This is the third and final essay on the horseshoe crab. The previous two diaries were Horseshoe Crab Basics and Horseshoe Crab Anatomy, in case you need to get caught up. Today I’d like to spend some time examining the conservation angle of this ancient animal, especially since in some areas populations are barely holding on. Fortunately there is a tremendous amount of knowledge about this creature’s ecological needs in the scientific community, which is half the battle of preserving any species.
Each spring around the world millions and millions of horseshoe crabs wait for the sun to set on the evening of the first full moon, when the gravitational effect of our satellite is strongest and the ocean rises far up onto the beach. Males and females have spent the previous few days pairing off using a process known as aplexus, with the males laying claim to the much larger female by attaching to her exoskeleton using special hook-like appendages on the front pair of legs. When the tide has reached its peak the females haul themselves and their mates out of the water at the tide line. Here she digs a shallow pit and deposits tens of thousands of tiny gray eggs in the sand.
Once the cluster of eggs is laid she lumbers forward to position the male over the pit, and he promptly fertilizes them with milt deposited from seminal glands attached to the book gills. The female then pushes her way back towards the water line, covering the eggs with sand as she leaves. This is repeated several times during the night as the tide recedes, each nest positioned a bit closer to the low tide line each time. In all, each female will have laid over a quarter million eggs.
This image, taken in the early 1980’s,
shows a huge mass of spawning crabs
on a beach.
Every adult that was originally born on this beach has migrated from deeper water to converge on this shore. On some beaches as many as ten thousand individuals could once be seen spawning on the same night. There won’t be another tide this high until the next full moon and the embryo's development is timed so that hatching occurs during this next tidal event, allowing the tail-less young to be washed away as they emerge from their shallow nests.
It is sadly ironic that this mass-spawning behavior that has served this animal so well for 350 million years was nearly, and could still be, its downfall. For a person wishing to harvest these animals in large numbers not only knows where they will arrive, but also when. Like the passenger pigeon, whose seemingly unlimited numbers did them little good when it came to human predation so long as they existed in highly concentrated and predictable flocks, this similar behavior is what makes the horseshoe crab so vulnerable as well. Let’s look at some of the ways Limulus polyphemus has been exploited.
There is not much meat on the bones (so to speak) of the horseshoe crab, and it is rarely if ever eaten in this country. Above is a photo from a grilled horseshoe crab vendor in Thailand. So, although the impact on this species for human food is minimal, the eggs are vital to the millions of migratory seabirds that use the Atlantic Flyway each year as they travel to their nesting grounds. In fact, there is a good chance that this mass bird migration has evolved to coincide with the horseshoe crab’s spawning season. Many of the bird species feeding on the eggs along their route would simply not be able to survive the trip without this rich food source. A graph showing the importance of Limulus eggs to shorebirds can be seen here.
Historically, horseshoe crabs were used for fertilizer since colonial times. Commercial harvesting took a heavy toll on the species as from the mid 1800’s up to the 1960’s millions of animals each year were collected by hand, dried and ground up into powder to be dispersed on crop fields. This practice ended as the advancement of newer, petroleum-based fertilizers became less expensive than the harvesting process.
So you’d think with the new high-tech fertilizers the horseshoe crab populations would have rebounded. They did for a while, but two situations have arisen since the crab’s heyday, both relating to the relentless human push towards the coasts since the end of World War II, a migration that continues today. This steady influx of people demand food and they demand land. Beachfront development has robbed the species of suitable breeding beaches. Not any old beach can be used for spawning. The sand must have the proper degree of compaction, solid enough to keep the developing eggs in place so they don’t wash away before hatching, yet porous enough to allow oxygen to penetrate down to the eggs. The spawning beach must also be close to tidal marshes or other shallow estuaries to supply sufficient food and protection for the first several years of the juvenile’s life.
Even on those beaches that have been spared from being covered in blacktop or beach blankets, an increasing demand for seafood such as striped bass and channeled whelks is perhaps the greatest threat of all to the horseshoe crab. The mainly recreational striped bass fishery depends on using young American Eels (Anguilla rostrata) as bait. The eels are impaled live on a hook and lowered by rod and reel to the sea bed. The boat remains unanchored, in a fishing method known as "drifting", allowing the hooked eel to wiggle it’s way along the bottom in the hope that it attracts a large striper. No striped bass can resist a live eel.
