The Strange Nature of Horseshoe Crabs

Covered in eyes and sensors, their blood has served medicine for decades

By Dana Wilde

Ages ago in the sea-green clear water of Chandler’s Cove on Chebeague Island, strange dark shapes were scuttling around the bottom. My seven-year-old face peeked out over the edge of the wharf, watching them. I remember my hands gripping the splintery planks.

Horseshoe crabs, the big people called them. Gliding along the sand down there in gangs. They didn’t look like horseshoes. The name puzzled me, the way you’re not sure what to think when someone tells you the bright star off Orion’s foot is a dog’s shoulder. It does not look like a dog. In the beginning, anyway. Horseshoe crabs were shaped only vaguely like horseshoes and definitely were not crabs, which run sideways on dangerous-looking legs and do not have tails like big stingers.

Well, more than a half-century later, I clearly see a dog bounding along behind Orion, and Limulus polyphemus’ rounded, ridged shell is close enough to horseshoe-shape that I can see that too. But the deeper they get in time, the stranger they seem.

horseshoe crab

Horseshoe crabs are among the most ancient of still-living beings. The earliest horseshoe crab fossil is from about 445 million years ago, which means they scuttled across the floors of Earth’s silent seas roughly 345 million years before any flower blossomed. They are the closest living relatives of the trilobites, who flourished during the first evolutionary explosion of multi-celled animals about 530 million years ago. And still living today as Limulus polyphemus, the Atlantic horseshoe crab, from Maine down the East Coast and the Gulf of Mexico to the Yucatan Peninsula, and as three other species in the Asian Pacific (Tachypleus gigas, Tachypleus tridentatus, and Carcinoscorpius rotundicauda).

Their hard shells led early biologists to classify them as crabs, but it’s been suspected for decades that horseshoe crabs are more closely related to spiders than to crustaceans such as lobsters and crabs. And in fact, a recent genetic study established that horseshoe crabs are, in fact, arachnids, meaning they’re aquatic evolutionary cousins of spiders, and now classified as a sister group to hooded tick spiders (aka ricinuleids, spiderlike relatives of ticks and mites).

Anatomically they are a living fascination. They’re the only marine animal known to breathe with book lungs, similar to those of spiders. Like other arachnids they have two body parts. The front is the prosoma (or cephalothorax) covered by the carapace, or hard, ridged shell, and underneath is its mouth flanked by ten legs (spiders have eight) for scuttling along the bottom. Hinged to the prosoma is the opisthosoma (or abdomen). Unlike other arachnids, horseshoe crabs have a long tail known as a telson, which is not a stinger at all but functions as a sort of rudder and as a lever the horseshoe crab uses to right itself if it gets overturned.

Horseshoe crabs also have a lot of eyes, including photo-receptors on the telson and seven eyes atop the carapace. The two lateral eyes are located about midway along the shell, either side of the central ridge. These are compound eyes, meaning they’re made up of about 1,000 individual receptors, or ommatidia, that collect light. Human eyes are simple rather than compound, meaning light is collected through one receptor, or adjustable lens. Insects such as dragonflies have compound eyes, but horseshoe crabs’ lateral eyes are unusual because they contain rods and cones similar in structure to that of humans. (In 1967 the Nobel prize in medicine was awarded for studies involving similarities between horseshoe crab and human eyes.) The two lateral eyes are used mainly during spawning, and their ommatidia can adjust to daytime and nighttime light conditions to help locate mates.

Prehistoric horseshoe crab walks out of the water and onto the sand beach leaving a trail

A rudimentary eye, used primarily by the embryo inside its egg, is located just behind each lateral median eye. At the front of the carapace are three eyes that can pick up ultraviolet light, which human eyes can’t, and that have special sensitivity to moonlight, which is important during  spawning. Underneath, at the middle of the cephalothorax near the mouth and legs, are two more eyes whose function is not understood.

Along the top of the telson are arrays of simple photoreceptors, whose purpose also is not well understood, but they may help the animals synchronize light and dark cycles.

In fact, horseshoe crabs have sensory receptors all over their bodies, and those in the legs and two chelicerae (the small crablike claws used to place food such as worms, clams, even algae, in their mouths) can detect the chemical composition of seawater.

