If you’ve never encountered or learned about horseshoe crabs before, I have a treat for you. One of my absolute favorite marine species, these oddly charming marine arthropods get their name from their horseshoe-shaped exoskeleton, called a carapace. I’ve always thought that the exterior appearance of these animals was one of their most fascinating qualities, but it turns out there’s so much more to them than meets the eye. Read on to find out some of the most mind-blowing facts about one of our ocean’s most intriguing bottom-dwellers: the horseshoe crab.
They’re living fossils.
These ocean critters predate the dinosaurs! The oldest known horseshoe crab species, (Lunataspis aurora) was discovered by scientists in 2008 and is estimated to be nearly 450 million years old. While they’ve certainly experienced a few evolutionary adaptations, their physiology has remained largely unchanged over time, which is why they’re so often referred to as ‘living fossils.’ Four species are known to be alive today, and only one calls American waters home: the Atlantic horseshoe crab (Limulus polyphemus). This species can be found from Maine all the way to Mexico, typically populating deeper waters as adults but traveling to shorelines each year to spawn.
No, they’re not actually crabs.
Horseshoe crabs are much more closely related to arachnids like spiders and ticks than true crabs. They’re part of the Subphylum Chelicerata and are further classified into Class Merostomata, which includes horseshoe crab species as well as eurypterids (sea scorpions). Interestingly, the term Merostomata translates to “legs attached to the mouth.” Speaking of which…
They’ve got legs near their mouth.
While their primary sets of legs are used to help them move around, the pair of legs that’s closest to their mouth is actually a set of pincers, which they use to move food closer to their mouth and crush it up to be easier to consume. If you were wondering, some of their favorite foods are sea worms, mollusks and crustaceans. Adults tend to opt for bivalves once they’ve matured in size, but youngsters tend to like to go for the easy-to-maneuver marine worms.
They’re not dangerous.
While many people mistakenly associate these animals with rays or skates because of their shape, they’re actually not related and aren’t dangerous at all. Their tails, called telsons, aren’t poisonous or venomous; they’re mainly used to steer and flip themselves over in case they get stuck on their backs.
They can swim upside-down.
Flap-like structures near their abdomen, called book gills, enable horseshoe crabs to breathe underwater … but sometimes, they use them for the goofier purpose of swimming upside-down. It’s mainly the youngsters that engage in this behavior, but it’s entertaining to watch nonetheless.
Spawning season is an absolute spectacle.
Every spring when the tide is high, horseshoe crabs travel up to the shoreline to mate and lay their eggs at night. One of the most famous locations of these “love fests” occurs in Delaware Bay every year, where hundreds of thousands of these ancient critters can be seen on the shore. When it’s time to lay her eggs, a female horseshoe crab will deposit somewhere around 4,000 eggs in clusters. She’ll do this several times, laying up to around 20,000 eggs in one night and up to some 100,000 in each meeting season. That’s alot of horseshoe crab babies!
Their eggs are a main food source for many coastal species.
While the number of eggs a horseshoe crab mom lays is jaw-dropping, most of the eggs will not survive to adulthood, or even to their hatching point. Scientists calculate that a very small percentage make it to maturity because the eggs serve as an important source of food for wildlife like migratory birds, sea turtles and some fish species. While Atlantic horseshoe crab populations are currently listed as vulnerable by the IUCN (International Union for the Conservation of Union)’s Red List of Endangered Species, this listing is primarily a result of anthropogenic (human-caused) sources and habitat loss, and not as much a result of natural predation by other traditional species.
They can see ridiculously well.
With 9 eyes in total along with various light receptors near their telson, these creatures have insanely good vision. They have different core purposes, too; their two lateral compound eyes are often used to help locate potential mates, while their smaller median set is sensitive to UV light. The others are useful in detecting movement nearby as well as changes to surrounding light.
They’re all about personal growth.
Horseshoe crabs don’t reach full maturity until approximately age ten, and while they do experience consistent growth over those first ten years, their carapace (their tough exoskeleton that makes them look like tanks) doesn’t actually grow with them. Because of this, kiddo horseshoe crabs have to molt … and do so up to an average of 16 times before reaching maturity. Once they’re grown-ups, it’s unusual for them to shed their exoskeleton. Before they reach maturity, I guess they’re constantly shedding ‘who they were before’ to grow into who they were meant to be!
