Male and female horseshoe crabs emerge from the sea to spawn. Photo courtesy of USFWS.
They have ten eyes, hearts half the length of their bodies, breathe without lungs, chew without jaws, and cerulean blue blood that may have saved your life. With a hard shell and spiked tail, they resemble vintage Army helmets. They have been on this planet for 450 million years, long before dinosaurs walked the earth. Scientists call them living fossils, as they’ve remained virtually unchanged since their story began.
Native Americans used their tails for hunting and shells as serving bowls. In the past two centuries, millions have been ground up for fertilizer, used commercially for bait, and bashed in by watermen who consider them competitors for clams. Their eggs provide a rich source of food for migratory shorebirds, and their blood tests for medical contaminants. Meet Limulus Polyphemus, the American horseshoe crab.
In fact it isn’t a crab at all, but an arthropod; its closest living relatives are ticks, spiders, and scorpions. Fossil hunters and archeologists have found their encased remains in many parts of the world, but just four species survive today, and only one lives along the North American coast, from Maine to the Yucatan.
Massive horseshoe crab stranding. Photo courtesy of Jessica Quinn.
On a blustery May morning we arrived at our favorite Chesapeake Bay beach, anticipating a long solitary stroll, but discovered we were not alone. As far as the eye could see, dozens of horseshoe crabs lay stranded upside down. Some were already dead, others dying, the rest struggling to right themselves. For the next hour we prodded, flipped, and carried the stranded animals back into the surf. We mourned the dead and celebrated the living, watching them slowly crawl back into the water and disappear beneath the waves.
Horseshoe crab fossil.
For millennia, horseshoe crabs have emerged from the shallow waters of the Atlantic to spawn. Timing varies from early spring in the Southeast to late May further north. Author Anthony Fredericks describes the spring spawning as “a family reunion on steroids”.
In Delaware Bay, the epicenter of horseshoe crab spawning, volunteers for ‘Just Flip ‘Em’ assist thousands of stranded animals every spring. In New Jersey, ‘Return the Favor’ encourages communities along the Jersey shore to rescue horseshoe crabs stranded on their beaches or trapped behind riprap.
For most of the year they inhabit the bottoms of bays and shallow estuaries, feeding on worms and shellfish. But for a brief period of time they return to the beach, engaging in a mating frenzy of epic proportions. The flurry of activity occurs mostly at night or at dawn and dusk. Weather, water temperature, and surf conditions can have a profound effect on where and when they come ashore.
The large females, some the size of dinner plates weighing upwards of ten pounds, have no lack of suitors as the smaller males attach and vie for the prize. Once a male latches onto a female, she tows him up onto the beach where she burrows into the damp sand to make her nest. Digging down four to six inches, she uses her pusher legs to create a slurry of sand and water into which she releases a clutch of eggs, which the male fertilizes. She then moves forward, depositing another clutch. Each clutch takes approximately eight minutes to release, and she can lay as many as 88,000 eggs in a single season.
Rufa Red Knots gorging on horseshoe crab roe. Photo courtesy of USFWS.
Shorebirds and small mammals await this largesse, gorging on the roe that has been deposited too shallow or accidently dug up by other spawning horseshoe crabs. As eggs wash into the surf, other predators await, creating a vital link in the food chain that ensures the survival of numerous species. Scientists estimate that less than one egg out of 130,000 survives to adulthood.
The soft, gelatinous eggs soon harden. Sunshine, sand, and seawater keep them incubated for two to four weeks. For the first few days the horseshoe embryos are invisible to the naked eye, but the eggs gradually swell until their outer shells burst. At this stage, tiny, tailless, horseshoe crabs growing within their chitin shells will shed their casings four times within each egg. The eggshells soon split open, spilling the tiny larvae into the nest. During the next high tide, untold numbers wash into the bay.
A short paddle and they settle to the bottom, burrowing into the soft mud where they dine on rich worms. In their first year they will molt five or six times, at which time they will resemble their adult relatives, measuring about two inches across. The tiny horseshoe crabs spend most of their time eating and growing, molting several times a year during their first three years. It takes seventeen or eighteen molts over a period of seven to ten years to reach adulthood.
The first thing most people notice about a horseshoe crab is its shell. Its body armor is comprised of a thick, flexible, chitin-rich shell that shields it from most predators. The curved shell allows it to plow through the bay bottom like a miniature bulldozer, as it hunts for algae, marine worms, small clams, and dead fish.