But before all this can be played out, you need to get yourself some eels. This is done by placing steel-meshed eel pots in rivers and estuaries and baiting them with adult horseshoe crabs that have been hacked up with an ax. The scent of the slaughtered crabs drifts downstream and is detected by the eel’s keen sense of smell. As an aside, and I’m not sure how this affects eel populations, but if you read that eel essay linked above, you may notice that all eels used by bass fishermen are male.
Whelk fisherman also use horseshoe crabs, chopped up with an ax of course, to bait their traps. In this mollusk’s case, the target of the trap is used for food directly in the form of recipes such as snail salad and Scungilli. You may wonder what is so special about horseshoe crabs that they must be used as the sole bait in these snail and eel pots. There are several reasons: First, they are (or were) very abundant, second, they are free for the taking, and third, once hacked up they spew a lot of strong-smelling liquid and semi-liquid into the water (especially the females).
Channeled Whelk
So this is the main industry, the eel and whelk fishery, to concentrate on if the horseshoe crab is to be protected. And this is actually what is being done throughout the country. Although most regulations are left to the states, there are also now federal guidelines in place to require states to set quotas specifically on the harvesting of horseshoe crabs for bait. Even with these new regulations, close to three million horseshoe crabs are still legally slaughtered along the Eastern Seaboard each year, not counting those taken illegally or killed as fishery by-catch. On a hopeful note, this researcher is trying to isolate the component of the horseshoe crabs that make them so good at attracting eels and whelks in order to synthesize an artificial bait that can be used instead.
There is one last commercial use of this species left, one that although does not directly result in the death of the animals harvested, may still have a serious affect on their populations. The oxygen-carrying component of our blood is called hemoglobin, and it is red because this molecule is iron-based. Horseshoe crab blood, on the other hand, is blue. This is caused by the copper-based molecule hemocyanin, which serves the same purpose. Whether your blood is red or blue, carrying oxygen to the body’s cells is only one of many jobs our circulatory system has. Another is to protect us from pathogenic bacteria and other harmful agents by marching out our white blood cells to do battle with the invaders. These marching orders are issued by our immune system. This immune response is great for us. Horseshoe crabs, not so much. These primitive animals do not even possess an immune system so are obviously unable to launch an immune response.
Horseshoe crabs do, however, have a compound in their blood known as LAL, or Limulus Amebocyte Lysate. While our blood contains three components (white cells, red cells and platelets), the horseshoe crab makes do with only LAL. When the LAL detects bacterial endotoxins it immediately begins to bind to and clot around the foreign body. Essentially, it’s an extremely simple and primitive form of immune response. One that also proves to be very useful for pharmaceutical firms.
One of the major problems drug manufactures face, especially for drugs that are injected directly into a person’s bloodstream, is accidental bacterial contamination. The methods used to detect contaminants was costly and complicated until it was realized that the LAL of the horseshoe crab could do this work for us. Simply exposing samples of medication or medical equipment to refined LAL taken from the blood of a crab can tell you almost instantly if the meds are contaminated or not. If it clots, throw it out.
Of course, to obtain LAL you need horseshoe crabs. Lots of them. Fortunately the blood of a horseshoe crab can be extracted without killing, or even harming, the animal. Crabs are collected and transported to the LAL laboratory (the largest one is on Cape Cod, MA) to be bled. A needle is inserted into the joint muscle between the first two segments of the body, and up to 30% of the blood volume of the animal is extracted. It takes the crab a few months to build its blood level back up so it is released after bleeding. Horseshoe crabs can theoretically be bled up to three or four times a year and continue to do so for decades.
So, what’s my problem with this? Medical science benefits, patients benefit and the horseshoe crab lives. It’s a win/win/win. My only problem, one that can certainly be solved, is that the crabs are collected all along the East Coast and transfered to the lab. If they are not returned to the same bay or general area they were collected in, they do not have the ability to find their spawning grounds. A horseshoe crab from Chesapeake Bay that is bled and then released in Bar Harbor, ME may not be able to successfully spawn.
That’s really my only issue with this procedure. In fact, I can think of one very big reason, aside from the medical advantages of course, to support it. If the horseshoe crab proves to be a profitable and necessary species to the medical industry, that is a mighty big incentive to protect its habitat and keep the population as healthy as possible. This way everybody wins: The doctors, the fishermen, the migratory birds, and one of the most ancient animals that exists on this planet.
Fun Fact: The largest horseshoe crab colony in the world is in Delaware Bay.
Other diaries in this series can be found here.