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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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Animals
How Bats Protect Rare Books at This Portuguese Library
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Visit the Joanina Library at the University of Coimbra in Portugal at night and you might think the building has a bat problem. It's true that common pipistrelle bats live there, occupying the space behind the bookshelves by day and swooping beneath the arched ceilings and in and out of windows once the sun goes down, but they're not a problem. As Smithsonian reports, the bats play a vital role in preserving the institution's manuscripts, so librarians are in no hurry to get rid of them.

The bats that live in the library don't damage the books and, because they're nocturnal, they usually don't bother the human guests. The much bigger danger to the collection is the insect population. Many bug species are known to gnaw on paper, which could be disastrous for the library's rare items that date from before the 19th century. The bats act as a natural form of pest control: At night, they feast on the insects that would otherwise feast on library books.

The Joanina Library is famous for being one of the most architecturally stunning libraries on earth. It was constructed before 1725, but when exactly the bats arrived is unknown. Librarians can say for sure they've been flapping around the halls since at least the 1800s.

Though bats have no reason to go after the materials, there is one threat they pose to the interior: falling feces. Librarians protect against this by covering their 18th-century tables with fabric made from animal skin at night and cleaning the floors of guano every morning.

[h/t Smithsonian]

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Honey Bees Can Understand the Concept of Zero
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The concept of zero—less than one, nothing, nada—is deceptively complex. The first placeholder zero dates back to around 300 BCE, and the notion didn’t make its way to Western Europe until the 12th century. It takes children until preschool to wrap their brains around the concept. But scientists in Australia recently discovered a new animal capable of understanding zero: the honey bee. According to Vox, a new study finds that the insects can be taught the concept of nothing.

A few other animals can understand zero, according to current research. Dolphins, parrots, and monkeys can all understand the difference between something and nothing, but honey bees are the first insects proven to be able to do it.

The new study, published in the journal Science, finds that honey bees can rank quantities based on “greater than” and “less than,” and can understand that nothing is less than one.

Left: A photo of a bee choosing between images with black dots on them. Right: an illustration of a bee choosing the image with fewer dots
© Scarlett Howard & Aurore Avarguès-Weber

The researchers trained bees to identify images in the lab that showed the fewest number of elements (in this case, dots). If they chose the image with the fewest circles from a set, they received sweetened water, whereas if they chose another image, they received bitter quinine.

Once the insects got that concept down, the researchers introduced another challenge: The bees had to choose between a blank image and one with dots on it. More than 60 percent of the time, the insects were successfully able to extrapolate that if they needed to choose the fewest dots between an image with a few dots and an image with no dots at all, no dots was the correct answer. They could grasp the concept that nothing can still be a numerical quantity.

It’s not entirely surprising that bees are capable of such feats of intelligence. We already know that they can count, teach each other skills, communicate via the “waggle dance,” and think abstractly. This is just more evidence that bees are strikingly intelligent creatures, despite the fact that their insect brains look nothing like our own.

Considering how far apart bees and primates are on the evolutionary tree, and how different their brains are from ours—they have fewer than 1 million neurons, while we have about 86 billion—this finding raises a lot of new questions about the neural basis of understanding numbers, and will no doubt lead to further research on how the brain processes concepts like zero.

[h/t Vox]

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