Banana Slug

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We have Banana Slugs all around our home in Boulder Creek, California, USA. For the most part they leave living plants alone, but there are some exceptions. Every once in a while when we are not looking closely enough, we will accidentally touch one. The slime is really hard to take off. The small ones are gray and they turn more yellow as the grow. The University of California at Santa Cruz adopted the Banana Slug as their mascot, which says a lot about the school.

Donna Hill writes:

"Banana Slug 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 sand-paper-like grinding mouth parts). They are Pulmonates, which means they have a small lung inside their bodies which opens to the outside with a pneumostone.

Banana Slugs are the second largest slug in the world, growing up to 25 cm long. They are so named because very often their coloring resembles a banana, bright yellow body with black spots. Solid greenish, pale brown and even almost white specimens can be found locally too. They can change their color slightly over time, becoming more intense or paler as the light, moisture and food allows. These colors help them to camouflage with the leaves on the forest floor.

They may be confused with an introduced species, the Black Garden Slug, as it is large and also comes in a brownish color. If you look closely at the Garden Slug, you can usually see an orange-striped edging around the foot and textured furrows on all upper parts of the body except the mantle. The Banana Slug does not have any orange on its body and its body is smooth.

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 soil. One of their favorite foods seems to be mushrooms. In the process of eating, they also spread seeds and spores.

They can be seen migrating across the path on dark damp days and at night or climbing up stumps looking for a dark place to hide. Once they find a dark damp spot to hide out on dry days, they will usually come back to that same spot, preferring to hide alone. They can fit into amazingly small spaces, this probably helps them to find enclosed areas that are still damp when most others have dried out.

With only one muscular foot, a slug moves slowly. How it moves can be seen when you place a slug on a piece of glass and look at it from underneath. Small horizontal waves of muscle move from the back to the front of the animal, allowing it to glide over a surface. A slug can travel over a razor blade or sharp edge of glass without cutting itself because of the slime that covers the foot.

Slime has many functions. One is to keep the slug's skin moist so it can breathe through it. A slug breathes through its skin and 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 make a bigger animal and produce a thick milky mucous. Most animals and birds do not like the slimey texture and the fact that it gets goey-er when it is put in their mouth. Also, when the slime comes in contact with a moist surface, it contains an anasthetic which temporarily causes the membranes to go numb. 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.

Another function of slime is it is used in movement. Slime on the underside of a slug's body comes in contact with leaves and sticks on the forest floor. This slime coats the leaves, allowing the slug to move over, under and around them easily. A trail of slime is left behind it as it moves along. As well, a slug that has climbed a tree can get down quickly by dropping to the ground safely from a string made of slime.

Slime is useful in mating too. A slug that is ready to find a mate and lay eggs leaves a chemical in the slime to attract other slugs. Another slug smells the chemical and follows the trail to the slug who left it.

Scientists have tried to reproduce slug slime because it is one of the best natural glues and may be of use in the medical field. But somehow, a slug is able to make what no human scientist with years of experience can. Perhaps if they can figure out what component is missing, we will one day have slug sticks instead of glue sticks!

To prevent themselves from drying out during long dry periods, slugs estivate, which is like hibernation except it occurs during hot dry spells. They become inactive, secrete a mucous shield and insulate themselves with a layer of soil and leafy debris.

Slugs are hemaphrodites, which means they contain both male and female organs. A slug that is ready to mate will smell the slime trail left by another slug and follows the trail until it finds it. The slugs exchange sperm and produce 20 or less translucent eggs which is 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 occurs several times throughout the year. Eggs and young are not protected by the parents.

Slugs use their two pairs of tentacles to sense their environment. The larger pair at the top of their head have a small black spot at each tip that 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 the their body. This pair acts like a nose, picking up chemical smells. Both of these tentacles can telecope in and out as they move along the forest floor to protect them from damage when they bump into leaves and twigs.

Sometimes as a slug is moving, you will notice a small hole, usually on its right side, near the front of the body. This hole is called a pneumostone. Air enters the hole and passes into a small lung-like cavity. Like us, when a slug is exercising hard (moving quickly), it needs more oxygen in its body. This extra lung provides more skin area for the slug to breathe through.

Most people think that a slug's cloaca is located at the back end of the slug, as it is in most animals. This is not true. The lump of material that often gets dragged behind the slug is actually dirt and leafy material which was collected by the slime and gets dropped off at the back end of the slug. The cloaca is located behind the pneumostone under the mantle so it cannot be seen except when the slug is getting rid of wastes.

Banana slugs may live several years, hibernating under a log when the temperature hovers near or below freezing. Scientists do not know exactly how long the average one lives in the wild, however.

Roll over a log (make sure to put it back) or dig through some leaves and pick up a slug to look a little closer. Touch it with a clean index finger, then touch a dry leaf or small twig. The leaf will stick to your finger. You'll be surprised at how glue-like the slime is! Just like for rubber cement, instead of washing it off, it is better to wave your hand in the air and roll the slime off between your hands or with paper towels. If you try to wash it off, it gets slimier as it gathers water.

It is best to do your slug studies out of doors and leave them where you find them as they make a somewhat slimey animal for classroom studies and should not be kept in a forest floor terrarium unless you are willing to invest the time to clean the slime off the sides of the container. As well, make sure to handle them with clean hands as the salts, natural oils and other contaminants on your hands can damage their skin.

Please do not kill slugs using salt. This is a terrible way to die! The thick goo you see on the slug after salt is put on the body is actually the fluid or blood of a slug that rushes to the skin's surface to dilute the salt. Dehydration is a nasty way to go. Simply remove the Banana Slug from your garden and place it back in a nearby forest where it can continue its important role as a forest floor decomposer.

Take time to look for one of these fascinating one-footed invertebrates next time you are walking in the forests of the Pacific Northwest. Watch them do their important role as forest floor recyclers and then walk on, leaving them to do their job."

Photo by Chuck Rogers.