Horseshoe Crabs
If you’ve ever walked along a Mid-Atlantic beach in late spring and stumbled upon something that looks more like a prehistoric helmet than a living animal, chances are you’ve met a horseshoe crab. These creatures might have been friends with dinosaurs, as they were around 450 million years ago, though dinos didn’t come around till about 170 million years later. They’ve become one of those species we can’t stop paying attention to, partly because of how strange they look, and partly because of how essential they are to coastal ecosystems.
What Horseshoe Crabs Eat
Horseshoe crabs are bottom feeders, and spend most of their lives foraging along shallow ocean floors. Their diet is made up largely of small invertebrates like marine worms, clams, snails, and crustaceans, along with decaying fish. They also ingest algae and organic debris as they sift through sediment. This feeding behavior helps recycle nutrients and keeps benthic ecosystems in balance, even though it often goes unnoticed by the humans walking above them.
Where to Find Them (Especially in Maryland)
Along the Atlantic coast of North America, horseshoe crabs are most visible during their breeding season, which runs from late spring into summer. Here in Maryland, beaches and coastal wetlands become hotspots for activity. Places like Assateague Island, Ocean City, and Sandy Point State Park or Terrapin Nature Park are all areas where you’re likely to encounter them during this time of year.
Mating season is when horseshoe crabs come ashore to lay their eggs, and this annual event is about more than just reproduction. The eggs they deposit in the sand become a crucial food source for migratory shorebirds, many of which rely on these high-energy meals to fuel long journeys. In other words, protecting horseshoe crabs also means protecting the birds that depend on them.
Quite the Unconventional Body
Let’s be honest: horseshoe crabs look a little odd. Their domed shells are shaped—unsurprisingly—like horseshoes and are usually a muted brown that blends easily into sand and mud. Around the edges of the shell are small spines, and extending from the back is a long, pointed tail called a telson. Despite how it looks, the telson isn’t a weapon; it’s used for balance and righting themselves if they get flipped over.
What to Do If You Find One on the Beach
If you come across a live horseshoe crab stranded upside down, it’s worth taking a closer look—briefly. Flip it over only if it seems unable to right itself, and do so by gently turning it sideways, never by grabbing the tail. Pulling on the telson can seriously injure the animal.
While you’re down there, you might notice something else fascinating: their ten legs. Males have modified front legs that resemble tiny boxing gloves, which they use to latch onto females during mating. Females, on the other hand, have more uniform, spider-like legs. It’s a small detail we like, because even if you haven’t seen the underside of a horseshoe crab, it’s fun to think about their little legs. And to imagine male horseshoe crabs boxing each other. WWE but for crabs.
A Species Under Threat
Despite having survived for hundreds of millions of years, horseshoe crabs are now facing decline. Conservation organizations classify them as vulnerable, citing a combination of overharvesting, accidental capture in fishing operations, and the loss of suitable habitat due to shoreline development. On top of that, climate change is reshaping coastlines through rising sea levels and increasingly severe storms, putting their spawning beaches at risk.
Their decline doesn’t happen in isolation. Horseshoe crabs are considered a keystone species, meaning their absence has a disproportionate impact on the ecosystem. Migratory shorebirds, such as piping plovers, depend heavily on horseshoe crab eggs during critical stages of their migration. As crab populations fall, birds feel the consequences.
A Childhood Memory
When I was eleven, I had the chance to help tag horseshoe crabs with my grandparents, and that’s when I first encountered their most surprising feature: bright blue blood. It jumped out at me at the time: both literally and figuratively. Unlike humans, whose blood relies on iron to carry oxygen, horseshoe crabs use copper-based hemocyanin, which gives their blood that distinctive blue color.
Years later, at the National Aquarium in Baltimore, I learned that this unusual blood does far more than transport oxygen. It reacts almost instantly to bacterial toxins, clotting around them in a way that makes it invaluable for testing medical equipment and vaccines for contamination. Nearly every injectable drug you’ve ever heard of has, at some point, depended on horseshoe crab blood for safety testing. The problem is that harvesting this blood places significant stress on wild populations. While alternatives are now being developed in labs, the transition has been slow, and the pressure on the species continues.
Conservation Efforts and Why They Matter
Efforts to protect horseshoe crabs focus on several key strategies. Regulations that limit or prohibit harvesting—particularly of females—are essential for stabilizing populations. Conservation groups are also working to restore and create suitable spawning habitats along developed shorelines. Managing human disturbance on busy beaches, especially during peak spawning and migration periods, helps reduce stress on both crabs and birds.
Public education plays a major role as well. The more people understand how interconnected these species are, the more likely they are to act responsibly when sharing coastal spaces. Something as simple as knowing how to properly help a stranded horseshoe crab can make a real difference.