Tuesday, July 8, 2014

THE PACIFIC NORTHWEST IS SLUG COUNTRY


With our very wet climate (favorable for terrestrial mollusks) and our relatively acid soils (not so favorable for forming snail shells), we furnish great habitat for slugs.

The ones most of us see are garden slugs (Arion ater). This slug, native to northern Europe, comes in two forms, a black and a reddish one. Long thought to be subspecies of one another, Arion ater ater and Arion ater rufus, they have recently been split as two separate species Arion ater and Arion rufus. Unfortunately for the field worker, both of them come in a great variety of colors and can only be distinguished by dissection or molecular analysis.


So we’ll just call them all garden slugs. They have proven to be very successful imports to our region but aren’t well liked by gardeners because of their predilection for garden plants. In fact, it’s the easiest thing in the world to go online to find out how to get rid of slugs. I am choosing to extol their virtues, perhaps the most important one just to familiarize people with slugs.

Our big native slug is the banana slug, Ariolimax columbianus. After a European species that grows to a foot (30 centimeters) in length, ours is the second largest in the world, reaching lengths of 25 centimeters. Four of these in a cup would weigh a pound! Rarely are such monsters seen, though; most that we encounter are in the range of 10-15 centimeters, just a bit larger than the much more familiar garden slugs.

But walk into a mature wet forest, and if it’s a moist day, you are likely to find lots of banana slugs. They come in a variety of colors, from white to plain yellow to heavily spotted with black. There must be some genetic differences among these color types, as often all the ones you see in one spot look about the same.

The big hole on the right side of these slugs is the pneumostome (breathing hole). An active slug shows two eyestalks above that detect light or movement and two tentacles below that are chemosensitive. They have mucous glands all over the body that keep them protected from dehydration and that can lay down a trail for easier locomotion.


Most slugs are herbivorous, feeding directly on plant tissue (garden slugs) or on detritus and mushrooms (banana slug). The leopard slug (Limax maximus), a large pale brown, black-spotted species that is also introduced in our region, feeds on other slugs as well as detritus and garden plants. In turn, garter snakes and ducks eat a lot of slugs, apparently able to combat the mucous that protects them from many other predators.

Slugs are hermaphrodites, both sexes present in the same animal, and when they mate each one contributes sperm to the other. The lack of separate sexes may make sense in slow-moving animals that might have trouble finding a mate. In this case, every slug encountered would be a potential mate, not every other one!

Dennis Paulson

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