Sunday, October 12, 2008
Moralea Milne. Feb 3, 2008
One of the creatures which most astounds visitors to our wet, west coast is our Pacific banana slug. Evincing gasps of astonishment and revulsion in equal measure, banana slugs can reach 25 cm in length and weigh 113 grams! Usually a light tan colour with random, darker blotches, like an over ripe banana, they can be anything from white to black, spotted or not. Although they are common in rural and natural areas, they have not adopted well to urbanization and are rarely found in developed regions; there you will find a selection of introduced, invasive species that bedevil gardeners and greenhouses operators alike.
The great grey garden slug (10 cm), milky slug (5 cm), midget milky slug and the greenhouse slug (7 cm) are formidable pests of the agricultural industry as well as home gardeners. Another introduced slug species, the black or licorice slug (15 cm), has strayed far from urban centres. This glossy, black pest can be found rapaciously feeding from ocean side to mountaintop. Wild native slugs are generally territorial and solitary animals, preferring to remain in the vicinity of a favoured, protected space.
All slugs are members of the mollusk family which also includes oysters, octopus and snails. Invertebrates with soft bodies, often with hard outer shells, mollusks are most often found in aquatic environments. Those that have adopted to land, particularly without shells, needs sufficient moisture to survive and you will most often find them out on rainy days or in the night.
Banana slugs can crawl along at the great speed of ten metres per hour, the locomotion provided by a powerful band of muscle in the sole of its foot. For interest, place a slug on a piece of plate glass and view the fascinating interplay of rippling muscles in action.
Slugs are voracious, some eating up to several times their weight daily. With the help of up to 27,000 backward pointing teeth-like projections (radula), they devour fungi, lichens and plants, while a few species prefer a modicum of protein in their diets and consume insects, carrion, even other slugs.
Slugs process information through sensory cells that are found throughout their bodies, especially around their mouths, tentacles and along the foot. Slugs have two sets of tentacles, the longer pair are tipped with eyes that can distinguish light from dark and are possibly able to detect sources of heat. A shorter set, close to the ground are used for taste and smell.
What is more synonymous with slugs and our sense of disgust than slug slime? Slugs produce two types of mucus that aid in locomotion, prevent dehydration and protect them from predators. Their mucus has the ability to absorb water up to 100 times its initial volume; in practical terms, this means do not try to remove slug slime from your hands with water, it will only increase the amount of mucus. Instead, roll it off your hands, the same way you would with glue or gum. Scientists are studying slug mucus and exploring ideas that use the mechanics of slug mucus in new drug delivery systems, as pollutant traps for sewage treatments plants and as water based lubricants. In recent scientific experiments it has been found that slug slime accelerates decomposition in leaf letter, contributing to soil accumulation and forest health.
Banana slugs hibernate through the hot, dry summer months (estivate). Wet weather triggers their mating instincts, so in the Pacific Northwest, that means mating can occur during much of the year.
WARNING!! The sex life of a slug is an x-rated affair and if you are easily offended or suffer from a delicate condition, you should skip this section!
Slugs are hermaphrodites, that is they are equipped with both male and female sex organs and if a potential partner is not in the vicinity, they can successfully reproduce by themselves. In the long term, however, the genetic vitality of the species as a whole is increased through the exchange of genes. Slugs have developed an elaborate courtship; they spend hours circling one another, while lunging, nipping and sideswiping each other with their tails. They have a disproportionately enlarged penis, up to half their total body length. The scientific species name for our banana slug is “dolichophallus” or long penis. As the mating progresses the slugs entwine into an S position, continuing to stimulate and encourage each other for hours, finally releasing and receiving sperm simultaneously. Banana slugs go even further into the world of the bizarre; after hours of sticky foreplay with such well endowed sex organs, they can become stuck together, like the two randy dogs I recently witnessed in a doorway of a Mexican hotel. To uncouple, one will chew off the penis of the other slug. Hmmm, apparently the penis-less slug will now act the role of a female, supplying eggs only, while the victorious penis endowed slug will be able to both provide and receive fertilisation, giving it a genetic advantage. Enough said!
Finally, three to fifty white or golden eggs are laid in a suitably moist location, under a rotting log or in a hole in the ground. Depending on weather conditions, the eggs will hatch in three to eight weeks, although eggs laid in the fall will usually overwinter.
Banana slugs are generally not a major problem for gardeners but the introduced species can destroy tender young shoots soon after planting. Slug bait formulations can be bought at garden supply outlets but they are EXTREMELY TOXIC to birds, cats, dogs and young children. Some kill off earthworms and other soil fauna. Instead, when waging battle against these ravenous mollusks, try some of the following, less harmful methods.
Alter the environment by removing slug hiding places; avoid overwatering; cultivate bare soil to destroy slug eggs; supply habitat for snakes, such as south facing rock piles (all of our snakes are harmless and feast on the slimy mollusks). Most slugs feed at night, hand pick and drop into soapy water; please do not use salt, which is a cruel and painful end. Build beer traps and let the dead slugs accumulate, they are an added enticement to the thirsty, alcoholic slugs. Erect three inch, solid copper barriers around your most sensitive and favoured plants; contact between slug and copper produces an unpleasant electric shock, which will deter all but the most dim-witted slugs!
There are several other native slug species on southern Vancouver Island. Taildroppers, as their name implies, have the ability to release a tasty section of their tail, cover themselves in a less appetizing mucus and hopefully gain time to escape their predator. In BC, the blue-listed scarlet-banded taildropper is found only in the Southern Vancouver Island area and the blue-grey taildropper is a rare, red-listed species, recently found at Mary Hill and Devonian Park in Metchosin. The more common yellow banded taildropper I recently found was about 2.5 cm long, with an almost teal blue foot that had a faint yellow edge. Actually quite remarkable when viewed in a magnified photo.
Another new discovery is the dromedary jumping slug, found at Muir Creek. Jumping slugs protect themselves by twisting, turning and leaping frantically when disturbed.
Don’t be repulsed by our native slugs, they play an important ecological role in our forests; by hastening decomposition of organic materials, by fertilizing the soil and by dispersing seeds and spores. Marvel in their unique lives and appreciate their contribution to our distinctive southern Vancouver Island ecosystems.