Horseshoe crabs have three main body parts. The first, which gives the crab its distinctive horseshoe shape, is called the cephalothorax, which is hinged to the midsection or abdomen. This hinge allows the body to bend and flex as it swims. The third section is called the telson, or tail. Unlike other marine creatures with barbed tails, this telson is harmless and helps the crab plow through sand or mud by sweeping from side to side. If a heavy wave should flip the animal onto its back, the telson can also save the horseshoe crab’s life by flipping it upright by powerful thrusts into the sand. Beach strandings, like the one we witnessed, are nature’s number one killer of adult horseshoe crabs.
Underneath, two small pincers serve as feeding grippers. Next come five pairs of walking legs. The back pair is tipped with fanlike structures that aid in burrowing. In the young and in females, the other four pairs of legs end in claws for holding food. In males, the first two legs are tipped with special claws called claspers, which are used to hold onto females during mating. Unlike blue crabs with formidable claws, the horseshoe crab cannot pinch.
Between the legs is a slit that is the horseshoe crab’s mouth. Like fish, they breathe with gills. Each of the ten gills holds stacks of tissue that resemble book pages. To breathe, it flaps its gills, forcing water past the pages, forcing oxygen in the water to pass through the gills and into its bloodstream.
Horseshoe crab's compound eye. Photo courtesy of Jessica Quinn.
Its ten eyes are multifaceted, but researchers stress these eyes “see” in different ways than our own. Two eyes underneath and five on top don’t see images per say, but can sense light and dark. Its two noticeable compound eyes detect movement and shapes in shades of black and white. Some eyes sense ultraviolet light and a group of photoreceptors line the telsons, allowing the crab to “see” behind.
Its most remarkable feature however, is its blue blood. If you have ever been hospitalized, had surgery, been to a medical clinic, received an injection, taken a prescription drug, used contact lenses, or had stitches and are alive and well, you have the horseshoe crab to thank. In the 1960s, researchers discovered that when marine bacteria were injected into the bloodstream of the North American horseshoe crab, massive clotting occurred. Clotting prevented bacteria or endotoxins from infecting an injured crab in water filled with germs. Endotoxins in a human’s bloodstream can cause dangerous infections. A test using a compound of horseshoe crab blood, referred to as LAL (Limulus amebocyte lysate), was developed that can detect one part per trillion endotoxins. This simple and fast test is now used worldwide to check if medicines, blood donations, surgical implants, and medical supplies are clean and safe. So horseshoe crab blood is big business. Unfortunately, the donors of LAL are live animals.
So what does this mean for horseshoe crabs? The subject is highly controversial. There are currently four laboratories on the East Coast certified to manufacture LAL, and these companies need large numbers of live crabs to meet demand. As the horseshoe crabs come ashore to spawn, collectors are waiting.
Laboratory technicians mount each crab into a fixture, pierce the tissue around their hearts, and drain up to 30% of their blood, before returning the crabs to the ocean, miles from where they were picked up to avoid re-bleeding too soon. Industry officials claim mortality is low, between ten and thirty percent.
Scientists and environmentalists claim the death rate is much higher, and many crabs that are bled do not recover quickly. In regions where crabs are heavily harvested, fewer and fewer females are showing up to spawn. For many environmentalists, any laboratory mortality is unacceptable. Synthetic substitutes for LAL have been developed, but are not yet widely used and, for those concerned individuals, an alternative chemical will only be viable once the crab population is gone.
It’s not just the horseshoe crab whose numbers are threatened. In spring, when thousands of shorebirds migrate from South America along the Atlantic Flyway, they arrive famished and exhausted and gorge on horseshoe crab roe before continuing on. Some stay in the Mid-Atlantic and include American oystercatchers, skimmers, sanderlings, ruddy turnstones, and the rufa red knot. Their numbers have plummeted in recent decades, and the red knot is listed as threatened.
To save the red knot, wildlife officials in New Jersey, Delaware, Maryland, and Virginia implemented horseshoe crab harvesting restrictions. Some allow no females to be harvested, while others have a strict quota. But other threats are on the horizon.
As sea levels rise, horseshoe crabs lose sandy beaches in which to lay their eggs. Warming waters affect spawning, plastic trash in the water poses hazards, and pollutants kill shellfish that the crabs rely on for food, as well as killing horseshoe eggs and larvae.
We returned to the beach the next day but the shore was empty, spawning over. But we will return again this May, and if you walk the beach and see a stranded horseshoe crab, remember what this animal has contributed to mankind. Kindly turn it over by its shell, not its telson, and perhaps even send it safely on its way into the life-giving waters from which it came.
Special thanks to Dr. Lawrence Niles, Niles Associates LLC; Debra Reynolds, USFWS; Rom Lipcius, VIMS Professor of Marine Science, and Jessica C. Quinn, photographer.