Photograph by Joel Sartore, National Geographic Photo Ark
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Banana slugs, like the pair seen here, are native to the North America's Pacific Northwest. Named for their uncanny resemblance to the fruit, they thrive on cool, moist forest floors.

Photograph by Joel Sartore, National Geographic Photo Ark

Banana slug


What is the banana slug?

The banana slug is named for its resemblance to a ripe (or overripe, in the case of spotted individuals) banana. It’s one of the slowest creatures on Earth, moving at a maximum speed of six and a half inches per minute. The gastropod has one lung, one foot, and no spine.

The slug is native to the dense, moist forest floors of the Pacific Northwest, ranging from Central California to Alaska. One subspecies, the Pacific banana slug, can grow up to 9 inches long, making it the second-largest slug in the world.

The banana slug is often bright yellow, though it can be brown, white, and green. Like all slugs, banana slugs use four tentacles to sense their environment. A pair of upper tentacles, protruding from the top of their heads, are optical. Tiny black dots, or “eyes,” at the ends of the tentacles detect light and movement. A pair of smaller, lower tentacles, protruding straight out in front of their “face” are able to feel and smell. If a banana slug loses a tentacle to a predator or accident, it’s able to grow back.

The banana slug has a single lung, which has an external breathing pore, and it eats with its radula, a tongue-like body part that’s covered in rows of microscopic teeth.

Slime

Like all gastropods (slugs and snails) whose bodies are made of mostly water, banana slugs must stay moist to stay alive. To avoid dehydration, gastropods secrete a layer of mucus, or slime, which covers their bodies.

While snails have shells to help protect their skin from drying out, slugs must find other ways to retain their moisture. In dry conditions, slugs insulate themselves in dirt and leaves until their environment becomes moist again, typically venturing out to eat at night.

The slime also helps banana slugs move—and simultaneously keep predators away. The creatures move by expanding and contracting their single foot, and the slime provides a slippery surface to glide over. The trail of slime can often imperil or slow down attackers such as Pacific giant salamanders and northwestern garter snakes—common banana slug predators. When ingested, the slime can make attackers’ tongues go numb. Often, to neutralize the mucus, predators will roll banana slugs in soil before eating them.

Human uses of slime

To make slime, gastropods produce special tiny grains, called mucins, which link together. When the mucin chain comes into contact with water, it expands to more than a hundred times its original size, creating a sticky mucus. By studying the versatile, unique properties of slug slime, scientists have produced several new medical glues, including an adhesive that can bond to bloody, moving tissue—a struggle for traditional, medical-grade adhesives. Another group of scientists, intrigued by the banana slug’s ability to thrive in California’s redwood forest, have sought to create slug-inspired architecture, equally adapted to the redwood environment. They built a greenhouse prototype, which is encased in special silicone units that store and release water, much like the banana slug’s mucus-coated body.

Role in ecosystem

Banana slugs are decomposers and play an important role in their ecosystem. They eat detritus (dead organic matter), including fallen leaves and plants, animal feces, moss, and mushroom spores, and then recycle their food into nutrient-dense waste, which fertilizes healthy soil.

Reproduction

Banana slugs, like all gastropods, are hermaphrodites, which means they possess both male and female sex organs simultaneously. They’re able to mate with themselves, though they will commonly court other individuals. They lay clutches of eggs under leaves and soil, and leave the clutch once laid.

Banana slug in culture

The banana slug’s funny appearance and charming name has inspired local pride: It’s long been the mascot of the University of California, Santa Cruz. In 1980, the school’s president changed the mascot to, he argued, a more “dignified” sea lion. Students resisited, even after officials painted a sea lion logo on the gymnasium floor. After a prolonged, six-year battle, the student body voted overwhelmingly in 1986 to make the banana slug the official—and only—mascot of the school. Santa Cruz city officials commemorated the 25th anniversary of the banana slug’s mascot victory by declaring September 27, 2011 Banana Slug Day.