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10 Hard-Shelled Facts About Horseshoe Crabs

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The plodding sea creatures have weird blood, weirder swimming habits, and a secret weapon that’s probably saved your life.  

1. HORSESHOE CRABS ARE INCREDIBLY OLD.

Discovered in 2008, the 25 millimeter-wide Lunataspis aurora crawled over Manitoba 445 million years ago. This makes it the world’s oldest-known horseshoe crab. Four species are with us today, all of which closely resemble their long-extinct ancestors.

Supposedly frozen in time, horseshoe crabs are often hailed as “living fossils” by the media. Yet, appearances can be misleading. Evolution didn’t really leave these invertebrates behind. They’ve transformed quite a bit over the past half-billion years. For instance, some prehistoric species had limbs that split out into two branches, but today's specimens have only one.

2. THEY'RE NOT CRABS.

In fact, they aren’t even crustaceans. Unlike real crabs and their kin, horseshoe “crabs” lack antennae. So, where do the strange ocean-dwellers belong on the arthropod family tree? Biologists classify them as chelicerates, a subphylum that also includes arachnids. Members possess two main body segments and a pair of unique, pincer-like feeding appendages called chelicerae (hence the name).

3. EACH ONE HAS A HUGE ARRAY OF SIGHT ORGANS.

Rachel Oh via Wikimedia Commons // CC BY-SA 3.0

Large compound eyes rest on the sides of their shells. Come mating season, these bean-shaped units help amorous crabs locate a partner. Behind each one, there’s a small, primitive photoreceptor called a lateral eye. Towards the front of the shell are two tiny median eyes and a single endoparietal eye. On its underside, a horseshoe has two “ventral eyes,” which presumably help it navigate while swimming.

Most interesting to scientists are the compound pair. By virtue of their relatively simple wiring, they’re easy to study and have taught us a great deal about how our own eyes function.

4. BABIES CAN SWIM UPSIDE DOWN.

Walking around on the ocean floor is generally how horseshoe crabs get from point A to point B. Nevertheless, young ones will often flip over and start propelling themselves through the water, using their gills as extra paddles. With age, they do this less frequently.

5. THE SPIKED TAIL HAS SEVERAL USES.


Stinging isn't one of them, despite what many falsely believe. Among its uses are assuming rudder duties and helping the arthropod right itself after getting stuck on its back. 

6. ADULTS MAINLY EAT BIVALVES.

Both larvae and fully grown horseshoes eat aquatic worms. Though adults will also devour algae and carrion, they predominantly consume clams and mussels. When feeding time comes, these low-profile predators mash food between the spiky upper regions of their legs before pushing it into the mouth. 

7. HUNDREDS OF THOUSANDS GATHER IN DELAWARE BAY FOR A MASSIVE ANNUAL ORGY.

Every year in May and June, the bay becomes the largest Atlantic horseshoe crab spawning zone on Earth. During the night, a female will climb ashore with a male (or several) in hot pursuit. After she digs a hole and deposits her eggs, the males fertilize them. Migratory shore birds descend upon the bay in huge numbers, fattening themselves on the nutrient-rich eggs. Among these avians are scores of red knots, which use the crab fest as a final pit stop during their yearly migration from the Arctic to South America.

8. VERY FEW SURVIVE INTO ADULTHOOD.

A mother can lay as many as 90,000 eggs per clutch. Even so, it’s estimated that only about 10 of those individual embryos will ever become adults. Before they get a chance to hatch, fish, sea turtles, and birds gorge themselves on the eggs. With so many animals utterly dependent on this fodder, nesting horseshoe crabs are vital to the ecology of Delaware Bay and countless other regions around the world.   

9. ATLANTIC HORSESHOE CRAB FEMALES ARE 25 TO 30 PERCENT BIGGER THAN MALES.


When it comes to reproduction, females also mature more slowly: While males are ready to mate by age 8 or 9, their counterparts don’t start breeding until age 10 or 11. 

10. IF YOU'RE UNDER 40 AND HAVE BEEN VACCINATED, THANK A HORSESHOE CRAB.

While human beings don’t have blue blood, horseshoe crabs do. Whereas our blood uses iron-based hemoglobin to transport oxygen throughout the body, horseshoe crabs rely on hemocyanin, which contains copper. Copper turns bluish-green when it oxidizes.

Strange coloration isn’t the only remarkable thing about horseshoe crab blood. Shorelines are downright squalid: a single gram of undersea sediment contains roughly 1 billion bacteria. Unlike us, the arthropods lack infection-fighting white blood cells. Instead, special cells called amebocytes attack pathogens in the horseshoe crab's body by sealing them inside a gooey physical barrier, thus halting the malady’s spread.

Ever since Johns Hopkins University physician Frederick Bang discovered this characteristic in 1956, medical scientists have been capitalizing on it. To ensure that a vaccine or injectable drug is safe, they introduce horseshoe crab amebocytes into a sample. If the cells start releasing their goo, it’s because they’ve encountered bacteria and, therefore, the product isn’t ready yet. In the 1970s, the FDA made this test mandatory for experimental drugs and surgical implants. Without these magnificent animals, thousands—perhaps even millions—of people might have died during the past four decades from unsanitary injections.

As you might expect, horseshoe crab blood is worth a pretty penny: sellers now command $15,000 per quart. The magic elixir is extracted from over 600,000 “donors” every year, each of whom parts with 30 percent of its blood before being released within 48 hours. Sadly, though, many don’t last that long. About 10–15 percent of captured crabs die somewhere in the process, and survivors can exhibit lethargic behavior down the road. Scientists aren’t insensitive to this problem. Researchers have been trying to develop synthetic amebocytes.

