It’s still a mystery how snails ended up scattered around the globe
How do organisms that are so sedentary end up being so incredibly widely dispersed?
Thom van Dooren is a field philosopher at the University of Sydney and the University of Oslo. He is the author of several books, including “Flight Ways: Life and Loss at the Edge of Extinction, “The Wake of Crows: Living and Dying in Shared Worlds,” and “A World in a Shell: Snail Stories for a Time of Extinctions,” from which this article is adapted.
This excerpt was originally featured on MIT Press Reader.
There have been surprisingly few experimental efforts to explore the possible avenues by which Hawai‘i’s snails might have crossed oceans to arrive in their new home. In fact, to date there has been precisely one study on this topic of which I am aware. In 2006, Brenden Holland, a researcher in the biology department at Hawai‘i Pacific University, placed a piece of tree bark with 12 live snails of the species Succinea caduca into a saltwater aquarium. This is one of Hawai‘i’s nonendangered snail species; in fact, it is one of the few species that is found on multiple islands and seems to be doing okay. It is a coastal species, and the individuals enrolled into the study were from populations living as little as 10 meters from the beach. Brenden explained to me: “After heavy rain, they are commonly seen in gullies by the coast so there’s no question that they are going to get washed down pretty frequently.”
The purpose of Brenden’s experiment was to determine whether, when this happens, it might be possible for these snails to move around by sea and successfully establish themselves in new places. The answer, it seems, is yes. Brenden and his colleague Rob Cowie reported that: “After 12 h of immersion, all specimens were alive, indicating that sea water is not immediately lethal and suggesting the potential for rafting between islands on logs and vegetation.”
Why, you might wonder, does this matter? Far from being an abstract, albeit fascinating, scientific curiosity, I am convinced that attending to snail biogeography and evolution is particularly important at our present juncture. Hawai‘i was once home to one of the most diverse assemblages of land snails found anywhere on the planet, over 750 species. Today, however, the vast majority of these species are extinct, and most of those that remain are headed in the same direction. As they disappear from their island homes en masse, my hope is that paying attention to the deep-time processes of snail movement that brought them all here in the first place could help us to understand and appreciate these snails in new ways. As the writer Robert Macfarlane has argued, a deep time perspective can offer “a means not of escaping our troubled present, but rather of re-imagining it; countermanding its quick greeds and furies with older, slower stories of making and unmaking.”
Beyond Hawai‘i’s shores there have been numerous efforts to experimentally explore or otherwise interrogate the puzzle that is the evolution and distribution of island land snails. Charles Darwin, in a letter to Alfred Russel Wallace in 1857, summed up the situation succinctly: “One of the subjects on which I have been experimentising and which cost me much trouble, is the means of distribution of all organic beings found on oceanic islands and any facts on this subject would be most gratefully received: Land-Molluscs are a great perplexity to me.” Or, as he put it in a letter to another correspondent a year earlier: “No facts seem to me so difficult as those connected with the dispersal of land Mollusca.”
“No facts seem to me so difficult as those connected with the dispersal of land Mollusca,” Darwin wrote in 1857.
In an effort to address this perplexity, Darwin submerged land snails in saltwater to discover whether and how long they might survive. Among his other findings was the fact that estivating snails of the species Helix pomatia recovered after 20 days in seawater. The fact that these snails were estivating is important. During these periods snails can create a thin layer of mucus to cover their aperture and prevent them drying out. As long as they are sealed up inside their shells in this way, it seems that many snails can survive being submerged in saltwater for weeks at a time.
Inspired by Darwin, a French study in the 1860s placed 100 land snails of 10 different species in a box with holes and immersed it in seawater. Roughly a quarter of the snails, from six different species, survived for 14 days — which was calculated to be about half the time it would take for an object like a log to float across the Atlantic.
All of these years of submerging snails — of gastropods drowned and survived — have produced one primary, albeit tentative, finding: It is at least possible that land snails are floating around the world to establish themselves in distant places. We just don’t know enough about Hawai‘i’s snails to know how likely a vector this is for their movements; we have a single, short-term study on one of the over 750 known species.
But floating is by no means the only mode of transportation open to snails. In fact, most of the biologists I spoke to were of the view that it probably isn’t the primary way in which they have moved across large distances. While snails have possibly floated around within the Hawaiian archipelago, between islands, it is thought to be unlikely that the first snails to arrive did so in this way: The distances of open ocean are just too vast. But here, things get even stranger, and even less amendable to experimentation.
As we walked along a winding path around the summit of Pu‘u ‘Ōhi‘a on a cool, rainy, afternoon, Brenden Holland and I discussed some of these other potential modes of snail movement across oceans. He explained to me that not all of these possibilities are immediately obvious if we look only at organisms in their current forms. Many species change after arriving on islands; some, for example, undergo processes of “gigantism” or “dwarfism” in which their new environmental conditions lead to a significantly increased or decreased body size. Alongside these kinds of changes, many entirely new species evolve on islands after initial arrival events. In the case of Hawai‘i’s snails, phylogenetic analysis indicates that the vast majority of species evolved in the islands in this way, a single arrival giving rise to multiple new species over a few million years (these analyses compare genetic material to determine how closely related species on different islands are to one another, and in this way piece together their histories of arrival and evolutionary divergence). Some of these new island species will continue to look a great deal like the ancestor that made that initial oceanic crossing; others will not.
