40 Fun Facts About Horseshoe Crab

Despite the name, horseshoe crabs are not true crabs and are more closely related to which animals?

Spiders, along with ticks and mites, are far closer kin than any lobster. Some genetic studies even place horseshoe crabs inside the arachnid family tree.

Which colour does a horseshoe crab's blood turn when it meets the oxygen in the air?

Blue is the colour it turns in air, while inside the body the blood is closer to grey-white or pale yellow. The protein that carries its oxygen has no colour until oxygen binds to it.

What does a horseshoe crab mainly use its long, spiky tail for?

Flipping itself upright is the tail's main job when a wave rolls the animal onto its back, and it doubles as a rudder. The tail cannot sting, but lifting a crab by it can injure the animal.

What does horseshoe crab blood do on meeting bacterial toxins that makes it vital for testing drugs?

Clots into a gel is the reaction, and even a tiny trace of toxin sets it off. Drug makers use an extract of the blood cells to check that vaccines and injectable medicines are free of contamination.

Which body of water on the US Atlantic coast hosts the world's largest population of horseshoe crabs?

Delaware Bay draws hundreds of thousands of spawning crabs to its shores each May and June. The gathering is so dependable that scientists run a bay-wide spawning survey every year.

Because a horseshoe crab has no jaws or teeth, which body parts does it use to crush its food?

Its legs do the chewing: bristly bases on the walking limbs grind up prey and push the mush toward the mouth. A tougher pair at the back even works like a nutcracker on hard-shelled food.

Which nickname do horseshoe crabs carry for having barely changed since before the dinosaurs?

Living fossils is the label, since forms almost identical to today's animals were already crawling the seabed in the Triassic. Only a handful of species survive from a once much larger group.

Although horseshoe crabs live in the sea, where do females go to lay their thousands of eggs?

Sandy beaches are the nursery: females crawl ashore at high tide around new and full moons and bury clusters of eggs. The young emerge from the nests several weeks later.

When a horseshoe crab swims instead of walking the seabed, in which odd position does it travel?

Upside down is how they swim, tilted at an angle and steering with the tail. Youngsters do it most, as do adults heading for shore to breed.

Three of the four surviving horseshoe crab species live along the coasts of which continent?

Asia has three species, ranging from India to Borneo and beyond. The fourth is the lone American one, found from Maine to Mexico.

Which shorebird plans its long spring migration around a feast of horseshoe crab eggs on US shores?

Red knot flocks arrive thin after flying up from South America and can as much as double their weight on the eggs in about two weeks. They then push on to nest in the Arctic.

Human blood carries oxygen with iron, but which metal does the same job in a horseshoe crab's blood?

Copper sits at the heart of the animal's oxygen-carrying protein, which drifts freely in the blood instead of riding inside cells. Octopuses, squid and many snails use the same chemistry.

Which term names the rigid tail spine of a horseshoe crab and also the sting of a scorpion?

Telson comes from a Greek word for a limit, fitting for the very last division of the body. Lobsters and shrimp have one too, as the core of the tail fan.

Which feature of true crabs and lobsters do horseshoe crabs lack, one reason they are not crustaceans?

Antennae are absent, and so are the grinding mouthparts called mandibles. In their place the animal has a pair of small pincers for moving food into its mouth, as its land-dwelling relatives do.

Newly hatched horseshoe crabs are named after which extinct sea creature that they closely resemble?

Trilobite larvae is the name given to the hatchlings, which are about a centimetre long with only a stub of a tail. They can already swim and burrow as soon as they leave the egg.

Research on which sense of the horseshoe crab helped Haldan Keffer Hartline share a 1967 Nobel Prize?

Sight was the sense Hartline probed, recording signals from single nerve fibres in the animal's eye. He showed that neighbouring light sensors damp each other, which sharpens edges and contrast.

Which kind of light, invisible to people, can the small middle eyes on a horseshoe crab's shell detect?

Ultraviolet light is picked up by a second pigment in the middle pair of eyes, which the big side eyes lack. Together with light sensors in the tail, those eyes help set the animal's daily body clock.

After dark, a horseshoe crab's eyes become roughly how much more sensitive to light than by day?

A million times more sensitive is the night-time boost, switched on by the animal's own body clock. Its light-catching cells are giants too, about 100 times the size of a human's.

What do horseshoe crabs mainly eat as they forage along the sea floor during the night?

Worms and clams top the menu, though the animals are not picky and will also take small crustaceans, bits of fish and even algae.

A male horseshoe crab's front claws, used to cling to a female, are often likened to which sports gear?

Boxing gloves is the usual comparison for the bulbous claspers a male gains at his final molt. He may ride on a female's shell for months, and several rivals can cluster around the same female.