They’re medical heroes.
Hang on tight, because we’re going for a wild science ride. Firstly, horseshoe crabs don’t have white blood cells like we do to help fight off infection. Instead, they have what’s called amebocytes, which are feisty little cells that attack pathogens (the bad guys in the cellular world, which cause disease) by trapping them in what’s basically a wall of goo. This keeps the disease from spreading throughout the organism. Why is this important for modern-day medicine, though? In the late 1950s, a physician at Johns Hopkins discovered that you can use the amoebocytes found in horseshoe crab blood to test the safety of vaccines and other drugs. Essentially, amebocytes are added to the contents of the vaccine or drug, and if they excrete their gooey defense mechanism, it means the product isn’t ready for use yet, because the specimen contains bacteria encountered by the amebocytes. If it weren’t for horseshoe crabs and this discovery, who knows how many people may have been harmed from drugs doctors otherwise would have thought were ready for administration!
They’re super helpful to scientists,
but they could also use scientists’ help.
While the medical bleeding process is for the most part not harmful to horseshoe crabs, scientists are still hoping to work toward using synthetic amebocytes in the future so they won’t have to put horseshoe crabs through the process of extraction. The animals go back into the water mostly unharmed after nearly 30% of their blood is drained over the course of two days, but not all horseshoe crabs make it back healthy. Research shows various data reporting that between 10% and 30% of horseshoe crabs that are used for medical bleeding don’t survive once returned to the water, and some that do survive don’t return to exhibiting healthy behavior, often appearing lethargic with a decreased ability to thrive. The process is a careful one where scientists do all they can to make sure the animals are unharmed, but they still hope to move to a synthetic option that replicates these original cells so horseshoe crabs won’t be subject to the stress that may come from the practice.
All in all, I honestly think horseshoe crabs are some of the most fascinating and resilient species on Earth. From their ancient-looking exteriors to the part they’ve played in medical advances, their preservation proves important to both the health of the ecosystems they inhabit as well as factors far beyond their own homes.
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FAQs
How much blood is in a horseshoe crab? ›
Blood is collected by direct cardiac puncture under conditions that minimize contamination by lipopolysaccharide (a.k.a., endotoxin, LPS), a product of the Gram-negative bacteria. A large animal can yield 200 - 400 mL of blood.
What is the biggest threat to horseshoe crabs? ›The world's horseshoe crab populations are imperiled, because of overharvesting for use as food, bait and biomedical testing, and because of habitat loss from coastal reclamation and development.
What is special about a horseshoe crab? ›Horseshoe crabs are also extremely important to the biomedical industry because their unique, copper-based blue blood contains a substance called "Limulus Amebocyte Lysate", or "LAL".
Why is horseshoe crab blood blue? ›Why is the Horseshoe Crab the original "blue blood"? A horseshoe crab's blood has a blue to blue-green color when exposed to the air. The blood is blue because it contains a copper-based respiratory pigment called hemocyanin.
Are horseshoe crabs poisonous? ›It's long and pointed, and although it looks intimidating, it is not dangerous, poisonous, or used to sting. Horseshoe crabs use the telson to flip themselves over if they happen to be pushed on their backs.
Do crabs have blood? ›Yes, most of the crabs have blue-shaded blood. The blue color results from a copper-containing protein called hemocyanin, which is present in the blood of many invertebrates.
Are horseshoe crabs safe to touch? ›When handled properly horseshoe crabs are harmless. Grasp both sides of the shell of the head portion of the animal, pick the animal up, and set it down with its legs facing the sand. Don't ever pick up a horseshoe crab by its tail! You risk damaging its tail and it won't be able to flip itself over in the future.
Are horseshoe crabs killed for their blood? ›Horseshoe crabs are not intended to be killed when they are used for their blood, however, this does not mean it doesn't occur. After being drained of nearly a third of their blood, the animals are supposed to be released back into the ocean. When this does occur, the sea creatures are not necessarily able to survive.
Do crabs hurt you? ›even if you were find a crab, and if it were big enough to pinch you, and you managed to corner it and aggravate it enough to pinch you....all you would have is a painful pinch. Crabs aren't aggressive nor poisonous -- nor can they hurt you badly enough to even seek medical attention.