As we walked that day, Brenden pointed out to me tiny snails of the species Auriculella diaphana, moving around among the introduced ginger plants. It was these snails he had brought me here to see. He explained that despite their very different appearance, these snails are actually close relatives of the much larger, brightly colored, Achatinella tree snails that have become the poster-children of endangered snail conservation in Hawai‘i. The former is about 7 millimeters in length, the latter about 2 centimeters. But, Brenden told me, Auriculella and Achatinella have a smaller common relative still, and phylogenetic analysis indicates that it is an even more likely candidate for having made the initial trip to the islands. There, among the ginger leaves, we were lucky enough to also encounter some of these tiny beings, members of the subfamily Tornatellidinae.
The Tornatellidinae snails we saw that day, along with some other species within this subfamily, reach a maximum size of about 2 millimeters in length, roughly the size of a grain of rice. But this size difference is more significant than these simple length measurements imply. As Rob Cowie explained to me, the mass of a snail is roughly equivalent to the cube of its length. As such, one of the tiny Tornatellidinae snails might be as much as 1,000 times lighter than its Achatinella brethren. If a minute creature similar to these tiny snails was the ancestor that first made its way to the Hawaiian Islands, then it might have had many other modes of transportation open to it. It might even have arrived by bird.
At some point in the distant past, a tiny snail climbed on board a migratory bird, perhaps a golden plover, as it perched or nested overnight.
In numerous conversations with biologists, again and again I was told with varying degrees of confidence that the most likely answer to the puzzle of Hawai‘i’s snails is that the first ones flew here. Everybody narrated this hypothetical scene a little differently, but the main events remained the same. At some point in the distant past, a tiny snail climbed on board a migratory bird, perhaps a golden plover, as it perched or nested overnight. As snails are nocturnal, it makes sense that they might encounter a perched bird in this way, and that this wayward passenger might then be able to hunker down, deep in the bird’s feathers, sealing itself up. Days or weeks later, having rested through the exhausting crossing, the snail then climbed off the bird in its new home.
I must admit that on first hearing this explanation I was somewhat dubious. This sequence of events just seemed so horribly unlikely. I reminded myself, though, that in the vastness of evolutionary time, “horribly unlikely” is actually pretty decent odds. But as I continued to talk to scientists and read the literature, I discovered an unseen world of surprising snail journeys. For the most part, scientists have not deliberately gone looking for snails on birds, but in a handful of articles published over the last several decades they have nonetheless reported on their accidental encounters with them, usually in the course of routine bird banding or observation. In these cases, it seems, snails have sometimes been present with surprising regularity and abundance.
Across several studies, the snail Vitrina pellucida has been found on a variety of migratory birds in Europe, while Succinea riisei has been found on three different types of birds in North America, with anywhere from one to 10 snails on a single bird. In one particular study, focused on migratory birds in Louisiana, snails were found on three different bird species. The main focus of the research was the woodcock, and it was only on these birds that the researchers really monitored snail presence: “Of the 96 woodcock checked, 11.4% had snails present,” they report. “Of those, the average number of snails per bird was 3.”
In Hawai‘i, there has never been a targeted scientific search for snails on birds, so it is hard to know which species might be climbing on board and with what kinds of frequency. Partway through my research, however, Nori Yeung at the Bishop Museum came across and shared with me a tantalizing snippet from a field notebook. The collecting note was made in 1949 by Yoshio Kondo who was at the time in Nori’s current position as curator of the museum’s malacology collection. There at the top of a grid-lined page, in neat cursive writing, he reported: “a juvenile sooty tern on which were Succinea and Elasmias. Brought bird back. Unfortunately, did not keep shells on bird separate.”
But there is another fascinating, albeit equally speculative, avenue by which tiny snails might move around the globe. They might fly without the aid of birds, blown on leaves and other debris, or just on their own, sealed up in their shells. Indeed, there is significant evidence from sampling, conducted with nets attached to airplanes, that rock particles the size and weight of some of these tiny snails can move around in this way, sometimes being found at altitudes of more than 2,000 meters. Drawing on these findings, some scientists have argued that it is not at all unreasonable to think that snails might travel in similar ways, definitely over shorter distances but perhaps also for transoceanic journeys. At least a couple of the scientists I spoke to, including Brenden and Rob, were holding open the possibility that the progenitors of at least some of Hawai‘i’s snail families may have blown to the islands in this way, perhaps even carried by the winds of a hurricane.
Of course, once a snail species has made that first giant leap across oceans, a range of other options open up for the shorter, inter-island, movements that genetic analysis indicates have taken place at various points in the past. As we have seen, some snails might survive a floating journey between islands. Others, it seems, might be making these briefer trips inside birds: studies in various parts of the world have now shown that a variety of snail species — including as least one species of the Tornatellidinae — can survive passage through avian digestive tracts at a relatively high frequency.
