American Scientist

What do recently retired scientists do for six weeks on a tiny Caribbean island? My husband and I are fortunate enough to have a holiday home on Little Cayman, where we spend our time swimming, snorkeling, gardening and walking. We supplement our activities with that staff of life, reading. And we go hunting for the rarest snail in the world.

Even before retirement, we had started building a library of books about the Cayman Islands. Little Cayman is about 15 kilometers long by two kilometers wide and has about 170 permanent human residents, so facilities are limited. There is no bookstore and the only library is a book exchange—heavy on beach books and thrillers—at the National Trust House. One of the best books we own about the Cayman Islands is a large compendium called The Cayman Islands: Natural History and Biogeography, published in 1994. Among other topics, it covers geology, vegetation, birds, invertebrates, reptiles and fossils. We dip into various chapters to answer our questions or sometimes simply to increase our knowledge of Little Cayman’s natural history.

That’s how we learned about Cerion nanus, the rarest snail in the world. That’s a big claim for a little snail only about a centimeter long. Cerion is a common genus of air-breathing land snails in the West Indies and the Florida Keys. Different species within the genus either have no common name or are lumped together as “peanut snails” for their general shape. The most common species on Little Cayman, Cerion pannosum, is everywhere: on grasses, bushes and trees, and lying dead on the beach. The second species on Little Cayman, C. nanus, is a most uncommon snail. When we read about C. nanus in Mike Hounsome’s chapter on terrestrial invertebrates in The Cayman Islands, we were hooked. As a young man, Mike had participated in the joint Royal Society and Cayman Islands Government Expedition to Little Cayman in 1975, when the island was little known scientifically. The permanent population then consisted of 18 people, “all of whom,” Mike later joked with us, “seemed to be named Bodden” (the name of one of the Cayman Islands’ most prominent families). Mike is an ornithologist, but, because there was another ornithologist on the expedition, he became responsible for all animals that weren’t birds—including snails.

First contact

One of the intriguing things about C. nanus is its colorful and enigmatic history in the scientific literature. The snail was first described in 1888 by Charles Johnson Maynard, an eccentric zoologist and professional specimen collector. Maynard somehow persuaded a schooner captain who was sailing several hundred kilometers from Kingston, Jamaica, to Grand Cayman to let him off on Cayman Brac, the middle-sized Cayman Island. How Maynard got to Little Cayman is unrecorded, but he probably got a fisherman or turtler to take him the remaining eight kilometers.

Maynard became a Cerion specialist, although he used an older generic name, Strophia. Maynard was not only eccentric, he was also an overly enthusiastic namer of new species. In his life, he named 248 species of Cerion, 10 of which came from Little Cayman. One of these was the dwarf Strophia, S. nana, which later became Cerion nanus. Only two of his Little Cayman species, Cerion pannosum and C. nanus, are accepted by modern scientists. Maynard published his snail work in 1889 in a journal of his own invention, Contributions to Science, which was printed in very small numbers. He explained in his inaugural issue:

I have long felt the need of some organ wherein I could publish papers upon various scientific subjects, where they would be accessible. To be sure, this could be done in the journals of various natural history societies, but this procedure would defeat one of the very objects that I have in view, that of placing my work of this description in a concentrated form, where it can be had by any one who chooses to procure it, without subscribing to the publications of half a dozen societies.

Reading this, my husband and I deduced that Maynard—self-educated in science with some decidedly peculiar ideas about taxonomy—had suffered from rejections by those journals of various natural-history societies. Nonetheless, what Maynard observed about C. nanus would be irresistible to any curious natural historian. He wrote:

The island of Little Cayman is only ten miles long with an average width of two miles [sic], and is thus a mere spot in the waters of the Caribbean Sea, and the Dwarf Strophias occur in a space which is only five or six yards wide by twenty long, on this little key…. I consider that this species has the most restricted range of any animal with which I am acquainted.

Not only did Maynard conclude that C. nanus existed in a single, small population, he also found it almost exclusively on one plant species now known as Evolvulus squamosus. Also called the rockyplains dwarf morning glory, the species is patchily distributed on Little Cayman but also lives on many other Caribbean islands and in Florida. Maynard pronounced C. nanus “dwarfed to an extreme degree, from feeding on the pungent leaves of the plant described.” At only about half the length of C. pannosum, C. nanus seemed to compete with the larger, more ubiquitous snail.

A Tenuous Existence

These observations raised a surprisingly complex question: What exactly does rare mean? Is it small in numbers, small in territory, highly specific in habitat—or some combination of those characteristics? Several scientific groups have addressed these questions. NatureServe assesses the rarity of a species by a combination of six factors: the population size, the extent of the species’ total range, the part of the range actually occupied by the species, the number of distinct populations, the percentage of the range with good viability or ecological integrity, and the variety of environments the species can inhabit. Because Maynard found only a single population of the species, with a very small range and high environmental specificity (it lives on one plant species), C. nanus is rare on several counts.

