Horseshoe Crabs, otherwise known as “living fossils“ are some of the oldest animals in the world. Their ancestors date back to 445 million years ago, which is 200 million years BEFORE dinosaurs. Because of their name, most people think they are crustaceans but they are actually more closely related to arachnids. There are four different species of horseshoe crabs including Carcinoscorpius rotundicaud, Tachypleus gigas, Tachypleus tridentatus, and Limulus polyphemus, which is the only species found in North America.
Photos taken during Invertebrate Zoology class.
Over the millions of years, the body structure of the horseshoe crab hasn’t changed drastically. Their body is surrounded by a hard carapace, which is used for protection. There are two compound lateral eyes that consist of one thousand ommatidia. Ommatidia are clusters of photoreceptor cells surrounded by support and pigment cells. Along with their two lateral eyes, they also have two median eyes that can see visible and UV light, one endoparietal eye, and two more rudimentary lateral eyes on the top of their body. Horseshoes crabs have five pairs of legs, and their mouth is located in the center of them. They have no endoskeleton, and have book gills which function as an exchange of respiratory gases.
Horseshoe Crabs importance in the Medical Industry
Human blood is iron based, and a horseshoe crab’s blood is copper based which give it its blue color. When bacteria or endotoxins try to enter their blood stream, the blood will “gel” and create a barrier against the bacteria. A horseshoe crab’s blood is made up of only amebocytes ,which are mobile cells that act as a defense like white blood cells do in vertebrates.
The biomedical industry has conducted a way to extract this compound from their hemolymph and use it for medical treatment. The preparation is called Limulus Amebocyte Lysate (LAL). The goal of this treatment is to detect endotoxins that are associated with gram negative bacteria. This bacteria activates a pyrogenic response when it comes in contact with human blood. Because of the detection function, it is used in biomedical companies to test vaccines, IV drugs and implants for any signs of bacteria contamination.
During the preparation of LAL, the horse shoe crabs are not killed. About one third of their blood is extracted. They are then released back into their habitat where they came from. Researchers estimate that it takes thirty days for the horseshoe crab to replace their missing blood. There are about 500,000 animals collected annually for medical testing. The mortality rate is about 10%, which scientists argue “could be a lot more”. Some argue that this preparation is in dire need to make sure medical equipment is sterile, while others argue it is unethical and animal cruelty. Bleeding of the females may prevent them from being able to spawn and can decrease the amount of eggs laid. Some biomedical companies get accused of not putting them back to the ocean but instead selling them as fishing bait, which is the horseshoe crabs other main “use”. As a result of this controversy, the legislation has put a limit on how many horseshoe crabs can be harvested, and the horseshoe population is now increasing. Personally, I understand the health benefits of using horseshoe crab blood to test for sterile medical treatment, but I don’t understand why we are so advanced in technology but cannot come up with a different way to test that does not use animals.