A conversation with a former grad school colleague in Oregon got me thinking about slugs recently. So, I made a trip over to Discovery Park in Seattle for a slug walk. It was, I think, a perfect day to look for slugs. Cool, overcast, and a little rainy — the kind of day where a slug doesn’t have to worry much about dehydration. Because water relations are an issue for slugs, they are found in the moist parts of the world. This makes the Pacific Northwest ideal slug territory. I didn’t see any slugs initially on my walk. But it was a pleasure to walk slowly, scanning the ground on either side of the trail for signs of slugs, and turning over pieces of wood to look for them. It occurred to me that much of what I do is hurried and goal directed — a slow walk, being present to what’s occurring alongside the trail, is a happy antidote to that. I found my first slug under a decomposing fallen branch. It was scrunched-up in a defensive posture (picture below), a lovely representative of the non-native, European red slug — or possibly the European black slug — apparently you can’t know which it is for certain without some dissection. These slugs, and other imports like the great grey garden slug, are responsible for much of the slug activity that so annoys farmers and gardeners. Yet slugs are multi-dimensional — not just pests, but living things with much to celebrate. And, slugs, particularly those who have been imported by humans are intimately associated with us, feeding in our gardens and sliming their way into our basements.
The great grey garden slug is an import carried by humans from Europe into North America (as well as New Zealand, South America, and Australia). This slug has a variety of aliases, including leopard slug, tiger slug, greenhouse slug, and great grey slug. The variety of common names for this animal demonstrates the virtue of the scientific naming process, in which there is a single Latin name for each species — in this case, Limax maximus, literally meaning greatest or largest slug. The great grey garden slug seems to thrive in areas around human habitation, where it is most commonly found. Slugs introduced to the Pacific Northwest like the great grey garden slug, the European black slug, and the European red slug, seem to be a threat to native slugs, like the banana slug. The banana slug has achieved iconic status on the western coast of North America from Alaska to central California. It’s the mascot of the University of California at Santa Cruz — which to me is reason enough to consider attending that school. Fortunately for UC Santa Cruz, none of it’s competitors have chosen the great grey garden slug as a mascot. This slug is faster, more aggressive, and sometimes eats banana slugs. Displacement or extinction of native species, like the banana slug, may result when humans intentionally or inadvertently introduce a new species in an area. These introductions are characteristic of the Anthropocene, in which human beings modify not only landforms, but also the diversity of living things on the earth.
Slugs are molluscs, a group (phylum) that includes snails, clams, squid, and octopus. Slugs evolved from snails, dispensing with most or all of the external snail shell. Losing the shell allows slugs to be more flexible than snails. They don’t require calcium for shell building, which may be hard to come by in a terrestrial environment. And, they can squeeze into small spaces that won’t accommodate a snail shell. The price of shell-less freedom, though, is exposure to predators and greater potential for dehydration. So, slugs tend to be nocturnal, and produce copious amounts of slime (mucus). The mucus is hygroscopic. It attracts and holds water on the surface of the slug’s body. The mucus also deters predators and assists the slug in gliding along on a variety of surfaces. Snails first moved to land about 350 million years ago, and are early ancestors of terrestrial slugs. Snails and slugs have had long term evolutionary success that is far beyond that of human beings.
I can’t complete this post on slugs without saying something about slug sex. All slugs are hermaphrodites. They each have both male and female reproductive organs, which is convenient since a slug doesn’t need to find a member of the opposite sex. The great grey garden slug begins it’s courtship by following and nipping at the tail of another slug. If receptive to the advances, the leading slug climbs up a vertical surface, like a tree. The slugs meet on a perch and form a circle, each mouthing the other’s tail — all the while secreting mucus. This may last for an hour before the two slugs drop down, hanging from a mucus string, and intertwine mid-air. Their sexual organs come together below them, forming what some observers refer to as a bell or as a flower. Sperm is transferred from each slug to the other in this process. There are many amazing photos and videos of great grey garden slug mating available online — David Attenborough documented it for the BBC.
I encourage you to go for a slug walk, and appreciate the contributions of these creatures, many of whom serve as important recyclers of dead plant and animal materials. Slugs are a fascinating part of the diversity of living things, whose forms and life strategies offer delight as we observe and learn more about them.