I can still remember the first time I saw a horseshoe crab (Limulus polyphemus) in the wild. I didn’t grow up on the coast, and owe much of my early and deep love of marine ecosystems to a brave and energetic elementary school science teacher. When I was in the 4th grade, she led a couple dozen of us on a field trip to the Mississippi Gulf Coast. The highlight of the trip was a ferry ride out to Ship Island. After disembarking the boat, we walked out onto a boardwalk that led to a fort. Below us in the sand, I was blown away by the sheer number of horseshoe crabs. There must have been thousands. Only a few were moving, and I learned soon that most of what I saw were horseshoe crab molts. Like every other arthropod—crabs, insects, spiders, etc.—to grow larger, the horseshoe crab must shed its exoskeleton and grow a new one. As a 9-year-old future scientist, I was fully captivated by such an extraordinary sight.
These ancient creatures have survived for a very, very long time. Fossil records show horseshoe crabs in their current form from 450 million years ago. Their sturdy “tank-like” shells with the serrated edges and intimidating tail ensure most creatures leave it alone. While the tail, called a telson, looks dangerous, it’s used primarily to flip itself over in case the crab lands on its back, or as a rudder when swimming. I will admit their five pairs of walking legs are a bit cockroachy; the first time my son (also probably 9 or so) saw the underside of one with its segmented, wiggling appendages, he screamed. I think he expected a body more like a stingray, so he was equally surprised and grossed out!
Horseshoe crabs play an important role in human health. An enzyme in their blood (which is bright blue!) reacts to bacterial exposure, so medical companies utilize this enzyme to test vaccines and other medical interventions to prevent infection in people. Currently, horseshoe crabs are taken from the wild as blood donors, then returned to their native habitat. Not every organism survives this process, though, so there are efforts to find a new way. Research on a synthetic version of the enzyme, along with efforts to create an aquaculture industry for horseshoe crabs, could reduce the impacts on wild populations. The tens of thousands of fatty, nutrient-rich eggs horseshoe crabs lay on beaches are also an important component of the coastal food web, providing nourishment for migrating and local birds. Horseshoe crabs can live to be 20 years old in the wild, but don’t reach maturity until age 10. This extended life span means they are vulnerable for a longer time.
Around here, we don’t see so many horseshoe crabs. Northwest Florida has never had massive numbers of them, but coastal shoreline development has led to reduced numbers of horseshoe crabs all over the state. Our Sea Grant Agent, Rick O’Connor, has volunteers that go out at peak mating season (May/June, September/October) during the high tide and full moon to look for them while they are active. When mating, larger females with male mates riding her coattails or hovering nearby will swim to the beach to lay eggs and spawn. A statewide tagging effort, called the Horseshoe Crab Watch, has been in place just over 10 years to keep records on the health of the wider population in Florida. This collaboration between the Florida Fish and Wildlife Conservation Commission, UF, and Florida Sea Grant entails walking beaches, taking photos and data, and tagging some of the crabs for research purposes. If you’re interested in becoming a citizen scientist and being “Linked with Limulus,” check the official page for annual collection data and more information on the monitoring program.
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