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Banana Slug (Ariolimax columbianus)

By Heather Beresford,

Whistler Naturalist Society

Some people know that it’s spring when the crocuses bloom and the robins return. For me, it’s when the skunk cabbage pops up in the marshes and the slugs show up on the Valley Trail. Banana slugs are a little gross at first glance, but are really fascinating if you spend a some time watching them.

Banana slugs are mollusks, which means they are soft-bodied with no visible skeleton. They also belong to the class Gastropoda, which can be recognized by having a muscular foot, a mantle with a cavity, a meaty hump on their back, and a radula, or grinding mouth parts like sand paper.

I remember being at Nitnat Lake on the island a few years ago, and watching a slug eat a lettuce leaf. Gradually, a hole opened up on the side of its head as it ingested the lettuce. What the heck was that? Well, it turns out that that hole, called a pneumostone, is actually another defining feature of banana slugs. They are Pulmonates, which means they have a small lung inside their bodies which opens to the outside. When a slug is working hard and needs more oxygen, it opens the pneumostone to provide more surface areas for the slug to breathe through.

The banana slug lives in moist forest floors along the Pacific Coast of North America from California to Alaska. It is a decomposer, which means it chews up leaves, and animal droppings and other dead plant material, and recycles it into the soil. But take note dog owners, this is no excuse for not scooping up after your dog! In the process of eating, slugs spread seeds and spores.

You can’t think about slugs without thinking about slime. Slime has many functions. One is to keep the slug’s skin moist so it can breathe through it because just like the insides of our lungs, the skin must be moist to exchange gases. The slime gathers moisture out of the air like a sponge on damp days, and out of the soil under logs on dry days.

A second function of the slime is to protect the slug from predators. They simply hump up their body to become bigger and produce a thick milky mucous. Most animals and birds do not like the slimey texture and the fact that it gets even more gooey when it gets in their mouth. Also, when the slime comes in contact with a moist surface it temporarily causes the membranes to go numb because it contains an anasthetic. Raccoons will eat slugs but roll them in dirt first to bind up the slime. Garter snakes, ducks, geese and some salamanders will also eat them too. Baby slugs are eaten by shrews, moles and birds.