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10 Slippery Facts About Slugs

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Slugs get bad press. Okay, they’re cold and slimy and sometimes eat your kale, but there’s so much more to them. They exude a liquid crystal, they have sex while dangling from a thread, and one time they beat a charismatic mammal in a dramatic popular vote.

1. SLUG OR SNAIL? IT’S A SPECTRUM.

People tend to call something a slug if it looks like a snail but has no shell. However, many distantly related critters among the gastropods—the group that contains snails and slugs—have independently evolved a sluggy, shell-free shape. So there’s no single, evolutionarily distinct slug lineage. To make things more complicated, a lot of slugs secretly do have shells—they’re just hidden within the slugs’ bodies.

And then there are the in-betweeners. So-called semi-slugs have tiny shells on the outside of their bodies that are way too small for them to retract into. (Honestly, they look pretty ridiculous.)

2. SLUGS HAVE TENTACLES, BLOWHOLES, AND THOUSANDS OF TEETH.

If you want to dream up an alien species for the next big sci-fi blockbuster, start with slug anatomy. First, check out the tentacles. Slugs have four, and they’re retractable. Two are for seeing and smelling, and they can be operated independently: a slug can gaze at you (or smell you) and a friend simultaneously. The other two are for touching and tasting.

Slugs also have thousands and thousands of teeth. These tiny chompers are part of a rasping structure called a radula that’s unique to mollusks. And in case that doesn't seem weird enough, slugs essentially breathe through a blowhole that opens up on one side of their bodies. This round pore is called a pneumostome.

But that’s just the anatomy of land-living slugs. Sea slugs have their own incredible features. For example, some breathe using delicate feather-like gills that surround their butt holes, and they smell with neon-colored, bizarrely shaped protrusions called rhinophores.

3. WHEN ATTACKED, SOME SLUGS LOSE THEIR TAILS.

Melibe leonina from Santa Cruz. Image credit: Robin Agarwal via Wikimedia // CC BY-SA 4.0

It’s an ingenious strategy for escaping a hungry predator: Break off a small tasty part of your body, and leave it behind as you make your escape. Some slugs do this. The aptly named taildropper slugs, such as the reticulated taildropper, can quickly amputate their own tails. And certain sea slugs have body parts that snap off safely and easily, leaving a would-be predator with a smaller, less desirable meal.

4. THEIR LOVE LIVES ARE ACROBATIC AND GORY.

Leopard slugs and their relatives will only mate when they’re dangling upside down from a thread of mucus. This position enables them to extend their gigantic, body-length penises and wrap them around each other. And yes, that’s penises plural: slugs have both male and female body parts.

And that’s just one example of surprising slug sex. The banana slug, a fixture of the Pacific Northwest, sometimes chews off its partner’s penis after mating. Then there’s the sea slug that removes its own penis post-sex and rapidly grows a new one. Another sea slug sets the mood by stabbing its partner in the head.

5. SLUG SLIME IS A LIQUID CRYSTAL.

Slugs are gooey and sticky, and they leave a trail of slime wherever they go. But that goo is pretty remarkable. It’s a liquid crystal, a substance that’s somewhere between a liquid and a solid. It flows a bit like a liquid, but at the molecular level, it’s more organized. It can be both adhesive and lubricating, and it actually slurps up water.

Why all the goo? Slug slime is multi-purpose. It helps these critters move and climb challenging surfaces. It also protects them from fungi and bacteria. Plus, slugs can learn about each other—and find potential mates—by examining slime trails. And, of course, mucus is a key part of a leopard slug’s daredevil upside-down sex life.

6. THEY CAN GET UP TO 30 POUNDS.

Some land-living slugs can get pretty large. Europe’s ashy-grey slug is 10 inches long. But that’s nothing compared to the sheer size of some sea slugs. Found in California, the black sea hare can reach nearly 40 inches and weigh 30 pounds.

7. SOME SLUGS ARE FIERCE PREDATORS.

Amgueddfa Cymru, National Museum Wales via Wikimedia // CC BY-SA 3.0 

Sure, many eat a salad-like diet of plants, mushrooms, and fruits, or chew up dead and rotting plants and sometimes even rotting animals. But others have a taste for living flesh. The ghost slug slurps up worms. The stunningly colored Spanish shawl (a type of sea slug) chows down on a particular marine creature called a hydroid, eating everything except the hydroid’s stinging cells, which the slug then uses for its own defense. The sea slug Pleurobranchaea californica eats other sea slugs, and it’s shown a remarkable ability to remember which species—like that stinging Spanish shawl—are no good to eat.

