Aug. 14, 2012

Elusive Snails

by Kara Rogers

Click to enlarge images
At the ends of two long stalks, a pair of curious eyes peers down at the moist, moss-covered ground. The stalks form a V with the head, which extends forward searchingly, its touch-sensitive tentacles and mouth exploring. It explores methodically, patiently, its black foot inching forward, its spiraled shell teetering above it. Like other snails, there is something endearing about the noonday globe.
 
The noonday globe snail (Patera clarki nantahala) inhabits a two-mile stretch of wet forest along the Nantahala River Gorge in western North Carolina. Its named is derived from the Cherokee word nantahala, which means “land of the noonday sun,” a reference to the time of the day when the sun's rays finally grace the valley of the gorge. The noonday globe is considered to be  a threatened species in the state of North Carolina, but biologists do not know how many of the snails remain. It is an elusive animal, owing to its reddish shell and black body, which camouflage it in the forest, and thus the actual size of its range is uncertain.
 
There are other elusive snails in the United States, 26 species of which are listed as endangered. The Kanab ambersnail (Oxyloma haydeni kanabensis), for instance, is critically endangered. It is limited to springs along the Colorado River in Grand Canyon National Park in northern Arizona and to a wetland near Kanab in southern Utah. Like many other types of snails, the Kanab ambersnail is hermaphroditic (having both female and male reproductive parts) and is capable of self-fertilization, meaning that a single individual can establish a colony. But it is no match for the hooves of bighorn sheep, which stomp through the snail's habitat and are a major threat to the Grand Canyon population. Drought and changes in plant communities have also contributed to the decline of this population, as well as to the one in Utah, where encroachment by human development has further endangered the species.
 
The lacy elimia (Elimia crenatella), a freshwater snail with a grooved cone-shaped shell, is another species that is threatened in the United States. Once abundant in the Coosa River in east-central Alabama, it is now restricted to just several of the river's tributaries. The construction of dams on the river, changes in sediment transport, and water pollution are factors cited in the species' decline.
 
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Snails are delightful subjects to observe. In them, we discover a quirky tilt of the tentacles that makes us smile, a curl in the shell that captivates us. They carry their homes on their backs for their entire lives, traveling day in and day out with a roof literally over their heads, something that we have neither the strength nor the stamina to do, much less the biological mechanism necessary to self-generate that roof. But most snails are small, and because we tend to overlook the little things around us, they have suffered from our actions -- from development and pollution, the introduction of nonnative species, and overharvesting for their shells.
 
Fortunately, biologists are keeping a close eye on snails that inhabit the land, rivers, and oceans. Captive breeding and reintroduction programs, along with habitat preservation, are helping stabilize populations of some endangered species.
 
Having large numbers and diverse groups of snails around is important for several reasons. Field investigations, for instance, have revealed that certain species, like gilled snails, serve as key indicators of water quality and ecosystem health. Research has also shown that snails are models of evolution in progress. One need only look at the variation of colors and patterns on shells of individuals of a single species to see natural selection at work. And best of all, we can do this right in our own backyards. If conditions are sufficiently moist, just turn over a rock or a leaf, and let the thrill of discovery begin.
About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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