Banana Slug

A yellow banana slug on a redwood tree.
A yellow banana slug on a redwood tree.


The Banana Slug, Ariolimax Columbianus, is one of the Largest slugs in north America, reaching lengths of up to ten inches. Their range consists of the dense temperate rainforest that spans the west coast of the United States and Canada, and Alaska. Though they are typically active year-round, drier summer weather usually restricts activity, to cool moist nights, until wetter daytime weather allows them to move about without desiccation.

Banana Slugs are Gastropods, propelling themselves by a muscular “foot” underside that also serves as the slug’s stomach. A pebble sized hole on the side of their head gives their single lung breathing access, while their radula, a long thin row of microscopic teeth, is located on the ground at the front of the stomach. A set of 4 antennae allow the slug to navigate across the forest floor, reaching speeds of 6 and a half inches per minute. The top two longer antennae act as eyes, sensing light and movement, while the bottom, shorter pair are for feeling and smell. Although their yellow color and spotted pattern make these creatures resemble the fruit for which they are named, this coloration also helps banana slugs blend into the dense coat of leaves on the forest floor. To keep themselves moist, they constantly secrete a layer of mucus over their bodies. This mucus also serves as a defense against predators, numbing the mouth and tongue if ingested.

Slugs are decomposers, spending their time and energy on consuming dead plant and animal matter, as well as snacking on the occasional mushroom or young plant seedling. In redwood forests like Muir Woods, they have a symbiotic connection to the redwoods. Banana slugs will eat the young shoots of other trees and plants that compete with redwoods for shade, while ignoring growing redwood saplings. As these redwoods grow taller and more massive, they create a cooler microclimate in the forest beneath them, giving banana slugs the cool, moist shady atmosphere they need to thrive. This is an example of a mutualistic relationship, one where both participants are supported by each other. It is one of the many examples of how a diverse ecosystem harmonizes and improves the local environment.

Last updated: September 22, 2020

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