Carefully laid out on a table, but still somewhat jumbled, are curiosities of history: seashells, coins, stones, small ceramic vessels, and maybe even seeds. Books and paintings are propped up on the table and the wall behind the table, including two paintings of insects. Everything makes you want to stop and stare. At the very top left corner hangs something truly curious: a dark, armored tear-drop with a long, spiked tail. For the artist who painted this “cabinet of curiosities” in 1617, the horseshoe crab was definitely unfamiliar. Frans Francken the Younger was a Dutch painter in the Antwerp guild, and horseshoe crabs, then like now, are native to Asia and the North American Atlantic. This dead crab may have been the only one he’d ever see.
Today on the Eastern Shore, we can see hundreds of horseshoe crabs descend onto the beaches yearly, but that somehow doesn’t make them any more familiar. They look alien, and they kind of are. These ancient, magical animals have lived fascinating lives on our planet for hundreds of millions of years.
Although our Limulus polyphemus, or the Atlantic or American horseshoe crab, isn’t actually ancient, paleontologists have discovered very similar species like the Lunataspis aurora, which lived 455 million years ago. The body of the horseshoe crab looks like it could be imprinted in stone in a natural history museum. The army helmet-like exoskeleton protects organs including brain, heart, and nervous system. The crab scuttles around on ten legs, with a long spike tail dragging behind, and swims up-side down. They have nine eyes across their body, and light receptors near their tails to help them detect moonlight. Males usually grow to around fifteen inches, while females are about four inches longer. Once a year, the males and females follow rituals in hopes of continuing on the species for millions more years.
The Delaware and Chesapeake Bay provide two of the favorite spawning grounds for Atlantic horseshoe crabs. During the full moons of May and June, the males emerge from the water. The females release a scent to announce their arrival. Soon, hundreds or thousands of these creatures cover the ground. The males grab on, and the females drag them onto the beach, where they begin digging a nest. The females lay their 20,000-or-so eggs, and the males fertilize them.
The Maryland Department of Natural Resources warns that this won’t result in an enormous happy family: “Ultimately, many of the horseshoe crabs eggs never reach maturity, and instead are devoured by crustaceans, fish, and migrating shorebirds such as the red knot and sandpiper; who depend on this feast for survival.”
Humans depend on the horseshoe crab for survival, too.
Another fascinating, and slightly creepy, fact about horseshoe crabs: their blood is blue. In human blood, iron-based hemoglobin carries our oxygen, which becomes bright red when packed with oxygen. The oxygen-carrying molecule in the blood of horseshoe crabs is copper-based hemocyanin. The copper’s contact with oxygen makes the blood blue. This blue blood also has a magical quality. A chemical in the blood can detect the presence of bacteria and trap the harmful cells in a gel. For the crab, this chemical protects them from bacterial infections. For humans, it provides a vital step in detecting bacteria during the quality assurance process of injectable drugs and medical devices. During the Limulus amebocyte lysate (LAL) test, scientists isolate the chemical, introduce it to a solution, and if no gel appears, they know the solution is free from bacteria.
To get the chemical from the crabs, labs collect the blood while the crabs are still alive. They catch them by thousands, piece their small tubular hearts, and drain thirty percent of their blood. Then, they release them at a different location to make sure they are not re-bled. This doesn’t sound like a pleasant process for the crabs, and the blood harvesting might not be good for them, or the environment. The industry admits that ten to thirty percent of crabs may die after the process, while female crab populations have declined in heavily harvested areas. In 2013, researchers found that harvested horseshoe crabs are slower moving and had changes in their, “expression of circatidal behavioral rhythms,” or normal daily behavior. The researchers also found that the crabs had lower levels of hemocyanin, the molecule that transports oxygen, which “may alter immune function.”
Although on a full moon in June you may not notice, horseshoe crab populations are on the decline. So what happens when females aren’t laying hundreds of thousands of eggs for fish and birds to feast on every year? The ecosystem is a system, and conservationists now worry that declining shorebird populations are a result of declining horseshoe crab populations. The red knot, a threatened species, loves feasting on the eggs during migration through the Eastern Shore. Conservation groups want pharmaceutical companies to use recombinant Factor C (rFC), a synthetical alternative to LAL.
If you see a crab this summer, don’t step on him or her (for your sake and theirs), and flip them over if they’re belly-side up. Maybe a little help can make this prehistoric creature a feature of the future.