The most bizarre creatures on Sonoma Mountain aren’t the ones you’d expect. They don’t have claws, venom, or a haunting call. They barely even have a face, and if they wrote romance novels, you wouldn’t believe them. Yes, “them” — the pronoun fits. They’re hermaphrodites.
Ariolimax buttoni, or Button’s banana slug, is our local endemic species, found from Humboldt County south to the North and East Bay. They are the only banana slug species found on Sonoma Mountain. Until 2006, they were considered Ariolimax columbianus, like their northern cousins. But DNA testing revealed two distinct species, with the southern slugs taking the name Ariolimax buttoni, in honor of Fred L. Button, the man who brought them to scientific attention.
Whether spotted or spotless, their bright yellow-to-tan bodies bring a cheerful splash of color to the deep greens and browns of the forest floor. All button’s banana slugs are born with speckles on their tails, but after about three weeks, the pigment either darkens into spots or fades away completely. Scientists have yet to confirm what mechanism leads to spots.
Slime (what it is and what it does)
Slugs use slime for locomotion and also to defy gravity by sticking to surfaces. The slime contains pheromones that attract other slugs for mating. This “slime,” aka mucus, is technically a liquid crystal; its structure is more ordered than a liquid but less rigid than a solid. Banana slugs excrete sugar molecules and mucin proteins as dry granules that expand hundreds of times upon absorbing water. Their pheromone-laden slime crystals are also an anesthetic. Lick a banana slug, and it will make your tongue go numb. I would dare you, but who knows where that slug has been! And just what gives banana slug slime its anesthetic properties? Scientists haven’t figured that out either.
How they eat and breathe
Button’s banana slugs are primarily detritivores, feeding on decaying organic matter, but they also eat living plants, mushrooms, and even feces. A slug’s mouth is on the bottom of their head, and it has a strong jaw to bite off chunks of food. Their tongue, called a radula, is covered in microscopic teeth to help break down the food even more. Waste exits through the anus, which is tucked beneath the mantle just behind the head.
Like most land slugs, A. buttoni have a single lung. They breathe through an opening called a pneumostome, visible on the right side of the mantle. This opening closes and opens to regulate oxygen intake and conserve moisture.
Life Cycle
Button’s banana slugs mature at remarkably different rates. Some lay their first eggs before their first birthday, while others can take more than two years, according to a study where siblings were all raised under the identical conditions. This is another banana slug mystery scientists have yet to unravel.
Personality
Ariolimax buttoni is mostly a night owl, but in the winter you are likely to see an explosion in numbers, even in the daytime, especially after a good rain. During summer and fall, banana slugs may hunker down like newts, trying to stay as cool and moist as possible. November is when they really start coming out of their hiding places.
We aren’t sure if they are a peaceful species or just timid, but Button’s banana slugs are known to flee much smaller slugs, and even their own species during mating season, when some prospective partners can get a little overly amorous. Their most aggressive act is nibbling during courtship, which often leaves lasting gouges or, in rare cases, apophallation (chewing off the entire penis).
Reproduction
When mating, banana slugs curl around one another like a yin-yang symbol, nibbling on each other’s bodies. Then, from yet another hole in the mantle called a genital pore, each will extend a penis that can be as long as the slug itself.
The dance and ensuing exchange of genetic material typically can take seven or more hours or, in extreme cases, has been observed lasting 23 hours.
Banana slugs possess both male and female parts (which explains the dueling phalluses). Since the slugs are hermaphroditic, one slug will, hopefully, invert its penis so that sexual reproduction can take place. They lay clutches of 20 to 30 bead-sized eggs at a time.
Cross-fertilization is not always necessary, however. These cool creatures can self-fertilize if need be.
A quiet mass extinction
Most don’t think of slugs when they think of mass extinction. Banana slugs are land mollusks, the group that also includes snails and semi-slugs. In a 2004 study, scientists found that mollusks accounted for 42% of all recorded animal extinctions, mostly land and freshwater species. More recent research suggests that thousands more may have disappeared without being documented. This is the quiet, mass extinction that has not made it to the news.
It’s time we gave more respect to our native banana slugs. They may be slimy, but they are the ultimate survivors, and they call our favorite mountain home.
Sources and further reading:
https://www.sonomamag.com/winter-is-time-for-sonoma-countys-banana-slugs-to-shine/
https://baynature.org/magazine/fall2018/not-just-a-pretty-face-the-banana-slug-is-one-of-the-weirdest-animals-on-earth/
https://tucker933.com/guides/banana-slugs/
https://www.researchgate.net/publication/232673158_Phally_polymorphism_and_reproductive_biology_in_Ariolimax_Ariolimax_buttoni_Pilsbry_and_Vanatta_1896_Stylommatophora_Arionidae
https://bioone.org/journals/american-malacological-bulletin/volume-23/issue-1/0740-2783-23.1.121/Phally-polymorphism-and-reproductive-biology-in-Ariolimax-Ariolimax-buttoni-Pilsbry/10.4003/0740-2783-23.1.121.short
Image credits:
- Creative Commons: iNaturalist user: moranat, CC0, via Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Ariolimax_buttoni.jpg
