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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.


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.)


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.


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.


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.


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.


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.


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.



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.


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.


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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Owning a Dog May Add Years to Your Life, Study Shows
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We've said that having a furry friend can reduce depression, promote better sleep, and encourage more exercise. Now, research has indicated that caring for a canine might actually extend your lifespan.

Previous studies have shown that dog owners have an innate sense of comfort and increased well-being. A new paper published in Scientific Reports and conducted by Uppsala University in Sweden looked at the health records of 3.4 million of the country's residents. These records typically include personal data like marital status and whether the individual owns a pet. Researchers got additional insight from a national dog registry providing ownership information. According to the study, those with a dog for a housemate were less likely to die from cardiovascular disease or any other cause during the study's 12-year duration.

The study included adults 40 to 80 years old, with a mean age of 57. Researchers found that dogs were a positive predictor in health, particularly among singles. Those who had one were 33 percent less likely to die early than those who did not. Authors didn't conclude the exact reason behind the correlation: It could be active people are more likely to own dogs, that dogs promoted more activity, or that psychological factors like lowered incidences of depression might bolster overall well-being. Either way, having a pooch in your life could mean living a longer one.

[h/t Bloomberg]

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Big Questions
Why Don't We Eat Turkey Tails?
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Turkey sandwiches. Turkey soup. Roasted turkey. This year, Americans will consume roughly 245 million birds, with 46 million being prepared and presented on Thanksgiving. What we don’t eat will be repurposed into leftovers.

But there’s one part of the turkey that virtually no family will have on their table: the tail.

Despite our country’s obsession with fattening, dissecting, and searing turkeys, we almost inevitably pass up the fat-infused rear portion. According to Michael Carolan, professor of sociology and associate dean for research at the College for Liberal Arts at Colorado State University, that may have something to do with how Americans have traditionally perceived turkeys. Consumption was rare prior to World War II. When the birds were readily available, there was no demand for the tail because it had never been offered in the first place.

"Tails did and do not fit into what has become our culinary fascination with white meat," Carolan tells Mental Floss. "But also from a marketing [and] processor standpoint, if the consumer was just going to throw the tail away, or will not miss it if it was omitted, [suppliers] saw an opportunity to make additional money."

Indeed, the fact that Americans didn't have a taste for tail didn't prevent the poultry industry from moving on. Tails were being routed to Pacific Island consumers in the 1950s. Rich in protein and fat—a turkey tail is really a gland that produces oil used for grooming—suppliers were able to make use of the unwanted portion. And once consumers were exposed to it, they couldn't get enough.

“By 2007,” according to Carolan, “the average Samoan was consuming more than 44 pounds of turkey tails every year.” Perhaps not coincidentally, Samoans also have alarmingly high obesity rates of 75 percent. In an effort to stave off contributing factors, importing tails to the Islands was banned from 2007 until 2013, when it was argued that doing so violated World Trade Organization rules.

With tradition going hand-in-hand with commerce, poultry suppliers don’t really have a reason to try and change domestic consumer appetites for the tails. In preparing his research into the missing treat, Carolan says he had to search high and low before finally finding a source of tails at a Whole Foods that was about to discard them. "[You] can't expect the food to be accepted if people can't even find the piece!"

Unless the meat industry mounts a major campaign to shift American tastes, Thanksgiving will once again be filled with turkeys missing one of their juicier body parts.

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