June 25, 2009
Species of the Week - Banana Slug
By Rachel Shaw | Posted on June 25, 2009
Banana slugs are the second-largest terrestrial slug in the world.
Some of the more distinctive inhabitants of the Pacific coastal rainforest are the slugs of the genus Ariolomax, more commonly known as banana slugs. Banana slugs get their name because these large slugs resemble bananas, at least the yellow variants. They also come in brown, green (a variant I and my friends have dubbed “pickle slugs”), black and even white. Many of them are spotted as well, which further adds to the banana-like appearance. Reaching lengths of nearly 10 inches (25 centimeters), banana slugs are the second-largest land slug in the world, second only to the slugs of the Limax genus in Europe, who top out at 30 centimeters. They live in habitats ranging from southeastern Alaska down the central California coast. There are also a few relict populations stranded since the moister, cooler Pleistocene on Mount Palomar in San Diego county, in the Sierra Nevada north of Yosemite and in the Columbia Mountains of inland British Columbia.
Banana slugs are found primarily in coastal rainforests, where the damp environs and abundance of decaying leaves and mushrooms provide ideal habitats for slugs. Slugs breathe through their skin and a single lung, and both organs must be damp in order to function. The lung rests under the slug’s mantle along with their digestive and reproductive parts. Air moves through a hole in the mantle called a pneumostome. It opens and closes at a rate that is affected by the moisture in the slug’s environs; when the environment is drier, it opens and closes more slowly, in order to keep the slug from dehydrating. If conditions become too dry, a slug can cover itself in mucus, leaves and soil, and estivate until moister conditions return.
The mucus serves a range of functions for banana slugs in addition to keeping them moist. It lubricates the slug’s path as it traverses the forest floor, protecting the slug from sharp rocks and the like. It discourages predators, being both distasteful and an anesthetic when in contact with a moist surface, such as another animal’s mouth. (When I was in 5th grade, we attended Outdoor Ed, a nature-oriented camp, and one of the activities was to kiss a banana slug. Those who took the dare reported that it made their lips go numb. Supposedly native peoples took advantage of this for medicinal purposes, but sources are sketchy.) There have been reports of smaller slugs hanging from mucus strands from leaves (why, or how, this occurs isn’t clear). The slime also contains pheromones, which banana slugs use to find and attract mates.
Banana slugs, like all slugs, are hermaphrodites. Mating thus involves simultaneous penetration of both partners in an exchange of sperm that involves eating each other’s slime and may end in one or both slugs having their penes devoured in an act called apophallation. (And no, a penis so devoured does not grow back, according to one researcher who is studying the gender dynamics of hermaphroditic species like slugs.) This activity results in around 20-25 eggs (one source claimed as many as 75, but that seems high) which are deposited on logs or leaves and abandoned. Banana slugs will mate year round.
Banana slugs do not rank high on any predator’s list of desired foods due to their slime. That said, many animals do eat banana slugs, provided they can find a way to manage the mucus. (Rolling the slug in the dirt is one option.) Salamanders (who presumably have their own experience dealing with slime), raccoons, geese, ducks, crows and garter snakes are the main predators of banana slugs, as are shrews and moles when the slugs are young (and thus small). Human beings have also eaten banana slugs successfully, though the human approach usually involves cooking the animals. The slugs themselves are detritivores, meaning that they prefer decaying plant matter (and sometimes animals) to live plants for their food. They are particularly fond of mushrooms, whose spores they spread as they eat and move about.
Humans clearly find these harmless, brightly colored animals intriguing. In addition to kissing them and taking many, many pictures of them, people have also raced them, named musical groups after them, crafted magnets and shirts and glass sculptures in their image, and adopted them as a college mascot. Go slugs!
Sources:
EOL
Wikipedia
Animal Diversity Web
Banana slug sexuality research. Includes photographs and videos of mating slugs, and an erect slug penis. Perhaps NSFW?
Video of a banana slug on the move.
Catalog for slug educational film. Here you can order an educational film to teach children about nature, including banana slugs.
Home page of the Banana Slug String Band, an environmental musical group. (Very catchy!)
Article about the Russian River Banana Slug Festival 20 years ago, in which slugs were raced and consumed.
Article explaining “How the Banana Slug became UCSC’s official mascot.”
