Unusual mating behavior alerts us to an invasion of giant slugs

A pleasant outdoor evening for Cari and Jason Reynolds was interrupted by the antics of some rather large slugs. The slugs had entwined their bodies, then suspended themselves from a stiff strand of mucus. Two large translucent structures emerged from their heads and coiled together. Sex, slug style. More precisely, sex, European slug style; American slugs have more pedestrian ways of completing their unions. For readers with sluggy, salacious turn of mind, the Ever So Strange Animal Almanac has a description (note for the bemused: some British slang involved) and David Attenborough narrates the whole process with some “marvels of nature” music playing along. Bottom line: the translucent structures were penises, exchanging sperm between two hermaphrodites.

Cari alerted the world through Facebook and I requested a closer look at these creatures. Thanks to a prompt delivery by Jason, I’m now in possession of two specimens of Limax maximus (well named, they grow to eight inches long; photos below). These are the first that I have encountered on the Cumberland Plateau. This worries me a bit, not just because the spotless innocents of Sewanee and surrounding areas are perhaps not ready for regular exposure to giant penis-dangling hermaphrodites, but because these monster slugs may be about to invade our woodlands and out-compete native species. We’re in one of the world’s epicenters of gastropod diversity, so such an invasion would be one more loss in the ongoing worldwide (and local) erosion of biological diversity. In the northeastern U. S., non-native slugs have thoroughly disrupted the local ecology. We have far more native species, so the damage here would be that much more troubling.

The species has been present in the cities of the Northeast since at least the late nineteenth century. Tyron’s Manual of Conchology published in 1885 includes a plate featuring Limax maximus (top two animals in the plate). In these more northerly areas, the slug seems to prefer to live in disturbed habitats and gardens, so it might not invade the forests here. However, our climate is more slug-friendly that that of Boston and Philadelphia; we can’t assume that the species will behave in the same way here as it does elsewhere. Stay tuned for further adventures in the biology of invasive species. Up next: Arion, an exotic slug that is ubiquitous further east, but has not yet been seen in Sewanee (to my knowledge — keep the reports coming!).

Limax maximus. Junebug for scale.

One common name is “leopard slug.”

Five inches. Three more to go. They can live for several years.

14 thoughts on “Unusual mating behavior alerts us to an invasion of giant slugs

  1. Country Mouse

    Like many other skeptics you may not consider California to be a part of the U S of A, so you may not hold the banana slug as an American critter in good standing. Be that as it may, our beloved slug has similar mating habits, which can be viewed in photos and videos at this UC Santa Cruz grad student’s page: http://bio.research.ucsc.edu/grad/weaver/Pages/project.html (and elsewhere). You may also be unaware that the banana slug is UCSC’s mascot – hence the rousing sporting cry “Go Slugs!”

    1. David George Haskell Post author

      The Sewanee population seems to be restricted to Natural Bridge. I have hordes of two other species of slug in my garden, although the ducks do a good job of fighting back these hordes.

  2. David

    I too had a terrible time with those things in my Nashville garden this spring but I mostly eradicated them with beer traps. They might like cheap beer more than the average Sewanee student.

    1. David George Haskell Post author

      Sorry to hear that they have invaded Nashville. I’d say that some Sewanee students do indeed have a taste for cheap beer. Luckily for me, the biology crew seem a lot more interested in slugs and genes and trees. Far more mature than I was at 18!

      1. David George Haskell Post author

        Two ways: 1. Set up a comfortable chair in the garden, open a beer and drink it while waiting for the slugs to emerge. When they arrive, grab ’em. Then open another beer and wait some more. Repeat as needed. 2. Use jars or some other smooth-sided vessel to construct pit-fall traps: bury the container so that its lip is level with the ground, bait it with some beer, then leave it overnight. Slugs will be drawn in and will drown in the beer. Unfortunately, these traps are indiscriminate and will drown beetles and other creatures (including small mammals on occasion). I don’t use them for that reason. Another option is “Escar-Go” slug pellets that are (to my knowledge) non-toxic to other animals. I think they have some kind of iron compound in them that poisons the slugs.

  3. Craig Chernos

    I’m in the middle of reading ‘The Forest Unseen’. One of the very first books I read in this genre, many decades ago, was Aldo Leopold’s ‘A Sand Country Almanac’, and Dr Haskell’s work is a wonderful successor to it.

    The author’s use of a limited scope of observation – a ‘mandala’ only a metre square – reminds me of Thoreau’s description of his tiny hand-built cabin in the woods. By limiting its scope, the greatness of the literature rises taller. (How music works this way as well! In the hands of a capable composer, a small ‘leitmotif’ may be elaborated into a great work of art.)

    Thank you, Dr Haskell, for a beautiful, thoughtful, and artistic work of scientific observation.

    1. David George Haskell Post author

      Dear Craig,

      Thank you for these very generous words, especially the comparisons that you draw. I can’t hope to approach the achievement of Leopold or Thoreau, but I’m honored that my writing should remind you a little of them. Your comparison to music is a perceptive one: I think that one of the most beautiful features of language is its musicality and I try to honor that music in my writing.

      I hope that you will enjoy the second half of the book as much as the first.

      With best best wishes, David

  4. Miriam Keener

    David, How many native slugs are there? We get some pretty giant ones in the shiitake patch. How can I tell the non native from the native? -Miriam Keener I love your blog (if that’s what you call it)


    1. David George Haskell Post author

      Can you get a photo? In general, the natives have a mantle (saddle-like thing on their back) that covers the entire back. The mantel in the exotics just sits over the front half of the slug’s back.


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