The International Union for the Conservation of Nature uses a different system to assess a species’ vulnerability to extinction, a measure that is roughly equivalent to rarity. A species is considered critically endangered—the category for the rarest organisms—if it is distributed over a range of less than 100 square kilometers or if it actually occupies less than 10 square kilometers within its range. Since Little Cayman has a total area of only about 26 square kilometers, all species restricted to that island automatically meet the first criterion and most meet the second. Mike Hounsome wrote the report that declared C. nanus to be critically endangered, not only because of its highly restricted range but also because of the ongoing loss of its shrubland habitat to development.

When Mike located Maynard’s study site on Little Cayman in 1975, he couldn’t find any living dwarf Cerion, only dead ones. The larger C. pannosum, however, was abundant. To Mike, it looked like Maynard’s hypothesis might be correct: Perhaps the large Cerion had outcompeted and replaced the smaller species. But at another location only about a kilometer away, Mike found that both species were alive after all—and their relationship was not so clear. C. nanus lived only on Evolvulus, sometimes alone and sometimes sharing a plant with C. pannosum. Occasionally the larger snail was the only species on an individual Evolvulus shrub, but it could also live on different plants altogether. Furthermore, 30 percent of the Evolvulus plants at Mike’s new site had no snails at all. It seemed there was food to spare, but the two species still appeared together on the same plants more often than would be expected by chance. Certainly, the dwarf Cerion had not (yet?) been displaced and gone extinct at this location, as it had at Maynard’s old site. Mike wondered “Why not?” but found no definitive answer.

A species that is confined to an island, as C. nanus is, can rapidly become much smaller in size when natural selection favors little individuals that thrive with limited resources. This biological phenomenon, known as island dwarfing, is fairly common among mammals and birds. But Mike’s observations suggested that, on Little Cayman, the larger snail may have had the advantage. When he found Maynard’s original site, it was crowded with snails—but rather than outcompete the larger snails, the dwarf species had disappeared entirely. And at the second site, why were one-third of the plants free of snails? If competition triggered dwarfing, why would the larger Cerion be more common than the smaller species, which could surely prosper on a smaller food supply?

Roadblocks

With these tantalizing bits of information in hand, my husband and I decided to find the rarest snail in the world, if we could. Was it still living, or had it gone extinct since Mike last saw it in 1975? Was it really confined to the Evolvulus plant? Was the dwarf species actually more common than it seemed, rare only because of the difficulty of searching the uncut, dry scrubland? To find the snail and answer our questions, we would have to resolve a few problems. Maynard’s description of his collecting site was vague: “on the west end of Little Cayman, on the eastern most of the two paths that cross the island.” There were three modern candidates for these two paths, one of which is now paved, one of which is still a path and the third of which is enormously overgrown. We opted to search the easy places first. Mike had given a good description of the habitat in which he had found C. nanus and we knew a place that sounded ideal.

The second problem was that neither of us knew enough botany. Evolvulus is in the same family as morning glories but “looks nothing like one,” Mike informed us helpfully. In our photocopy of one of Mike’s publications, the picture of Evolvulus was not clear. Maynard had described the plant as “heathlike” and about “18 inches high.” He mentioned that its small gray leaves were about the same color as the snail shells and gave off a strong odor when crushed. A botanical description of the plant mentioned its “scalelike leaves”—a phrase we did not know how to interpret. Google Images produced wildly varying depictions of plants said to be Evolvulus.

The third complication was that we didn’t intend to take live specimens of such a rare species without a permit, so even if we found the snails, we wouldn’t be able to answer all our questions. For example, if C. nanus and C. pannosum were closely related (which a careful DNA study would reveal—if only we had a DNA lab), perhaps the dwarf species had evolved from the larger one. With more samples of Cerion from other islands, genetic analyses could unravel the fascinating story of the spread and subsequent isolation and evolution of Cerion on different land masses.

There are three main ways a snail could travel from island to island: in the beak or talons of some bird, attached to a floating log or other flotsam during hurricanes, or in a boat, as an inadvertent stowaway. The stowaway concept is not as unlikely as it seems. A number of snails crawled into some of Maynard’s collecting barrels full of corals, and he discovered them when he got home to Newton, Massachusetts. Charmingly, the snails woke up after six months’ travel and crawled around his garden, apparently quite happy, until cold weather came. But such unreliable modes of long-distance transportation mean that snails from islands separated by long water crossings rarely meet. These islands tend to evolve their own endemic species of Cerion. A scientist with the right permits and tools might find a fascinating evolutionary history in the DNA of C. nanus and its relatives. After all, the renowned evolutionary biologist Stephen J. Gould started his career by studying the morphology of Cerion and its adaptability in the Bahamas. But first, we simply wanted to find out if the little snail was still alive.