What are the horseshoe crab's breathing organs, five pairs of leafy flaps beneath its abdomen, called?

Book gills get the name from the thin sheets stacked inside like pages, as many as 200 in each one. The flaps also beat like paddles to help the animal swim.

Before a horseshoe crab blood test won US approval in 1977, which animals were used to check drug safety?

Rabbits were the stand-ins: a contaminated batch gave them a fever, a slower and costlier check. The crab-based test can return a result in as little as 45 minutes.

The mangrove horseshoe crab can poison diners because it carries which toxin, better known from pufferfish?

Tetrodotoxin is found in both the meat and the eggs of this one species, so cooks must stick to its relatives. The same nerve poison also turns up in certain octopuses and newts.

Like a bird, a horseshoe crab grinds its swallowed food in which muscular organ?

Gizzard is the answer: the toothed pouch pulverises each meal and spits back any bits that cannot be digested. The animal needs it because it has no jaws to chew with.

Where on a horseshoe crab's body would you find its mouth, which faces slightly backwards?

Centre of the underside is the spot, well back from the shell's front edge. Food reaches it along a groove that starts between the rearmost legs.

The horseshoe crab's order, Xiphosura, is named in Greek after which weapon that its tail resembles?

Sword is the meaning of the Greek xiphos, joined here to oura, a tail. Only one family in the whole order is still alive today.

Native Americans once put empty horseshoe crab shells to which practical use in their canoes?

As bailing scoops, the domed shells tipped unwanted water out of a canoe. The stiff tail spines were put to work too, as spear tips, and the meat was eaten.

Cape Cod towns once paid a bounty on dead horseshoe crabs because locals blamed the animals for ruining what?

Shellfish stocks were the supposed victims, since folklore held that the animals drilled into their prey with their long spines. Towns paid a few cents for each one turned in.

Early 20th-century harvests of horseshoe crabs went to feed farm animals and to make which other product?

Fertilizer was one fate of the harvested animals, spread on farm fields. Feeding them to livestock was abandoned once it turned out to give the meat a bad taste.

Ancient DNA suggests that Atlantic horseshoe crabs once lived as far north as which Arctic island?

Greenland's far north held the animals in a much warmer age, when the surrounding sea was about 8 °C warmer than now. The DNA traces are among the oldest ever sequenced.

The Atlantic horseshoe crab's species name honours Polyphemus, which kind of creature from Greek mythology?

Cyclops is the answer: the name was chosen in the mistaken belief that the animal had a single eye. In fact it has two large compound eyes and several small ones.

Which sea turtle is a noted predator of adult horseshoe crabs along the Atlantic coast of North America?

Loggerhead turtles feed on young and adult horseshoe crabs, so they suffer when the crabs grow scarce. Sharks and even alligators prey on the adults too.

The horseshoe crab's outer armour contains no hard minerals and is built from which tough natural material?

Chitin forms the cuticle of every arthropod, from beetles to shrimp. To grow, a horseshoe crab must shed the whole suit and swell into a larger one, and it gets through about 17 such molts before adulthood.

Which US state banned horseshoe crab harvests in 2008 to protect a migrating shorebird that eats the eggs?

New Jersey acted after counts of the bird crashed; the crabs had been taken as bait for other fisheries. A programme there also trains volunteers to rescue crabs stranded on their backs.

Unpaired male horseshoe crabs that crowd round a mating couple to fertilize some eggs are known as what?

Satellites, or satellite males, hang around an attached pair and still manage to father a share of the eggs. That works because fertilization happens outside the body, in the nest.

Which oxygen-carrying protein in horseshoe crab blood does the job that hemoglobin does in people?

Hemocyanin does the work, and each giant molecule in a horseshoe crab is built from 48 protein chains. Tarantulas get by with a version half that size.

Which name is given to the small pincers in front of a horseshoe crab's mouth that pass food inside?

Chelicerae give their name to the chelicerates, the wider group that unites horseshoe crabs with their land-dwelling kin. In some of those relatives the same pair became venom-injecting fangs.

US fishers use horseshoe crabs as bait in traps set for eels and which large sea snails?

Whelks are trapped with the crabs as bait, a harvest that kills far more animals than the medical trade does. A mesh bait bag designed by a conservationist roughly halved the amount needed.

Which blood cells of the horseshoe crab, the A in the LAL drug-safety test, act like our white blood cells?

Amebocytes carry granules of a defensive agent that they release on meeting invaders. For the test, the cells are burst open in distilled water to free it.

When a horseshoe crab molts, its old shell splits open along which part so the animal can crawl out?

The forward rim splits and the animal pushes ahead, where a true crab would back out instead. The cast-off shells wash ashore whole, and people often mistake them for dead crabs.