Buzz
Slugs often receive a negative reputation. While they may be cold, slimy, and occasionally feast on your garden greens, they possess fascinating traits. Their slime is a form of liquid crystal, they engage in aerial mating rituals, and they once triumphed over a beloved mammal in a surprising public vote.
1. SLUG OR SNAIL? IT’S NOT SO BLACK AND WHITE.
Many assume a slug is simply a snail without a shell, but the reality is more nuanced. Within the gastropod family, numerous unrelated species have independently evolved into shell-less, slug-like forms. This means there isn’t a single evolutionary line defining slugs. Adding to the complexity, some slugs actually have internal shells hidden within their bodies.
Then there are the intermediates. Known as semi-slugs, they sport tiny external shells that are far too small to retreat into. (Frankly, they appear quite absurd.)
2. SLUGS POSSESS TENTACLES, BLOWHOLES, AND THOUSANDS OF TEETH.
Stuart Brown via Flickr // CC BY-NC-ND 2.0
If you’re brainstorming an alien species for a sci-fi epic, take inspiration from slug anatomy. Begin with their tentacles: slugs have four retractable ones. Two handle sight and smell, functioning independently—allowing a slug to observe (or sniff) you and another target at the same time. The remaining two are dedicated to touch and taste.
Slugs also boast thousands of teeth, part of a unique mollusk feature called a radula, designed for scraping. To add to the oddities, slugs breathe through a blowhole—a circular pore known as a pneumostome—located on one side of their bodies.
However, this only covers land-dwelling slugs. Sea slugs boast their own astonishing traits. For instance, some utilize delicate feather-like gills near their rear ends for breathing, while detecting scents with vibrant, oddly shaped structures known as rhinophores.
3. WHEN THREATENED, SOME SLUGS SACRIFICE THEIR TAILS.
Melibe leonina from Santa Cruz. Image credit: Robin Agarwal via Wikimedia // CC BY-SA 4.0
This is a clever survival tactic: shed a small, appetizing part of your body to distract a predator while you flee. Certain slugs, like the aptly named reticulated taildropper, can swiftly detach their tails. Similarly, some sea slugs have body parts that easily break off, leaving predators with a less satisfying meal.
4. THEIR MATING RITUALS ARE BOTH ACROBATIC AND GRUESOME.
Leopard slugs and their close relatives engage in mating exclusively while hanging inverted from a mucus thread. This unique posture allows them to unfurl their enormous, full-body-length penises and intertwine them. Indeed, these creatures possess dual reproductive organs, meaning they have both male and female parts.
This is merely one instance of the bizarre mating behaviors of slugs. The banana slug, commonly found in the Pacific Northwest, occasionally bites off its mate’s penis after copulation. Another species, the sea slug, sheds its own penis after mating and quickly regenerates a new one. Yet another sea slug species initiates the process by piercing its partner’s head.
5. SLUG SLIME IS A FORM OF LIQUID CRYSTAL.
Steve Elgersma via Flickr // CC BY-NC-ND 2.0
Slugs are known for their sticky, slimy texture, leaving a trail of mucus wherever they move. However, this slime is quite extraordinary. It’s classified as a liquid crystal, a state of matter that bridges the gap between liquid and solid. While it flows like a liquid, its molecular structure is more ordered. It serves dual purposes, acting as both an adhesive and a lubricant, and even has the ability to absorb water.
What’s the purpose of all that slime? Slug mucus serves multiple functions. It aids in movement and scaling difficult terrains. Additionally, it shields them from harmful fungi and bacteria. Slugs can also gather information about one another—and locate potential partners—by analyzing mucus trails. Moreover, this goo plays a crucial role in the acrobatic, inverted mating rituals of leopard slugs.
6. THEY CAN WEIGH AS MUCH AS 30 POUNDS.
Certain terrestrial slugs can grow to impressive sizes. The European ashy-grey slug, for instance, measures up to 10 inches in length. However, this pales in comparison to some sea slugs. The black sea hare, native to California, can grow nearly 40 inches long and tip the scales at 30 pounds.
7. CERTAIN SLUGS ARE FORMIDABLE HUNTERS.
Amgueddfa Cymru, National Museum Wales via Wikimedia // CC BY-SA 3.0
While many slugs enjoy a plant-based diet, feasting on vegetation, fungi, fruits, and decaying organic matter, some have a penchant for live prey. The ghost slug, for instance, preys on worms. The vibrantly hued Spanish shawl (a sea slug species) devours hydroids, consuming everything except their stinging cells, which it then repurposes for self-defense. Another sea slug, Pleurobranchaea californica, dines on fellow sea slugs and has demonstrated an impressive ability to recall which species, like the stinging Spanish shawl, are best avoided.
8. CERTAIN SLUGS ARE PROBLEMATIC.
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Human migration has inadvertently introduced slug species to new regions. Without natural predators in these areas, these slimy invaders thrive, often outcompeting native slugs. The culprits munching on your garden vegetables might well be invasive species like the Spanish slug or leopard slug—the latter of which carries a parasite linked to meningitis.
Slugs aren’t just expanding their territories on land. Ocean vessels have also accidentally introduced exotic sea slugs to new marine environments.
Invasive slugs also disrupt ecosystems by interfering with other species. Certain plants depend on ants to disperse their seeds, offering a nutrient-rich reward called an elaiosome attached to each seed. However, invasive slugs often consume these rewards before ants can retrieve them.
9. SOME SLUGS ARE FACING THREATS.
While only a handful of slug species are considered pests, the majority play vital roles in ecosystems worldwide. Unfortunately, like many other creatures, their populations are declining. One example is the small yet strikingly vibrant blue-grey taildropper from the Pacific Northwest. Another, the evocatively named snake skin hunter slug, is confined to a few locations in South Africa. Though they may lack the appeal of animals like cheetahs or blue whales, their ecological importance is undeniable.
10. THE SLUG IS THE UNCONVENTIONAL MASCOT OF THE UNIVERSITY OF CALIFORNIA AT SANTA CRUZ.
For an example of slug admiration, look no further than UCSC. For years, students regarded the banana slug, a peaceful resident of the local redwood forests, as their unofficial mascot. Little did they know this humble creature would become the center of a spirited debate.
In 1980, when UCSC joined the National Collegiate Athletic Association (NCAA), the university sought an official mascot. Some administrators pushed for a more dynamic and traditionally appealing symbol, leading athletes to propose the sea lion as the new mascot. However, many students resisted the change, persistently chanting “Slime 'em!” and “Go slugs!” during basketball games. This sparked a heated debate that garnered national attention. The issue was ultimately resolved in 1986 through a campus-wide vote, where slug enthusiasts triumphed with a decisive five-to-one majority.
