The first time I saw a horseshoe crab (Limulus polyphemus) up close was while swimming on the sandy beaches where Bass Creek empties into Buzzards Bay on West Island in Massachusetts. I pulled it off the sandy bottom and marveled at its smooth shell, spiny tail, and the many spider-like legs on the crab’s underside.
Though these creatures have been in existence on earth for 350 million years (the earliest human fossils appear about 200,000 years ago) they still look like something out of a science fiction alien movie. Despite their apparent ability to survive for millions of years on Earth, there is evidence that New England populations of horseshoe crabs are declining and that some of this decline is linked to the harvest of horseshoe crab blood for biomedical testing materials. This blood harvest is not all that fatal to the crabs, so their decrease in numbers may not be due to just mortality alone. A recent study by scientists at the University of New Hampshire and Plymouth State University suggests that the harvest of blood during spawning season may be influencing the horseshoe crab’s behavior and ability to reproduce.
Humans have harvested horseshoe crabs for hundreds of years for various purposes. Until the 1970s they were commonly harvested to be ground up as fertilizer and they are still a common bait source for the conch and eel fisheries. The Atlantic States Marine Fisheries commission reports that horseshoe crab populations in New England have been declining since 2004 despite the fact that bait harvests in the area have been cut in half. However, during this same time period, the harvest of horseshoe crab blood for biomedical use has increased by 76%. It is estimated that about 200,000 crabs were collected in the 1990s to harvest blood while more than 610,000 were collected in 2012. The downward trend in horseshoe crab populations is even more apparent in Pleasant Bay, Massachusetts where the crabs have been harvested for biomedical use for 30 years and a moratorium on collection of horseshoe crabs for bait purposes has been in place since 2006.
If you have ever had surgery, received vaccines or other injections, had a joint replaced, or received any kind of implantable biomedical device, you were probably protected by horseshoe crab blood. The substance that is harvested from the blue blood of horseshoe crabs is called Limulus Amebocyte Lysate (LAL) and it is used for the detection of bacterial endotoxin, a complex molecule found in the cell walls of gram-negative bacteria. LAL is essentially a series of proteins that cause coagulation in the presence of gram-negative bacteria. In horseshoe crabs this clot traps bacteria while antimicrobial agents in the clot neutralize the invader. A variety of sensitive biomedical assays have been developed around this horseshoe crab blood product. Scientists are working on alternatives to LAL in biomedical testing and some are currently available such as the Pyrogene assay by Lonza that uses a recombinant form of horseshoe crab factor c to detect endotoxin. This recombinant protein is produced in the lab and not harvested from horseshoe crab blood. However, the LAL test remains the most widely used and it is extremely effective as it is capable of detecting picograms to nanograms of bacterial endotoxin (one millionth of a billionth of a gram of endotoxin). Thus horseshoe crab blood is very valuable and LAL can cost $15,000 per liter (1 liter = ~1 quart).
When horseshoe crabs are harvested for LAL only a small percentage of them actually die. Crabs are harvested by trawling nets or by hand capture from beaches and shallow water and 50% of the LAL harvest occurs during the spring spawning season when the crabs return from deeper water to reproduce on sandy beaches. Crabs are transported to labs for blood harvest usually in open-air containers and not in seawater aquariums. At the lab, about 1/3 of each crab’s blood is drained and the crab is returned to the point of capture. The whole process usually takes 24-72 hours. The mortality rates are 8-15% in male and 10-29% in female horseshoe crabs.
Despite the fact that mortality is pretty low during the LAL harvest, fewer females are showing up to spawn in areas where crabs are harvested for biomedical purposes. Perhaps the stress of the bleeding procedure is affecting the horseshoe crabs’ behavior and physiology during their breeding season. To make a crude analogy, suppose you were in your bedroom about to have sex and somebody grabbed you, threw you in a truck for a half a day, stuck a needle in you and drew off about 1.5 liters of your blood. After waiting around for another 6 hours or so they threw you in the truck again and dropped you off in your bedroom or at least someplace close to your house. Chances are you might have lost your mojo and be a little disoriented for a while. If you were a horseshoe crab you might have even lost out on the once a year spawning opportunity.
Rebecca Anderson, Win Watson, and Chris Chabot, researchers at the University of New Hampshire and Plymouth State University, recently tested the idea that the biomedical bleeding process affects the behavior and physiology of female horseshoe crabs and published their findings in The Biological Bulletin. They fitted crabs with accelerometers to monitor their activity and measured hemocyanin concentrations both before and after the bleeding procedure. Hemocyanin is a molecule that carries oxygen in horseshoe crab blood it contains copper and accounts for the bright blue hue of their blood. It is similar to hemoglobin, which carries oxygen bound to iron in human blood and accounts for its red color. Their study saw similar mortality (18%) as has been seen before in studies of the horseshoe crab bleeding procedures. They also saw significant reductions in the amount of circulating hemocyanin in crabs six weeks after the bleeding procedure. Bled crabs were slow and sluggish and overall activity was reduced for several weeks. Human activity has a circadian rhythm that is governed by light-dark cycles, we are more active during the day than we are at night. Horseshoe crabs follow a circatidal behavioral rhythm. Their activity levels follow the patterns of tides coming in and out. This study showed that circatidal activity patterns were not as strongly shown for several weeks after bleeding. In summary, the bleeding procedure befuddled female crabs enough that it could affect their ability to reproduce during the several week long spawning period in the spring.
Why should we care about dwindling numbers of horseshoe crabs on Atlantic shores? Declining numbers of crabs carry serious changes for the ecosystems they inhabit. Horseshoe crab burrowing moves sediments around in marine and estuary environments, so their activity can influence nutrients in the water. They are also an important source of food for shorebirds, fish, and crustaceans. The eggs of Limulus are an important source of nutrients and energy for migrating shorebirds.
Red knots (Calidris canutus rufa) stop on beaches in Delaware Bay during their spring migration from South America to their breeding grounds in the Arctic. While at these beaches they refuel by consuming eggs of breeding horseshoe crabs. Several studies link the severe decline of red knot populations to declines in horseshoe crab populations due to overharvesting. So, dwindling numbers of these crabs may cause ecosystem and population changes that could alter other marine and estuarine species.
The development of alternatives to LAL for biomedical testing may mean that someday we will not have to harvest horseshoe crabs for their blood and that their populations might rebound naturally. However, until those tests become economically viable and widely used our impact on the horseshoe crabs, which are a vital resource for humans and other animals, will need to be carefully monitored. In their study, Anderson, Watson, and Chabot indicate that, “to maintain the integrity of the stock needed to supply the industry, adaptive or flexible management strategies may need to be considered.” The Atlantic States Marine Fisheries Commission has developed a set of “best management practices” that suggest reduced out-of-water transport and holding times and cooler, less stressful temperatures for crabs during the bleeding procedure. Anderson, Watson, and Chabot’s study focused on current “industry standard” bleeding procedures and in a recent interview Chabot indicated that he is very interested in studying the “best management practices” and whether or not they will make a difference in how the crabs behave after bleeding.