How old is the oldest horseshoe crab? ›The oldest known horseshoe crab species, (Lunataspis aurora) was discovered by scientists in 2008 and is estimated to be nearly 450 million years old.
How big was the biggest horseshoe crab? ›
The largest horseshoe crab the museum confirmed in Japan to date measured 63 cm in length and weighed about 3.5 kilograms. "There's a high possibility that this one is the biggest," an official from the museum said.
Do crabs have hearts? ›A powerful heart is clearly a necessity to move blood throughout larger crustaceans such as crabs and lobsters.
What animal has purple blood? ›What animal has purple blood? Peanut worms, which are a kind of marine worm, have purple blood. This is due to the presence of hemerythrin, an oxygen-binding protein.
What bug has purple blood? ›| Cochineal | |
|---|---|
| Order: | Hemiptera |
| Family: | Dactylopiidae |
| Genus: | Dactylopius |
| Species: | D. coccus |
They're not really meaty, but you can eat their roe, which apparently tastes like briny rubber.
Can horseshoe crabs swim? ›Horseshoe crabs swim upside down and can survive out of the water for an extended period of time if their gills are kept moist.
Do horseshoe crabs have eyes? ›Horseshoe crabs have a total of 10 eyes used for finding mates and sensing light. The most obvious eyes are the 2 lateral compound eyes. These are used for finding mates during the spawning season. Each compound eye has about 1,000 receptors or ommatidia.
What happens if a crab pinches you? ›Their sharp and strong grip can be quite painful, as anyone who has ever been pinched by one can confirm. And if threatened, a crab may break off a claw or leg to try to escape predators; the limb will later regrow through a process called regeneration.
What is the color of crab blood? ›The blood of a horseshoe crab is blue because of a mol- ecule called a respiratory pig- ment. In humans, this molecule contains iron, while the crab version contains copper, and that affects the color of blood. A respiratory pigment binds to oxygen and carries it around the body, keeping organs and tissues oxygenated.
Do crabs have teeth? ›Lobsters and crabs have teeth— in their stomachs. These are used to crush its food, but they also have a strange secondary function in ghost crabs: making a noise that wards off predators.
Do crabs have brains? ›
The nervous system of a crab differs from that of vertebrates (mammals, birds, fish, etc.) in that it has a dorsal ganglion (brain) and a ventral ganglion. The two nervous centers are connected by a circumesophageal ganglion, i.e., it circles the esophagus.
Are horseshoe crabs painful? ›The spikes on the horseshoe crab's shell are also not dangerous, although they can hurt. The spikes help protect the horseshoe crab from predators because they make the crab look less appetizing. What is this? Like most crabs, horseshoe crabs have pincers and a mouth, but neither can hurt humans.
What eats horseshoe crabs? ›Adult horseshoe crabs are preyed upon by sharks, sea turtles, gulls and humans for use as bait or fertilizer.
Can I sell horseshoe crab blood? ›The blood of horseshoe crabs is harvested on a massive scale in order to retrieve a cell critical to medical research. However, recent innovations might make this practice obsolete. Horseshoe crabs' blue blood is so valuable that a quart of it can be sold for $15,000.
How much is a gallon of horseshoe crab blood? ›Horseshoe crabs, a 450-million-year-old living fossil, are on the verge of extinction, according to conservationists, due to the pharmaceutical industry's need for their blood.
Why do crabs have blue blood? ›Horseshoe crabs have a different oxygen-carrying protein, called hemocyanin. Hemocyanin has copper rather than iron, which gives horseshoe crab blood its bright blue color.
How many horseshoe crabs are left? ›In the Delaware Bay, home to the largest population in the US, horseshoe crab numbers declined from 1.24 million in 1990 to less than 334,000 in 2002. In more recent years, the population appears to have stabilized, growing steadily and reaching approximately 725,000 in 2019.
Can crabs break skin? ›If your crabby encounter involves broken skin, such as a cut you got while you cleaned your crab, you should proceed with caution. People are at risk to develop blood poisoning or necrotizing fasciitis -- ghoulishly known as flesh eating bacteria -- if bacteria enters a wound from some seafood or seawater.
