Atlantic Horseshoe crabs facing new threats
by Hannah Deadman
Atlantic horseshoe crabs are continually making their way into the medical and pharmaceutical spotlight. From their morphology to the medical research continually being conducted on them, these little arthropods seem to have a lot to offer.
Interestingly enough, these primal-looking critters are not true crabs. In fact, they’re more closely related to spiders than crustaceans.
There are four species that exist today. One of the four, called Limulus polyphemus, better known as the Atlantic horseshoe crab, lives on the Atlantic and Gulf coasts of North America, while the rest of the species are found in Southeast Asia. This article will focus on Limulus polyphemus.
The Atlantic horseshoe crab has 10 pairs of eyes, which hold light receptors 100 times larger than vertebrate eyes, like our own. The horseshoe crab’s body is divided into three parts – the cephalothorax (the front), the abdomen and the telson (tail). The telson is the horseshoe crab’s tail, or “stinger”, that actually doesn’t sting or cause any harm. Instead, a horseshoe crab uses the telson to propel or flip itself over in the mud. The animal also has six pairs of gills and five pairs of walking legs.
Like many of its relatives, the horseshoe crab sheds its exoskeleton as it grows and reaches adulthood around the 10-year mark. But many horseshoe crab larvae will not survive until adulthood, especially because many birds, reptiles and fish prey on the larvae.
For those that do survive, larvae travel into the water and live on the bottom of tidal flats for more than a year, feeding on worms and clams. Once the crabs reach adulthood, they live in deeper waters until they are ready to return to the shore to breed. They can live longer than 20 years.
Perhaps one of the most interesting features of the horseshoe crab is the opportunity to use the animal’s structural features for bio-medical purposes. The animal’s blood, a blue color as a result of using a copper-based substance called hemocyanin to carry oxygen, coagulates when it contracts gram-negative bacteria, which are also called endotoxins or lipopolysaccharides.
The animal’s response to such endotoxins continues to allow researchers to use the crab’s blood in drug testing to ensure that a drug is completely sterile before it’s placed on the market.
But it’s not just about the horseshoe crab’s qualities. Horseshoe crabs could either have a bright or a dim future ahead of them, depending on how current trends continue.
The past few years has seen a rise in controversy over using the animal’s blood for scientific purposes. After the horseshoe crabs are systematically bled by pharmaceutical and medical industries – where about 30 percent of a crab’s blood is taken each time – they are re-released into their natural habitat.
However, between 20 to 30 percent do not survive, according to Chris Chabot, a professor of neurobiology at Plymouth State University (Nature World News, “Horseshoe Crab Blood Harvesting Practices Questioned by Researchers.”)
Chabot and a team composed of biologists and graduate students carried out a controlled lab experiment where they harvested the crabs’ blood and observed their behavior. The crabs showed signs of disorientation after they were bled.
In order to prevent such disorientation behaviors, Chabot said, the crabs should be bled with longer periods between each time, as well as transported back to the beach by water, rather than removing them from water before placing them on the beach again.
In addition to the dangers of over-bleeding, horseshoe crabs also face the threat of over-harvesting. They’re used as conch and eel bait, according to the National Wildlife Federation’s list of species used in medicinal industries.
Another major threat to these animals is human development. Since the horseshoe crab uses the shoreline for breeding during the late spring and early summer, the development of houses and other buildings makes it harder for horseshoe crabs to breed.
With various legislation at state levels, horseshoe crabs might have a chance. Since 2000, the state of Florida requires people to hold a license in order to harvest the crabs for bait, at 100 animals a day per person. In 2002, the plan added the need for a bio-medical permit in order to harvest the crabs for research.
According to the Florida Fish and Wildlife Conservation Commission, since the plan was established, no one has applied for a bio-medical permit.
For more information about horseshoe crabs, visit myfwc.com/research/saltwater/crustaceans/horseshoe-crabs/facts.