November 15, 2010
Strange BiologyA Primitive Animal with a Modern Function
The origin of horseshoe crabs dates back over 200 million years. (Photo credit: Vanessa Vick/Photo Researchers, Inc.)
The horseshoe crab (Limulus polyphemus) is often referred to as a “living fossil.” This unusual animal’s origins date back over 200 million years—and, over that long period of time, not much has changed with regard to the horseshoe crab’s body form.
The horseshoe crab is not actually a crab at all. Instead, it is a close relative of spiders, ticks, and scorpions. These invertebrate animals can be identified by their helmet-like shape, long tail, and hard exoskeleton. Horseshoe crabs shed their outer shell as they grow larger. Each animal has six pairs of legs—one pair to feed with, four pairs used for both walking and feeding, and one pair of longer walking legs that is used to push the horseshoe crab forward.
Though primitive in looks and origin, surprisingly, these animals have a major role in modern medicine. Pharmaceutical and medical device companies use an extract in the horseshoe crab’s blood to test their products for possible bacterial contamination. A horseshoe crab’s blood contains antimicrobial cells (called amoebocytes). These cells work like a primitive immune system to help defend the crab against potential bacterial infections. Dr. Frederick Bang, a researcher at Johns Hopkins University, first discovered the antimicrobial properties of horseshoe crab blood in the 1950s. He found that injecting bacteria common to marine ecosystems into the bloodstream of a horseshoe crab caused massive clotting to occur. Further research indicated that cells called amoebocytes were responsible for this reaction. This reaction is important to horseshoe crabs since marine environments are notoriously rife with harmful bacteria. When the amoebocytes react to an invading pathogen, they bind to and inactivate the offending bacteria, fungus, or virus. In addition, the clot that forms helps to close up the wound and provides a physical barrier against further pathogen invasion into the horseshoe crab's body.
Through his research, Bang realized that amoebocytes could potentially be used to test drugs for the presence of gram-negative bacteria. Bang and a colleague named Dr. Jack Levin developed an extract from horseshoe crab blood called limulus amoebocyte lysate, or LAL, which could be used for this purpose. Their new method for detecting contamination using LAL was so successful that it was accepted by the Food and Drug Administration (FDA) as the primary test for endotoxins in 1983.
A technician removes blood from horseshoe crabs. Note that unlike a human's blood, which is red, a horseshoe crab's copper-based blood is blue. (Photo credit: Andrew J. Martinez/Photo Researchers, Inc.)
To produce LAL, a needle is inserted into a horseshoe crab’s circulatory system. During the extraction process, up to 30 percent of the horseshoe crab’s blood is removed. The animals are typically not harmed during this procedure. In fact, it’s in the extractor’s best interests to not harm the animals as a quart of horseshoe crab blood is worth $15,000.
Unfortunately, horseshoe crab populations have been on the decline in recent years. Research indicates that though both the fishing (which uses horseshoe crabs for bait) and pharmaceutical industries have put pressure on the animals’ populations, climate change is also a prominent factor. Several states along the Atlantic Coast, including New York, Maryland, and Delaware have already placed limits on the number of horseshoe crabs that can be harvested each year. Conservation biologists hope that such limitations may help horseshoe crab populations stabilize.
Recently, a new species—the African clawed frog—has proven useful in the development of a sensor to detect drug and medical device contamination. The African clawed frog makes antibacterial peptides on its skin that protects it from harmful infections. Scientists at Princeton University developed a method to attach these peptides—produced artificially in the lab—to an electronic chip that emits an electrical signal following contact with harmful bacteria. Though the sensor is still in development, the engineers who developed it hope that one day it will replace the use of LAL to test drugs and medical devices for contamination.
Comments
Comment from: Sky
December 9, 2010 04:15 PM [#]
OMGOSH THIS IS COOL!
Comment from: Cloud
September 8, 2011 07:24 AM [#]
Is it because of the blood that the horseshoe crabs species were able to survive this long?