Seawater, of course, is vitally important to horseshoe crabs. Their migration cycle in Maine waters begins in spring, when the adults come in from deeper to shallower water (typically about ten to 12 feet, as under the Chandler’s Cove wharf). In May and June they make their way onto estuarine beaches to spawn, with the activity peaking on the full and new moon tides.

They typically come ashore as “amplexed mated pairs,” meaning the male is grasping the female from behind. She burrows into the sand to lay thousands of eggs. While she’s doing this, other males who are not clasping her may circulate nearby, releasing sperm in hopes of fertilizing the eggs too. This may happen over several nights and yield up to 100,000 eggs from a single female. The eggs hatch in two to four weeks, and the young horseshoe crabs head for the water during a high tide a month or so later.

painting of a horseshoe crab from above and from below

The young horseshoe crabs look like miniature versions of the adults. They spend the first few years of their lives on tidal flats and in shallower water, molting toward adulthood. Meanwhile, around September, the adults begin to make their way to deeper water for the winter. Horseshoe crabs can live to be 20 years old or more.

Strangely, horseshoe crabs have no immune system. In other words, they don’t produce antibodies to fight infections. Instead, they make chemicals that inactivate bacteria and viruses and that create clots to seal out further intruders. Researchers decades ago figured out how to use horseshoe blood (which is blue because of its copper content) to detect bacterial contamination, and it’s now used practically universally in medical serums, vaccines, intravenous drips, and even implant devices.

Horseshoe blood is harvested by capturing individuals and drawing it from their bodies with needles. Typically the animals are then returned to their waters, under the assumption that the horseshoe crabs are not permanently harmed. But studies have shown that in fact, tens of thousands of horseshoe crabs die despite harvesters’ efforts. Recently there has been a push for the medical community to use synthetic horseshoe blood, which has been around for more than a decade, and stop the harvest over conservation concerns.

Medical industry and bait harvests are playing a role in a concerning decline in horseshoe crab populations. Most reports indicate that Limulus polyphemus numbers overall have been dwindling significantly; at least one study concluded that populations south of roughly New England seem more or less stable, but they are declining in the Northeast. The Maine Wildlife Action plan of 2015 marks L. polyphemus as a Priority 1 Species of Greatest Conservation Need, listing among its threats unintentional catch by commercial trawling and agricultural, forestry, and other effluents. Other detrimental factors are habitat alteration (due, for example, to sea level rise and ocean acidification); storms and flooding (in which storms occurring during mating and egg laying result in mass strandings and loss of eggs); and temperature extremes. The Gulf of Maine is one of the fastest-warming bodies of salt water on Earth.

According to the Atlantic States Marine Fisheries Commission, which oversees the horseshoe crab fishery, 131,101 were landed for commercial bait in Maine, Massachusetts, and Rhode Island in 2016. The commission reports that along the East Coast, 575,760 horseshoe crabs were collected and brought to biomedical facilities in 2017, an estimated 15 percent of which subsequently died. The Maine Department of Marine Resources says it has no reports of horseshoe crab landings in Maine for the last ten years.

In Maine, a permit is required to harvest horseshoe crabs, with a daily limit of 25 per person. Possessing or damaging horseshoe crabs or their eggs is prohibited in Maine from
May 1 to Oct. 31.

horseshoe crabs in water

I wasn’t thinking about conservation when I was seven years old, peering into the green water beneath the Chandler’s Cove wharf at the strange, strange creatures on the bottom. It seems like I already knew, somehow, that horseshoe crabs were ancient beyond ancient. That they had survived, if I could have known, the mass extinctions of life on Earth 250 million and 66 million years ago.

I thought they had been there for some kind of forever. And probably I thought, deep in the beginning of my own evolutionary innocence, that they’d be there forever yet, like starlight. I hope so.

Dana Wilde is a columnist, book reviewer, former news editor and college professor. His recent books are A Backyard Book of Spiders in Maine; Summer to Fall: Notes and Numina in the Maine Woods; and Nebulae: A Backyard Cosmography. He lives in Troy.