I’m as thrilled as a snail in lettuce that The Forest Unseen‘s hexagonal sex scenes appear in Robert Krulwich’s blog on National Geographic‘s website. He has some fabulous drawings and also discusses Joan Roughgarden‘s work on the evolution of hermaphroditism:
This little snail is an evolutionary oddity, a fish out of water. I found this particular individual clinging to a tree in the upper reaches of Shakerag Hollow. The common name is “globular drop,” quite appropriate given its pea-sized shell and rotund shape. The shell is very thick and feels like a pebble.
Unlike all other land snails and slugs in our region, this species belongs to the family Helicinidae, a group of mostly tropical land snails that are more closely related to the marine snails than they are to other terrestrial snails. The animal’s body reveals this kinship: it has an operculum (a “trap door”) with which it seals its shell when withdrawn, it has just one pair of antennae (other land snails have two pairs), and its eyes are located at the base of the tentacles instead of at their tips. These seemingly minor differences indicate a deep divergence, analogous perhaps to the difference between a bird and a bat. Like birds and bats, the two lineages of snails independently evolved the ability to live in a new habitat — creeping onto land. Not quite as impressive as taking wing, but then what use is a radula in the air?
The following photographs are from another individual, one that lived in the limestone at the bottom of the mountain.
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!).
The hilly redwood forests of Santa Cruz are home to a spectacular gastropod, the Slender Banana Slug (Ariolimax dolichophallus). These sulfurous-yellow slugs are large: many are over six inches long. They creep through the forest floor and across trails in broad daylight, munching on fallen leaves, fungi, and low-growing plants. Apparently, they don’t eat redwood seedlings, so they keep the competition down in the understory, helping the redwoods to regenerate.
A general rule of natural history is that brightly colored animals that wander around in the open without any visible means of defense or escape are likely to be poisonous in some way. As far as I can tell, the chemical ecology of banana slugs has not been fully analyzed, but among Santa Cruz naturalists there is a tradition of experiential investigation of these slugs, an experience that is mediated through the tongue. So, eager to join the inner circle of initiates, I genuflected then prostrated myself before a large specimen on the trail. The animal was strangely unperturbed by my licking. The same could not be said about my tongue. I did not taste much in the way of noxious secretions, but for half an hour afterward I had a layer of gelatin firmly adhered to the top of my tongue.
Following this encounter, I learned that the tangy stalks of redwood sorrel (Oxalis ¿oregana?) do a great job of “cleansing the palate” (an expression that I believe originated somewhere a little more classy than among the Ariolimax-lickers of California). For those of you whose thoughts are turning to hallucinogens: you’re thinking of toad-licking. Believe me, lying flat out on a redwood forest floor licking a giant yellow slug is experience enough for me. What could a hallucination possibly add?
The slug is endemic to the Santa Cruz area (two other species are found elsewhere on the west coast) and is the mascot of UC Santa Cruz. The T-shirts say: “Banana Slugs: No Known Predators” which is catchy but not entirely true. The less well-informed Pacific giant salamanders eat them, as do snakes and some other creatures.
I looked into Mead’s original 1943 description of this species and the diagnostic character is the length of the penis: “not infrequently of greater length than the slug itself.” Mead was so breathless with amazement that he added an exclamation point in the scientific description, a form of punctuation that is as rare as the smiley face in taxonomic journals. Quite why the famously enterprising undergrads of UCSC have not developed a T-shirt emphasizing this zoological phenomenon in their hermaphroditic mascot, I don’t know.
Thank you to my friend and former student Leighton Reid for being my host for this visit and guiding me in the ways of the banana slug.
Our experience of the world is mediated through stories. Stories (also called theories by those who need a patina of scientific respectability) tell us how the world came to be, how it works, and what its fundamental rules are. Once is a while, someone comes along who so fundamentally changes the nature of our guiding stories that our life experience is transformed. Lynn Margulis, who died two days ago, is such a person.
Margulis taught us the importance of symbiosis in biology — the union of two or more different species into a new form. Her views were dismissed, ridiculed, and ignored for years. Finally, some of her ideas prevailed, although she continued to receive sniper fire for her penchant for questioning dogma.
Now, thanks largely to her, we understand that every living species is, at some level, the result of symbiotic fusion and union: all animal, plant and fungal cells have ancient bacterial cooperators hidden within; trees are united to fungal helpers below-ground; the animals that build coral reefs cannot survival without algal partners; insects are partly nourished by symbiotic gut bacteria; and even DNA itself appears to jump among species, intertwining radically “different” species into new entities. These processes happen in every part of life’s delta, but are particularly powerful among Margulis’ favorite creatures, the so-called “microbes” (the true “99%”).
Our metaphors have to shift. The “tree” of life? No, life has too many cross-connections among distant branches. An unstable, flowing delta is a better image. Evolution as a capitalistic competition among individuals? No, there are as many unions as robber barons; self-interested cooperation is rife.
As a small homage to Margulis, here are some of my DNA sequence data from the mitochondria of land snails. These are the color-coded “letters” of the DNA alphabet found within the ancient bacteria that live inside every cell in a snail’s body. But this description is misleading: the bacterial cells truly don’t “live inside,” instead they have melted their bodies into the other, creating an individuality-destroying symbiosis. Thank you, Lynn Margulis.
This snail was hiding under the broken edge of a fallen log, posed on an oak leaf. The lung is visible through the shell (the pulmonary vein makes an interesting pattern). This individual is probably a young Triodopsis (dark body rules out Mesodon thyroidus), but it has not yet grown the diagnostic lip on its shell opening, so I’m not 100% sure.
The Field Museum in Chicago has kindly lent me the only known museum collection of the tiger-snail variety that I’m writing up as a new species. Genetic data indicate that this population is distinct and quite different from the other species in our region. One of the shells in this tube will become the “holotype” of the new species, the physical specimen to which the species name is attached. This all depends on the approval of the peer-review process, so the holotype will not be official until (and if) the new species passes muster in review.