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NatureSpeak Articles

Banana Slug (Ariolimax columbianus)

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.

Slime also helps the slug move. Slime on the underside of a slug’s body comes in contact with leaves and sticks on the forest floor which coats the leaves allowing the slug to move along them easily. As well, they can drop down from a tree quickly on a string made of slime. Look out below!

Slugs are hermaphrodites, which means they contain both male and female organs. A slug that is ready to mate will excrete a chemical in its slime, which another slug will smell and follow. The slugs exchange sperm and produce 20 or fewer translucent eggs which are laid under a log or in leaves. Eggs are about half as big as your fingernail on your pinky finger and may be pearly white, pink or even yellowish. Mating and egg laying occur several times throughout the year. Once the eggs hatch, the young are on their own with no protection from their parents.

Slugs use their two pairs of tentacles to sense the environment. The larger pair at the top of their head have a small black spot at each tip and are used to detect light. Slugs cannot see images like we can, but instead rely on brightness or darkness to tell them which direction they should move. They also have a second pair of antennae located at the lower front of their body. This pair acts like a nose, picking up chemical smells. Both of these tentacles can telescope in and out as they move along the forest floor to protect them from damage when they bump into leaves and twigs.

Upcoming events:

May 4, 7 a.m.: Monthly bird count. Meet at the bottom of Lorimer Road.

May 7, 6:30-7:30 p.m.: Weekly nature walk hosted by Lisa Helmer. Meet at One-Mile Lake parking Lot in Pemberton.

May 10, 7:30 p.m.: Spruce Groove Park. International Astronomy Day.

May 2, 4: The first Squamish Songbird Festival continues this weekend with a variety of events. Friday, May 2 the Brackendale Art Gallery is hosting talks by birder Chris Dale, bat expert Blair Hammond and ornithologist Nancy Ricker at 8 p.m. On Sunday, May 4 birders will meet at the Squamish Estuary, Vancouver Street entrance, at 6 a.m. to listen to the Dawn Chorus. At 7 a.m. the monthly bird count starts from the brew pub. At 10:30 a.m. volunteers will be meeting at the brew pub again to do some streamkeeping and habitat restoration. And from 1 to 5 p.m. at the Brennan Park Rec Centre there will be a series of workshops and displays for the whole family.

Written by: Heather Beresford


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