fact, horseshoe crabs are commonly referred to as "living fossils" because they are one of the most ancient creatures still living today.
The species that currently calls Massachusetts home is the Atlantic Horseshoe Crab (Limulus polyphemus). Unfortunately, the Commonwealth's population of these incredible marine animals is in decline and facing increasing threats to their survival.
Horseshoe crabs are one of the most fascinating creatures in our oceans!
They have excellent eyesight thanks to 5 pairs of eyes, and can see just as well at night as they can during the day. Horseshoe crabs also have a wide field of vision, which means they can see their surroundings in all directions—in front, behind, both sides, and above!
Photoreceptors on their tails are sensitive to circadian rhythms, enabling horseshoe crabs to "tell time" by tracking the hours of daylight. Large chemical receptors on their legs gather sensory input in much the same way as insect antennae.
Mating & Nesting
In spring, adult crabs make their way onto beaches during full moons to mate. Females usually only come ashore to nest for a single tide cycle each year. Males use their front clasping claws to physically attach themselves to their chosen mate, and they will stay attached for the entire tide cycle (or longer!). The female digs shallow nests about 5"-10" deep in the sand, where she then lays 2-5 clusters that each consist of anywhere from 2,000-4,000 eggs.
Development takes 2-4 weeks, during which the eggs will molt four times before finally hatching. Once hatched, larvae remain in their clusters in the sand, not feeding, for several more weeks. They then molt into tiny, spiny juveniles and usually swim out to sea at the next moon cycle. Young crabs will spend anywhere from a few weeks to a full year near the beach where they hatched before heading out to new waters.
In Massachusetts, horseshoe crabs are harvested to be used as bait for the eel and conch fisheries. Additionally, their blood is the only source of a chemical that's used to test medical devices and injectable drugs for toxins. When harvested for medical use, the crabs are caught, bled, and then returned to the water.
Increased harvesting of these fascinating animals threatens their population. The problem has been compounded by closures of horseshoe crab fisheries in New Jersey, New York, and other neighboring states. As a result, there is increased harvest pressure on the dwindling populations of horseshoe crabs in Massachusetts waters.
It's crucial that state managers have a robust estimate of the number of crabs in Massachusetts before they can set appropriate harvest quotas to ensure a sustainable fishery. As a first response, Massachusetts has reduced the annual quota for horseshoe crabs and prohibited harvests around full moons from late April through June.
Research & Ways to Help
Mass Audubon has been conducting long-term surveys of spawning horseshoe crabs on Cape Cod and Martha's Vineyard since 2001 in collaboration with the University of Rhode Island, the National Park Service, the MA Division of Marine Fisheries, and several other organizations and institutions.
At our Felix Neck and Wellfleet Bay wildlife sanctuaries, conservation staff work with trained community science volunteers in the spring and early summer to count adult horseshoe crabs spawning at several sites on and around the new and full moons at high tide.
The data collected during these surveys is submitted to the MA Division of Marine Fisheries, which uses the information to determine the best conservation and management practices for Massachusetts horseshoe crabs and the horseshoe crab fishery.
We invite you to join our efforts to help preserve these very special marine animals! Volunteers are needed every year during April, May, and June to count horseshoe crabs as they come onto beaches to spawn at high tide during the new and full moons.