These are, undoubtedly, all rather unreliable ways to travel. For every snail that successfully arrived in a strange new land on a bird or a floating branch, countless millions must have been washed, blown, or flown out to sea without such luck. The odds must be slightly better traveling by bird than log: At least in theory, if you hop onto or into a migratory bird in a forest, you are reasonably likely to be taken to another forest. Of course, for those snails unfortunate enough to be traveling inside the bird, they would have to survive the journey through the digestive system too.
However they travel, snails are largely at the whim of external forces in these movements, subject to what biologists call “passive dispersal.” As Brenden helpfully summed it up for me: “biogeographically, snails are plants” — both groups share many of the same vectors for movement, the latter usually by seed or spore. This is clearly a “system” of island dispersal that can hope to achieve results only with immense periods of time at its disposal. Over millions of years, a few lucky snails made these journeys successfully. We can’t know for certain how many times this happened in the Hawaiian Islands. But by tracing species back to their common ancestors in Hawai‘i and beyond its shores, Brenden and Rob have estimated that things must have worked out for around 20, and likely fewer than 30, intrepid travelers, or groups of travelers, over roughly the past 5 million years (when Kaua‘i, the oldest of the current high islands with suitable snail habitat, formed). All of the rest of Hawai‘i’s incredible gastropod diversity is thought to have evolved in the islands from this small number of common ancestors.
While there is undoubtedly something very “passive” about this dispersal of snails — always at the whim of others, be they birds, storms, or tides, traveling under their steam and direction — this isn’t the whole of the story. Deep evolutionary histories have produced these possibilities. Snails’ modes of passive movement only “work” because they have evolved some remarkable traits for dispersal, survival, and reproduction, across and into isolated new lands: from epiphragms that seal them up inside their shells and sticky eggs that can attach themselves to birds and debris, to hermaphroditism, sperm storage, and self-fertilization which all potentially allow a single snail introduced to a new land to begin reproducing. While not all snails can do all of these things, where these traits are present, they are surely a huge advantage. Millions of years and countless generations of more or less successful journeying have selected for those individuals that survived and established themselves best.
There is a profound kind of evolutionary agency at work here, a creative, experimental, adaptive working-out of living forms with particular capacities and propensities. For the most part, individual snails are indeed relatively passive in all this. They’re not, however, irrelevant. The particular actions of those snails that crawled onto a bird, that opted to seal up their apertures, that safely stored away sperm for future use, mattered profoundly. But neither are snails involved in the more active, sometimes even deliberate, dispersal undertaken by many other animals.
Instead, if we pay attention, snails amaze with their capacity to move so far, to spread so widely, while doing so little. This, it seems to me, is one of the real marvels of snail biogeography. Individuals do not need to exert great effort because natural selection has acted for them, acted on them, acted with them, to produce these beings that are so unexpectedly but uniquely suited to a particular form of deep time travel, drifting. From such a perspective, rather than being any kind of deficiency, the highly successful passivity of snails might be seen as a remarkable evolutionary achievement.
It’s likely that in the history of these islands, on average one successful snail arrival event has taken place every few hundred thousand years.
There is so much more to learn here, so much to learn about not just the vectors but the patterns under which dispersal takes place: Are they laid down by atmospheric and oceanic currents, or by the inherited paths of avian migration? And yet to some extent this must remain a space of uncertainty and even mystery. How can one really study processes of biogeography that take place across such vast periods of time and space? As Brenden reminded me, it’s likely that in the history of these islands, on average one successful snail arrival event has taken place every few hundred thousand years. Put simply, it’s not something that any of us are likely to ever see, let alone study, firsthand.
It is hard to really make sense of the vast, deep-time assemblage of Hawaiian snail life. I imagine it as something like a giant network with strands stretching out across the Pacific Ocean and beyond, extending back over evolutionary and geological time frames. Each strand represents one of hundreds of unique species. Millions of years of unlikely journeys — nestled into a bird’s feathers, or perhaps tucked away in the crevice of a floating log — heading to destinations unknown. Millions of years that have produced these intrepid, even if somewhat unlikely, island dispersers with the reproductive and other adaptations that made these movements possible. These are at least some of the processes that have produced the breathtakingly diverse, utterly unrepeatable assemblage of snail life in Hawai‘i.
To labor to hold this network in mind, however imperfectly, however impossibly, might offer us a glimpse into one of the reasons why these snails matter, and so the significance of what is being lost in their extinction. Doing so might remind us that each of the fragile, fleshy, little individuals of Auriculella diaphana or Achatinella mustelina is not so much a “member” of a species as it is a “participant” in a lineage, one link in a vast, improbable, intergenerational project. These are projects — made up of the lives, histories, and possibilities of diverse snail species — that are today being radically truncated, or simply shorn off, all within the space of a few generations of human life. With them is disappearing countless unique ways of life and the vast evolutionary heritage — to borrow Loren Eiseley’s apt term, the “immense journey” — that they together comprise.