8. SOME SLUGS CAUSE TROUBLE.

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As people have spread across the globe, we’ve unintentionally introduced slug species into new locales. These slimy invaders lack natural predators in their new lands, so they flourish and even push out native slugs. The slugs that eat the veggies in your garden may very well be invaders, such as the Spanish slug or leopard slug—and that last species carries a parasite that can cause meningitis.

Slugs aren’t just spreading to new places on land, either. Ocean-going ships have also accidentally transported exotic sea slugs to new places.

And invasive slugs also cause trouble for other creatures. Some plants rely on ants to spread their seeds, and they reward the ants with a tasty treat called an elaiosome that’s attached to each seed. But invasive slugs can eat those treats before the ants find them.

9. SOME ARE IN TROUBLE.

Blue-grey Taildropper (Prophysaon coeruleum)

Only a few slug species are pests. Most are critical members of land and water ecosystems all around the world. And, like so many creatures, they’re suffering declines. One is the tiny and ridiculously colorful blue-grey taildropper of the Pacific Northwest. Another, the evocatively named snake skin hunter slug, is found in only a few spots in South Africa. Though these critters may not have the charisma of, say, a cheetah or a blue whale, they’re no less crucial to the health of ecosystems.

10. A SLUG IS THE SUBVERSIVE MASCOT OF THE UNIVERSITY OF CALIFORNIA AT SANTA CRUZ.

If you’re looking for a model of slug appreciation, take a look at UCSC. For a long time, students considered the banana slug, a gentle denizen of the area’s redwood forests, to be their unofficial mascot. Little did they know that their slug was in for a fight.

In 1980, UCSC entered the National Collegiate Athletic Association (NCAA). The university needed an official mascot—and some officials wanted a fiercer, sportier, and more traditionally charismatic species. A group of athletes proposed the sea lion, and it became the mascot. But other students refused to embrace the new choice. They continued to shout “Slime 'em!” and “Go slugs!” at basketball games. A fierce debate erupted, resulting in national media coverage. Finally, the matter was put to rest with a 1986 school-wide vote. Slug supporters slid into first place with an overwhelming five-to-one victory.

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The Real Bay of Pigs: Big Major Cay in the Bahamas
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When most people visit the Bahamas, they’re thinking about a vacation filled with sun, sand, and swimming—not swine. But you can get all four of those things if you visit Big Major Cay.

Big Major Cay, also now known as “Pig Island” for obvious reasons, is part of the Exuma Cays in the Bahamas. Exuma includes private islands owned by Johnny Depp, Tyler Perry, Faith Hill and Tim McGraw, and David Copperfield. Despite all of the local star power, the real attraction seems to be the family of feral pigs that has established Big Major Cay as their own. It’s hard to say how many are there—some reports say it’s a family of eight, while others say the numbers are up to 40. However big the band of roaming pigs is, none of them are shy: Their chief means of survival seems to be to swim right up to boats and beg for food, which the charmed tourists are happy to provide (although there are guidelines about the best way of feeding the pigs).

No one knows exactly how the pigs got there, but there are plenty of theories. Among them: 1) A nearby resort purposely released them more than a decade ago, hoping to attract tourists. 2) Sailors dropped them off on the island, intending to dine on pork once they were able to dock for a longer of period of time. For one reason or another, the sailors never returned. 3) They’re descendants of domesticated pigs from a nearby island. When residents complained about the original domesticated pigs, their owners solved the problem by dropping them off at Big Major Cay, which was uninhabited. 4) The pigs survived a shipwreck. The ship’s passengers did not.

The purposeful tourist trap theory is probably the least likely—VICE reports that the James Bond movie Thunderball was shot on a neighboring island in the 1960s, and the swimming swine were there then.

Though multiple articles reference how “adorable” the pigs are, don’t be fooled. One captain warns, “They’ll eat anything and everything—including fingers.”

Here they are in action in a video from National Geographic:

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13 Secrets From the Ravenmaster at the Tower of London
Christine Colby
Christine Colby

Christopher Skaife is a Yeoman Warder at the Tower of London, an ancient fortress that has been used as a jail, royal residence, and more. There are 37 Yeoman Warders, popularly known as Beefeaters, but Skaife has what might be the coolest title of them all: He is the Ravenmaster. His job is to maintain the health and safety of the flock of ravens (also called an “unkindness” or a “conspiracy”) that live within the Tower walls. According to a foreboding legend with many variations, if there aren’t at least six ravens living within the Tower, both the Tower and the monarchy will fall. (No pressure, Chris!)

Skaife has worked at the Tower for 11 years, and has many stories to tell. Recently, Mental Floss visited him to learn more about his life in service of the ravens.