Into the Thicket

We set out one morning, equipped with plastic bags, brushes, a sieve, sunscreen, a camera and a flask of water, to look for snails. We hiked to a habitat we thought sounded right: an area off one of the main roads where there had once been a historic house belonging to Captain Woody. The captain was a remarkable man who had cleared, by hand, the original airstrip on Little Cayman in the 1960s. The area where his house had stood was fairly open, with lots of exposed limestone bedrock and thickets of small trees and shrubs with sparse undergrowth.

After roughly 15 minutes of scouring the ground and picking up snails that were not C. nanus, my husband cried “Got it!” He’d found an empty C. nanus shell among many more C. pannosum shells. Whereas the C. pannosum shell is about the size of a large plump peanut and is white—sometimes with brown highlights on its rough surface—the C. nanus shells are only half as long and rather svelte, not as thick as a pencil. They are mostly grayish or white. Once we had finally seen a dwarf shell ourselves, we quickly found more. We went home that day with about a dozen dead specimens. We were very proud of our success in our first venture into malacology.

But where was the Evolvulus bush? Or rather, which was the bush? None of the plants near the dead shells fit the description. Many had grayish leaves and pungent odors when crushed, as do many plants in dry habitats. But most were taller than half a meter. None had any C. nanus shells clinging to them. Eventually we discovered that the heathlike bush with scalelike leaves looked like a tangle of dead twigs because we were searching during the dry season. Everything living in such a seasonal habitat must adapt to dramatic changes in moisture, and Evolvulus did so by dropping its leaves and dying back.

In the days that followed, we corresponded by e-mail with Mike, obtained more information online about our quarry, and spent a few more hours searching for snails. Checking former paths and likely habitats, we found dead dwarf Cerion intermingled with the larger live and dead Cerion and other snails. We hacked and squirmed our way through overgrown trails, so thick we could not see each other if we were more than 3 or 4 meters apart. But if we could see any Evolvulus plants, we checked the area out. We found those habitats to be scratchy, mosquito-ridden and mostly snail-free.

Survivors

The rarest snail in the world is also rare in institutions: Only four museums in the world have specimens. One is the Smithsonian Institution, which sent out an expedition in 1930 that included ornithologist Paul Bartsch, who spent two days collecting snails at Maynard’s original site. In an unpublished article, Bartsch wrote alarmingly that he and his team “found Maynard’s C. nanus quite abundant, and gathered two eight-ounce bottles full of them.” Two bottles full!? We were aghast. After days of fairly diligent searching, we had found about 75 dead snails. Mike had collected a total of 87. Bartsch had clearly taken thousands of snails from Maynard’s original site, which might well have been almost the entire living population in 1930. No wonder Mike found no live snails there in 1975! Had the acquisitiveness of a scientific collector pushed C. nanus into extinction?

This horrifying thought made us stop and reconsider what we were doing. We were picking up shells, like any tourist on the beach, and we hadn’t thought to inquire about a permit. (We did so the very next day.) Were we picking up the last shells ever of C. nanus?

Spurred on by our shared curiosity, Mike dug out and sent us his unpublished field map, which showed where he had found living and dead snails. This guide helped us relocate the old sites. Sadly, we found that a house had been built on Maynard’s original collecting site, where we saw no sign of dwarf Cerion. Next, we searched for the only site where Mike had found living snails in 1975. Following his map, we drove slowly along the road, watching the odometer. Then we crested a slight rise and saw it: an area that had been completely bulldozed and covered in sand for parking heavy construction machinery. Diligent but downhearted searching around its edges yielded only one or two dead C. nanus shells.

Just when we began to fear the worst for C. nanus, we took a hard look at the 2009 Cayman Islands National Biodiversity Plan. We discovered that Mat Cottam, a terrestrial ecologist in the Cayman Islands’ Department of the Environment, had been monitoring the C. nanus population for years, as has Fred Burton, a naturalist based on Grand Cayman. We felt rather foolish: We had been looking for a rare snail that we thought nobody knew about, but Caymanian ecologists had been on the job all along! We contacted Mat right away. At least he had good news. Mat happily told us that he had found live C. nanus in 2010 and expected to find them again in 2011. He also confirmed our hunch that we were hunting C. nanus at the wrong time of year. Mike’s old notes showed that he found live dwarf snails only during the rainy season, but the rains hadn’t yet begun by the time we had to leave the island. Mat verified that in the dry season, C. nanus estivates, going into a state similar to hibernation, and seeks hidden, protected areas. With our holiday snail hunt behind us, we waited for Mat to e-mail us with news that this little mollusk is still alive. We got it in August, with the rainy season in full swing!

What do retired scientists do on vacation? They search for the rarest snail in the world, even though hardly anyone knows it exists and fewer still know it is “missing.” The continued existence of C. nanus will be a quiet sort of triumph. Tiny countries and tiny islands often have big problems with the survival of rare species. Despite the odds, one very small, very uncommon snail—one seemingly insignificant creature of unusual habits and great vulnerability—has survived competition, the paving of roads, bulldozers and development for another year.