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Scatterbrained
Everything You Ever Wanted to Know About Dogs
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Dogs: They’re cute, they’re cuddly … and they can smell fear!

Today on Scatterbrained, John Green and friends go beyond the floof to reveal some fascinating facts about our canine pals—including the story of one Bloodhound who helped track down 600 criminals during his lifetime. (Move over, McGruff.) They’re also looking at the name origins of some of your favorite dog breeds, going behind the scenes of the Puppy Bowl, and dishing the details on how a breed gets to compete at the Westminster Kennel Club Dog Show.

You can watch the full episode below.

For more episodes like this one, be sure to subscribe here!

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Sploot 101: 12 Animal Slang Words Every Pet Parent Should Know
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For centuries, dogs were dogs and cats were cats. They did things like bark and drink water and lay down—actions that pet parents didn’t need a translator to understand.

Then the internet arrived. Scroll through the countless Facebook groups and Twitter accounts dedicated to sharing cute animal pictures and you’ll quickly see that dogs don’t have snouts, they have snoots, and cats come in a colorful assortment of shapes and sizes ranging from smol to floof.

Pet meme language has been around long enough to start leaking into everyday conversation. If you're a pet owner (or lover) who doesn’t want to be out of the loop, here are the terms you need to know.

1. SPLOOT

You know your pet is fully relaxed when they’re doing a sploot. Like a split but for the whole body, a sploot occurs when a dog or cat stretches so their bellies are flat on the ground and their back legs are pointing behind them. The amusing pose may be a way for them to take advantage of the cool ground on a hot day, or just to feel a satisfying stretch in their hip flexors. Corgis are famous for the sploot, but any quadruped can do it if they’re flexible enough.

2. DERP

Person holding Marnie the dog.
Emma McIntyre, Getty Images for ASPCA

Unlike most items on this list, the word derp isn’t limited to cats and dogs. It can also be a stand-in for such expressions of stupidity as “duh” or “dur.” In recent years the term has become associated with clumsy, clueless, or silly-looking cats and dogs. A pet with a tongue perpetually hanging out of its mouth, like Marnie or Lil Bub, is textbook derpy.

3. BLEP

Cat laying on desk chair.
PoppetCloset, Flickr // CC BY-NC-ND 2.0

If you’ve ever caught a cat or dog poking the tip of its tongue past its front teeth, you’ve seen a blep in action. Unlike a derpy tongue, a blep is subtle and often gone as quickly as it appears. Animal experts aren’t entirely sure why pets blep, but in cats it may have something to do with the Flehmen response, in which they use their tongues to “smell” the air.

4. MLEM

Mlems and bleps, though very closely related, aren’t exactly the same. While blep is a passive state of being, mlem is active. It’s what happens when a pet flicks its tongue in and out of its mouth, whether to slurp up water, taste food, or just lick the air in a derpy fashion. Dogs and cats do it, of course, but reptiles have also been known to mlem.

5. FLOOF

Very fluffy cat.
J. Sibiga Photography, Flickr // CC BY-NC-ND 2.0

Some pets barely have any fur, and others have coats so voluminous that hair appears to make up most of their bodyweight. Dogs and cats in the latter group are known as floofs. Floofy animals will famously leave a wake of fur wherever they sit and can squeeze through tight spaces despite their enormous mass. Samoyeds, Pomeranians, and Persian cats are all prime examples of floofs.

6. BORK

Dog outside barking.
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According to some corners of the internet, dogs don’t bark, they bork. Listen carefully next time you’re around a vocal doggo and you won’t be able to unhear it.

7. DOGGO

Shiba inu smiling up at the camera.
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Speaking of doggos: This word isn’t hard to decode. Every dog—regardless of size, floofiness, or derpiness—can be a doggo. If you’re willing to get creative, the word can even be applied to non-dog animals like fennec foxes (special doggos) or seals (water doggos). The usage of doggo saw a spike in 2016 thanks to the internet and by the end of 2017 it was listed as one of Merriam-Webster’s “Words We’re Watching.”

8. SMOL

Tiny kitten in grass.
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Some pets are so adorably, unbearably tiny that using proper English to describe them just doesn’t cut it. Not every small pet is smol: To earn the label, a cat or dog (or kitten or puppy) must excel in both the tiny and cute departments. A pet that’s truly smol is likely to induce excited squees from everyone around it.

9. PUPPER

Hands holding a puppy.
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Like doggo, pupper is self-explanatory: It can be used in place of the word puppy, but if you want to use it to describe a fully-grown doggo who’s particularly smol and cute, you can probably get away with it.

10. BOOF

We’ve already established that doggos go bork, but that’s not the only sound they make. A low, deep bark—perhaps from a dog that can’t decide if it wants to expend its energy on a full bark—is best described as a boof. Consider a boof a warning bark before the real thing.

11. SNOOT

Dog noses poking out beneath blanket.
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Snoot was already a dictionary-official synonym for nose by the time dog meme culture took the internet by storm. But while snoot is rarely used to describe human faces today, it’s quickly becoming the preferred term for pet snouts. There’s even a wholesome viral challenge dedicated to dogs poking their snoots through their owners' hands.

12. BOOP

Have you ever seen a dog snoot so cute you just had to reach out and tap it? And when you did, was your action accompanied by an involuntary “boop” sound? This urge is so universal that boop is now its own verb. Humans aren’t the only ones who can boop: Search the word on YouTube and treat yourself to hours of dogs, cats, and other animals exchanging the love tap.

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