Comox Valley Naturalists Society
Knowing Nature . . .
A Slug's Paradise
September 9, 2005
I went to the forest for a specific purpose this week - to find some slugs. It had just rained, and a yellowish, dark blotched banana slug was crossing the trail. It glistened slightly as it moved stealthily forward over needles and leaves. Two eye stalks on the front of the head moved alien-like, sensing the environment. As it entered my shadow, the stalks retracted, then slowly and cautiously emerged again. It made no sound at all, and left a curious slimy trail.
Slugs aren’t particularly popular with humans. Local author Des Kennedy explores this theme in his book “Living Things We Love to Hate”. Slugs are ranked high on a list of creatures humans detest: “Arguably the most despised creatures in creation, they are cold-blooded, slippery, slow, and most awfully, slimy”. Kennedy suggests that through knowledge and understanding, people may begin appreciate, and even like, these unusual creatures.
Slugs belong to large group called Mollusca. Molluscs are invertebrates (creatures with no backbone), which live in the ocean, fresh water and land. Grouped within the Mollusca are the Gastropods. Gastropods, meaning literally “stomach-foot”, have a flat foot for locomotion, a head with tentacles and eyes, and a mantle, which often bears a shell.
Some slugs retain faint remnants of a shell under the mantle, but for the most part they have evolved to be shell-less. Without a shell, slugs lack protection, and they run the risk of desiccation (drying out), which can be fatal. The moist and mild climate of the Pacific Northwest is ideal for slugs. It is a slug’s paradise.
The sheath-like mantle covers the front section of a slug’s body. The mantle partially covers the “visceral mass”, where most of the vital organs are. Under the mantle there is a cavity, which, in land creatures like slugs and snails, contains a simple lung-like organ. Gastropods that live on land breathe air are all called Pulmonates.
Slugs breathe through a big pore, called a pneumostome. The pore is clearly visible on the right side of the mantle. The eyes are located at the tips of the two pointy stalks at the front of the head. The eyes are sensitive to changes in light, but cannot see clear images as we do. Beneath the two eye stalks, are two sensory tentacles close to the ground, used for smell and touch.
Eating is the slug’s favourite activity. Slugs have a scraper-like tongue called a radula, with sharp, backward pointing teeth useful for tearing food. They consume algae, green plants, lichens, worms, insects, mushrooms, rotting carrion, and even animal feces! Slugs are recylers, which means that they can digest organic “garbage” which is then, through their droppings, turned into soil out of which new things grow.
Most of the slug’s body consists of a large foot, and the underside is appropriately named the “sole”. If the sole forms a fringe around the body, it is referred to as a “skirt”. Slugs and snails secrete mucous, or slime, which enables them to glide easily over rough and varied terrain. A combination of muscles causes wave-like motions of the sole to propel the slug forward. The mucous may be thin and watery, for smooth locomotion, or sticky, when more traction is required. Slime is a very useful thing, if you are a land loving Gastropod.
Life seems solitary in the woods, and it is a wonder that slugs ever get together. Slugs are hermaphrodites, meaning that they possess both male and female organs. Mating is a long, complex and slimy affair. Eventually eggs are laid in protected places, and these hatch into baby sluglets.
Little was known about slugs and snails in BC until the recent publication a book by Robert G Forsyth entitled “Land Snails of BC” (Royal Museum of BC Handbook: 2004). Forsyth reports that there are 92 species of land slugs and snails in BC, 25 of which are exotic, or accidentally introduced from other countries. Forsyth’s web site Land Snails of BC contains a checklist, and great photos of slugs.
One of the slugs native to our area, and commonly encountered
is the banana slug (Ariolimax columbianus), which is yellow-grey with
is usually found in the lowland forest, but may occupy a variety of habitats.
Slugs come out when the weather is cool and moist. Watch your step along trails and walkways. If you aren’t in a hurry, and no one is around, stop and take a closer at this amazing creature.
Click on a link below to view the CVNS newspaper column.