Are crab bite poisonous? ›even if you were find a crab, and if it were big enough to pinch you, and you managed to corner it and aggravate it enough to pinch you....all you would have is a painful pinch. Crabs aren't aggressive nor poisonous -- nor can they hurt you badly enough to even seek medical attention.
Can lobster hurt you? ›Lobsters and other shellfish have harmful bacteria naturally present in their flesh. Once the lobster is dead, these bacteria can rapidly multiply and release toxins that may not be destroyed by cooking. You therefore minimise the chance of food poisoning by cooking the lobster alive.
Why is horseshoe crab blood so valuable? ›
Horseshoe crab blood is a vital resource to the medical field. It's unique in more ways than one: the blue color and its ability to identify bacterial contamination in small quantities. Horseshoe crab blood contains a special amebocyte that is separated and then used in FDA testing.
Are horseshoe crabs killed for their blood? ›Horseshoe crabs are not intended to be killed when they are used for their blood, however, this does not mean it doesn't occur. After being drained of nearly a third of their blood, the animals are supposed to be released back into the ocean. When this does occur, the sea creatures are not necessarily able to survive.
Can I sell horseshoe crab blood? ›The blood of horseshoe crabs is harvested on a massive scale in order to retrieve a cell critical to medical research. However, recent innovations might make this practice obsolete. Horseshoe crabs' blue blood is so valuable that a quart of it can be sold for $15,000.
Can you eat a horseshoe crab? ›They're not really meaty, but you can eat their roe, which apparently tastes like briny rubber.
Why do crabs have blue blood? ›Horseshoe crabs have a different oxygen-carrying protein, called hemocyanin. Hemocyanin has copper rather than iron, which gives horseshoe crab blood its bright blue color.
Do horseshoe crabs have eyes? ›Horseshoe crabs have a total of 10 eyes used for finding mates and sensing light. The most obvious eyes are the 2 lateral compound eyes. These are used for finding mates during the spawning season. Each compound eye has about 1,000 receptors or ommatidia.
How many horseshoe crabs are left? ›In the Delaware Bay, home to the largest population in the US, horseshoe crab numbers declined from 1.24 million in 1990 to less than 334,000 in 2002. In more recent years, the population appears to have stabilized, growing steadily and reaching approximately 725,000 in 2019.
Do crabs have hearts? ›A powerful heart is clearly a necessity to move blood throughout larger crustaceans such as crabs and lobsters.
Do crabs have brains? ›The nervous system of a crab differs from that of vertebrates (mammals, birds, fish, etc.) in that it has a dorsal ganglion (brain) and a ventral ganglion. The two nervous centers are connected by a circumesophageal ganglion, i.e., it circles the esophagus.
Do crabs feel pain? ›A longstanding related question: Do they feel pain? Yes, researchers now say. Not only do crabs suffer pain, a new study found, but they retain a memory of it (assuming they aren't already dead on your dinner plate). The scientists say its time for new laws to consider the suffering of all crustaceans.
Who has blue blood? ›
Can you guess what animals might have blue blood? Lobsters, crabs, pillbugs, shrimp, octopus, crayfish, scallops, barnacles, snails, small worms (except earthworms), clams, squid, slugs, mussels, horseshoe crabs, most spiders.
What eats horseshoe crabs? ›Adult horseshoe crabs are preyed upon by sharks, sea turtles, gulls and humans for use as bait or fertilizer.
How much is a gallon of horseshoe crab blood? ›Horseshoe crabs, a 450-million-year-old living fossil, are on the verge of extinction, according to conservationists, due to the pharmaceutical industry's need for their blood.
Did horseshoe crabs evolve? ›Fossils of horseshoe crabs have been dated at 445 million years old. They evolved in the shallow seas of the Paleozoic Era (540-248 million years ago) with other primitive arthropods called trilobites, a long extinct close relative of the horseshoe crab.
How do you get the smell out of a horseshoe crab? ›Soak the crab in diluted bleach for about half an hour. You should use about half a cup of bleach for every three gallons of water that you soak the crab in.
Are horseshoe crabs Endangered 2022? ›Out of the four extant horseshoe crab species left on the planet today, only the tri-spine horseshoe crab found along the coast of India, Southeast Asia, China, and Japan, is classified as Endangered by the International Union for Conservation of Nature (IUCN).