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THE NATURALIST
by Dave Evans

Dances with Slugs
August 5, 2003

Sometimes the most mundane creatures on Kodiak get overlooked in favor of more active and attractive animals. For instance, Kodiak has slugs in abundance. Nobody finds them particularly enticing, even though moving along a string of mucus dribbling from their slug-chins has always seemed to me an ingenious way of getting around.

Otherwise the greatest attention is paid to them as garden pests, and most of the questions asked have to do with what kind of beer makes the best bait for the slimy molluscans.

Common as they are, there are still plenty of questions left to be answered about slugs. In islander remarked to me that he'd only seen slugs around human habitation, whether in town, villages or fish camps. I wonder if this is a real pattern, and if so, why it exists. Are they indigenous to the island, or have they been brought in by humans like so many of our other flora and fauna? How many slug species are there on Kodiak? I can't remember when I last saw an illustrated field guide to the Kodiak slugs (and I don't imagine the Alaska Natural History Association is doing hot and heavy recruiting of potential authors).

One of the big biological questions that intrigues me is one I usually ask at the beginning of nature walks for kids. I simply ask whether the slug on the path in front of us is a boy or a girl. Since the kids are often Baptist Mission campers, I admit to fretting a bit over whether the question and the answer are quite appropriate. But the answer, as it turns out, is "both," and depending on the ages of the daycampers I get responses like "Weird" or "Gross" or just "hmmmmmm..."

Slugs are indeed hermaphroditic. Although they are capable of fertilizing themselves when in dire straits, they usually have an elaborate courtship consisting of circling each other, sideswiping one another with their tails, lunging, playfully nipping, and ultimately mating. The result is that each slug can produce a batch of up to 100 eggs. Appealing little baby slugs hatch after a couple of weeks and immediately begin their slimy crawling and feeding.

I begin wondering why the slugs here are so successful, and whether their hermaphroditic reproduction has anything to do with this success. In a population of hermaphrodites, any two individuals that encounter each other are potential mates. This type of reproduction is sometimes seen in low population species where possible reproductive encounters are few and far between. Another set of conditions in which hermaphroditic reproduction may be an advantage is that of fluctuating habitat, a situation which certainly describes Kodiak's seasonality. With every individual in a population being a potential mate for every other individual, a population can build up quickly during the limited time of environmental favorability. There are actually animals in fluctuating habitats that do away with the male part of the population, ramping up their reproduction by females alone when the conditions become favorable. That's true, for instance, of the zooplankton that are such an important part of the fish diet in Kodiak's lakes.

I'd go with this answer to my question, except all of the slugs and their snail relatives are hermaphrodites, even the ones from lower latitudes. Hermaphroditism could represent an evolutionary heritage, which, while not particularly advantageous, is at least not sufficiently costly to disappear over slug generations.

Energy expended on maintaining two separate reproductive systems in the same body is considerable, and there's that slime, too, which is energy-expensive to produce. All of this maintenance energy could otherwise be devoted to producing new baby slugs. My question about slug success doesn't get resolved, particularly. About the most we can say is that hermaphroditic reproduction seems to be an biological strategy that works on Kodiak, at least for slugs.

In Kodiak's slugs, we do have a fine example of how a common and, to put it kindly, unappreciated animal can pose some of the most interesting biological questions. And I'm also left wondering if a slug, perceiving our own arrangement of separate sexes, would formulate in its cerebral ganglion cluster the molluscan equivalent of the thought, "Ewww, gross!"

David A. Evans, Naturalist, Fort Abercrombie State Historical Park


http://www.kadiak.org/naturalist/danceswithslugs.html Updated 2003 August 19

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