1. MILITARY SERVICE IS REQUIRED.

All Yeoman Warders must have at least 22 years of military service to qualify for the position and have earned a good-conduct medal. Skaife served for 24 years—he was a machine-gun specialist and is an expert in survival and interrogation resistance. He is also a qualified falconer.

Skaife started out as a regular Yeoman Warder who had no particular experience with birds. The Ravenmaster at the time "saw something in him," Skaife says, and introduced him to the ravens, who apparently liked him—and the rest is history. He did, however, have to complete a five-year apprenticeship with the previous Ravenmaster.

2. HE LIVES ON-SITE.

The Tower of London photographed at night
Christine Colby

As tradition going back 700 years, all Yeoman Warders and their families live within the Tower walls. Right now about 150 people, including a doctor and a chaplain, claim the Tower of London as their home address.

3. BUT HE’S HAD TO MOVE.

Skaife used to live next to the Bloody Tower, but had to move to a different apartment within the grounds because his first one was “too haunted.” He doesn’t really believe in ghosts, he says, but does put stock in “echoes of the past.” He once spoke to a little girl who was sitting near the raven cages, and when he turned around, she had disappeared. He also claims that things in his apartment inexplicably move around, particularly Christmas-related items.

4. THE RAVENS ENJOY SOME UNUSUAL SNACKS.

The Ravenmaster at the Tower of London bending down to feed one of his ravens
Christine Colby

The birds are fed nuts, berries, fruit, mice, rats, chicken, and blood-soaked biscuits. (“And what they nick off the tourists,” Skaife says.) He has also seen a raven attack and kill a pigeon in three minutes.

5. THEY GET A LULLABY.

Each evening, Skaife whistles a special tone to call the ravens to bed—they’re tucked into spacious, airy cages to protect them from predators such as foxes.

6. THERE’S A DIVA.

One of the ravens doesn’t join the others in their nighttime lodgings. Merlina, the star raven, is a bit friendlier to humans but doesn’t get on with the rest of the birds. She has her own private box inside the Queen’s House, which she reaches by climbing a tiny ladder.

7. ONE OF THEM HAS EARNED THE NICKNAME “THE BLACK WIDOW.”

Ravens normally pair off for life, but one of the birds at the Tower, Munin, has managed to get her first two mates killed. With both, she lured them high atop the White Tower, higher than they were capable of flying down from, since their wings are kept trimmed. Husband #1 fell to his death. The second one had better luck coasting down on his wings, but went too far and fell into the Thames, where he drowned. Munin is now partnered with a much younger male.

8. THERE IS A SECRET PUB INSIDE THE TOWER.

Only the Yeoman Warders, their families, and invited guests can go inside a secret pub on the Tower grounds. Naturally, the Yeoman Warder’s Club offers Beefeater Bitter beer and Beefeater gin. It’s lavishly decorated in police and military memorabilia, such as patches from U.S. police departments. There is also an area by the bar where a section of the wall has been dug into and encased in glass, showing items found in an archaeological excavation of the moat, such as soldiers’ discarded clay pipes, a cannonball, and some mouse skeletons.

9. … AND A SECRET HAND.

The Byward Tower, which was built in the 13th century by King Henry III, is now used as the main entrance to the Tower for visitors. It has a secret glass brick set into the wall that most people don’t notice. When you peer inside, you’ll see it contains a human hand (presumably fake). It was put in there at some point as a bit of a joke to scare children, but ended up being walled in from the other side, so is now in there permanently.

10. HE HAS A SIDE PROJECT.

Skaife considers himself primarily a storyteller, and loves sharing tales of what he calls “Victorian melodrama.” In addition to his work at the Tower, he also runs Grave Matters, a Facebook page and a blog, as a collaboration with medical historian and writer Dr. Lindsey Fitzharris. Together they post about the history of executions, torture, and punishment.

11. THE TOWER IS MUPPET-FAMOUS.

2013’s Muppets Most Wanted was the first major film to shoot inside the Tower walls. At the Yeoman Warder’s Club, you can still sit in the same booth the Muppets occupied while they were in the pub.

12. IF YOU VISIT, KEEP AN EYE ON YOUR MONEY.

Ravens are very clever and known for stealing things from tourists, especially coins. They will strut around with the coin in their beak and then bury it, while trying to hide the site from the other birds.

13. … AND ON YOUR EYES.

Skaife, who’s covered in scars from raven bites, says, “They don’t like humans at all unless they’re dying or dead. Although they do love eyes.” He once had a Twitter follower, who is an organ donor, offer his eyes to the ravens after his death. Skaife declined.

This story first ran in 2015.

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