Tammy M. Cardeira posed for a memorable photo with a horseshoe crab sunning himself at Falmouth Heights Beach.
Horseshoe Crabs:
Living Fossils of the
Sea
The horseshoe crab is called a "living fossil" because it has been on earth for millions of years. Although called a crab, it is not a crustacean, but belongs to the arachnid or spider family. Their name is almost identical with their apprearance. These brown-shelled creatures of the beach have been around since before the dinosaur age. Horseshoe crabs are mainly found on the western side of the Atlantic Ocean anywhere from Maine to Florida.
After many years of being harvested for bait, horseshoes have been a source of controversy for many scientists, fishermen and naturalists. In recent years, many states along the Atlantic coast have declared the horseshoe crab an endangered species.
The march of the horseshoe crabs! BRGPhoto.
Horseshoe crabs have also been harvested for medicinal purposes. Over 30 years ago, scientists working out of the Marine Biological Lab in Woods Hole discovered that the blue blood found in horseshoe crabs clots when exposed to endotoxin - a chemical poison toxic to humans. Lysate, another chemical found in their blood, is helpful for diagnosing spinal meningitis. Horseshoe crab shells are also a good source of chitin, which has been shown to speed healing.
Unlike the method used for extracting endotoxins, lysate can be taken from the crabs without harm. However, when extracting endotoxin, roughly 1/3 of the blood in the horseshoe crab's body is removed, leading many to question their safety. Last year, Massachusetts laws were enacted regulating the return of horseshoe crabs back to sea. While many argue that it is safe for crabs to be sent back to sea in either case, many crabs don't survive the scientific testing and removal processes. Fishermen argue that returning horseshoe crabs out to sea endangers soft shell clams and other shellfish along Cape Cod's shores. Other uses for the crab include grinding them for use as a fertilizer and chicken feed.
Horseshoe crabs have an interesting sense of territory. Those that live in Cape Cod Bay don't mix with those that live in Nantucket Sound. The horseshoe crab's shell serves the him well for burrowing in sand and mud. The horseshoe's tail is known as the telson and is attached to the crab via a ball and socket joint making the spike tail easily movable. Since the horseshoe crab has no known enemies and doesn't use its tail to defend itself, the telson's main function is to dig into the sand and to turn the crab over if it gets overturned by waves. A horseshoe crab cannot live without its tail, so if you see one on the beach and want to pick it up, hold the crab by the sides of its shell.
Horseshoe crabs have four pairs of
walking legs with a claw at the end of each. There is also a fifth pair that
helps the crab lurch forward. Its mouth is actually located in between the base
of the legs and the horseshoe crab can only eat when it is moving. They tend to
eat mostly worms, dead fish and bottom-dwelling algae. Although horseshoe crabs
have 10 eyes, during the day they can't see very well. At night their eyes
become super sensitive. Therefore, horseshoe crabs only use their eyes at night
- usually to find other crabs to mate!
One motivation for the ban on harvesting horseshoe crabs can be attributed to the shore birds who eat their eggs. In recent years, shore birds have been migrating north only to find a depleting supply of food.
During April, May and June, horseshoe crabs come on the beach to reproduce. Female horsehoe crabs lay about 4,000 eggs in shallow nests in the sand. The smaller male crabs fertilize them and in about two weeks baby horseshoe crabs (minus telsons) are hatched. They can live anywhere from 16 to 40 years, but don't reproduce until they are at least 10 years old. When summer comes, the crabs go back out to sea.
If you happen to find a horseshoe crab while you are out enjoying one of our beautiful beaches, feel free to look and admire it, even take a photo, but please do not remove them from their natural habitat. They are quite happy where they are.
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