Not many people are pleased to see slugs moving around in their backyard vegetable gardens. In fact, slugs are recognized by many as pests of vegetables and other plant material. However, there are numerous good reasons that we should pay due attention to slugs and their biology.
Taxonomically, slugs are close relatives of snails, which typically have a shell. It would be more accurate to say that slugs are a group of snails with or without shell. They are both classified as gastropods, or Class Gastropoda (Greek: gaster = stomach, + poda = foot). Shiny slime trails left behind on various surfaces may make you unhappy, but that is how they evolved to move along with their "foot" most effectively. Interestingly, slugs and snails do not creep, but glide on mucus. In their long history of evolution, their ancestors went off on a great adventure by leaving the sea and started living on land. Consquently, they lost the advantage of buoyancy, but instead adapted to gliding on mucus in the terrestrial environment. The diversity seen within the group of gastropods is phenomenal, second only to insects (Class Insecta) in its number of known species in the animal kingdom. Next time you have a chance to take a closer look at slugs, you may find out something new about these biologically very diverse and interesting organisms.
For example, the Great Gray Garden Slug, also known as the Leopard Slug (Limax maximus) is known for its amazing mating behavior. Believe or not, they mate in midair! They are hermaphroditic, meaning individuals have both male and female reproductive organs in their body. A pair of slugs, while being suspended from a mucous string, exchanges their sperms and fertilize eggs in each body. Watch a 2-minute video clip of mating Leopard Slugs from the BBC Science & Nature Program series Life In The Undergrowth (RealPlayer® is required to view the movie).
Dr. Shi-Kuei Wu, Emeritus Curator of Zoology, spotted and took a picture of a Great Gray Garden Slug in his neighbor's garden in north Boulder in early September 2008. The slug was collected and carefully preserved in ethanol in our Mollusk Collection at the University of Colorado Museum of Natural History. According to Dr. Wu, when it was live and extended, it was 6 inches long!
The Great Gray Garden Slug is not native to the United States. It was introduced from Europe to west and east coastal areas of North America at some point in human history. Apparently, the species has extended its distribution range deep into the mountains and plains regions of the country, as far inland as the Colorado Front Range. The slug in this picture is the second specimen of the species from Colorado to add to the UCM Mollusk Collection. There is not enough information as to how widely the Great Gray Garden Slugs are spread and thriving today in Colorado. If you have seen a giant slug with a dark brown spot pattern on its back as shown in this photo, please tell us where you found it!