Meet the Prehistoric Animal That Still Lives Among Us

Written by Conservation Associate Lariah Maynard

When visiting the Dalton Discovery Center here at the Conservancy, you may see a strange creature with an armored shell and long tail buried in the sands of the touch tank. What you are seeing is the Limulus polyphemus; more commonly known as the American horseshoe crab.

A horseshoe crab ambassador in the Conservancy’s touch tank aquarium

But don’t let the name fool you! They aren’t actually crustaceans, but rather merostomes; a separate class that is more closely related to scorpions than it is to crabs. There are four species of horseshoe crab. One is the American Horseshoe crab, typically found along the eastern coast of North and Central America.

A comparison of the currently living American Horseshoe crab and a similar species from the Late Jurassic period.

From Kin, A., Błażejowski, B. 2014. The horseshoe crab of the genus Limulus: Living fossil or stabilomorph?

Horseshoe crabs are often referred to as “living fossils”. They get this title because their distinct three-piece shell has remained virtually unchanged for over 200 million years. Fossil forms of them have even been found as far back as the late Paleozoic era, 450 million years ago!

So why have they been able to live practically unchanged for so long? A big part has to do with their feeding behavior. Horseshoe crabs are generalist feeders. This means that they are able to eat a variety of different foods depending on what is available to them. Typically, they go along the bottom eating things like clams and worms, but they are able to eat a variety of prey items. At the Conservancy of Southwest Florida, you may see staff feeding them bits of fish. Their feeding behavior isn’t the only thing that has allowed the structure of the horseshoe crab’s body to remain unchanged for so long.

Horseshoe crabs are also environmental generalists. They usually live in the intertidal zone, not too far from shore. However, they are also able to live in much deeper waters. The ability of the horseshoe crab to live in an extensive geographic range with waters shallow and deep, cool and warm is what makes them an evolutionary success story.

Horseshoe crabs play an important role as a food source for many animals. Migratory birds, such as the red knot, stop along Florida’s beaches gobbling up the small blue horseshoe crab eggs they find along the shore. Adult loggerhead sea turtles use their strong beaks to crush the horseshoe crab shell and eat the meat inside. They are also a key multiple-use recourse for us. Historically they were used as fertilizer and by commercial fisheries as bait, but now they have a much more important role. Biomedical companies now use horseshoe crabs; more specifically, it’s their blue blood that’s used. Horseshoe crab blood contains the clotting agent Limulus Amebocyte Lysate, LAL for short. LAL can detect the minutest amounts of bacterial toxins, making it invaluable to the pharmaceutical industry.

A volunteer presents the underside of a horseshoe crab to guests. Showing off its ten legs, mouth, and book gills. Staff and volunteers at the Conservancy are trained to properly handle the animals at our touch tank.

At the Conservancy we have four horseshoe crab ambassadors that allow guests to get hands-on experience and a closer look at the anatomy of these prehistoric animals. Do not be too surprised if you don’t see them right away; they spend most of their time buried in the sand. A member of our staff can help you find one.

We can all take actions that will help ensure that the horseshoe crab continues to survive for thousands of more years, allowing it to continue benefiting the many species that rely on it. Start by cleaning up and restoring the sandy beaches that horseshoe crabs use for spawning. Studies have shown that female horseshoe crabs may avoid nesting on eroded beaches that lack natural nurseries, such as mangroves. In Florida, horseshoe crab spawning often occurs near Indian River, Seashore Key, and the around the Florida panhandle. Most adult horseshoe crab deaths occur due to beach stranding. If you happen to find a live horseshoe crab or accidentally catch one, release it back to the ocean. Haven’t seen a horseshoe crab yet? Visiting the Conservancy of Southwest Florida will allow you to get a closer look at one and support environmental conservation efforts!