In this issue we focus on slugs and snails, the familiar "belly-footed"
gastropods of land and fresh water. Despite their familiarity,
these creatures are widely misunderstood; they exhibit beauty, sensory ability,
and a range of intriguing behaviors that
may be surprising to people who are accustomed to the stereotypical image
of slow, slimy garden pests.
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Our cover photograph shows Hawaiian tree snails in the genus Achatinella. They were all declared endangered under the United States Endangered Species Act at the same time, the first time and an entire genus has been listed. Photographed in a temporary studio at the University of Hawai'i by David Littschwager and Susan Middleton. |
Living in a World of Tastes and Smells We make fun of them. They symbolize the slow. In an electronic age of e-mail at nanosecond speeds, ground-based postal service is called "snail mail." When we feel bogged down, unable to work efficiently, we say we are "sluggish," and we call our lazy brother-in-law a "sluggard." Their marine cousins are valued for their beautiful shells or the colors and taste of their flesh, and fresh-water species are valued for keeping our fish tanks clean of algal film. But our land snails and slugs, if noticed at all, are seen as pests who will eat our gardens or destroy the value of a farmer's crops. True, the tree snails of Florida with their bright bands of color rival the marine snails in attracting collectors, and some people like snails with butter and plenty of garlic. However, these are rare exceptions, since such animals are usually treated with indifference or hostility. Yet the land snails and slugs of our meadows, marshes, and woodlands not only play vital roles in the balance of these ecosystems, they provide us with an opportunity to investigate creatures which may share our space but live in a world vastly different from our own. Land snails and slugs live in a world made up almost exclusively of smells and tastes. The vast majority are lung bearing, pulmonates, (over 25,000 species) with eyes at the tips of their large posterior (or optic) tentacles; but these are generally thought to be capable of detecting light and dark only. Their main sensory organs are three pairs of chemosensory pads capable of detecting odors or tastes, with each pair of pads appearing to fulfil a different function. The first pair is in the long posterior tentacles and is capable of picking up odors from a distance. The second pair is in the short and stubby anterior tentacles, apparently able to detect odors much closer to the animal. Finally, there is a pair of labial pads adjacent to the mouth, which seem to be involved in taste. (scroll down for complete article, and a list of contents from
this issue) |
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These three pairs of chemosensory pads all connect to the
same portion of the snail's brain, a pair of cerebral ganglia. Do these
animals compare and combine the sensory inputs from their tentacles and
labial pads to construct a world of odor objects in a three-dimensional
odor landscape, much as vertebrates use visual inputs to construct a world
of visual objects in three-dimensional space? Certainly, both pairs of
tentacles are highly mobile, and might be used to sense patterns of odors
and tastes in their environment. They can be extended to different lengths;
they can be held close together or wide apart; they can be pointed in
particular directions. Tentacles can be retracted into the snail's body
by contraction of a muscle that attaches to the inside of the tip, rather
like pulling the finger of a glove inside-out. Each one can be contracted
rapidly and independently of or in synchrony with each of the others.
We don't know for sure the function of this retraction. It may protect
the tentacles from injury or serve to moisten the sensory pad or, perhaps,
serve to clean the pad of odorants. |
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We can determine what odors the animal is sensitive to by inserting
microelectrodes into different nerve cells, then exposing the animal
to an odor and measuring electrical activity in the nerves. Also, we
can trace the paths different nerves take by using tracer dyes. Nevertheless,
we can't really understand the way the animal "sees" its world
unless we pay careful attention to its behavior. We must begin by watching
the animal in its natural habitat-finding food or a potential mate,
avoiding predators, or responding to changes in light or moisture during
the day or during the change in seasons. Then we must challenge the
animal with experiments designed so that the animal itself can tell
us what it smells or sees or tastes by responding to these experimental
challenges. |
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It may seem odd to think in such complex terms about organisms as apparently simple as land snails or slugs. After all, don't they merely smell the nearest food, crawl over to it, and begin to eat it? The more we observe them, the more we realize how much more complex snails and slugs are. For one thing, not all snails are herbivores or fungivores; some are carnivores, eating worms or other snails and slugs. The kauri snail (Paryphanta busbyi) of New Zealand is a carnivore that eats earthworms. Catching a worm by one end, it uses its ribbon of little teeth called the radula (the feeding structure found in all snails and slugs) to pull the worm into its mouth like a piece of spaghetti. We don't know how this snail finds the worms-whether by smell, vibrations in the soil, or other means-or whether it prefers one species of worm over others. In contrast, we do know how the rosey wolfsnail (Euglandina rosea), found in the southeastern United States, finds its prey of other snails and slugs. Using large labial pads, which resemble handlebar moustaches, it tastes the prey's slime trail, then follows the trail to its source. If the source is a small snail or slug, the wolfsnail will swallow it whole; if large, it will rip off hunks of flesh with its radula.
The lancetooths are another taxa of North American carnivorous land snails. The gray-foot lancetooth (Haplotrema concavum) is found in the eastern United States and, like the rosey wolfsnail, seems to find its snail and slug prey by following their trails. However, this lancetooth has never been seen to swallow its prey whole and will seldom attack prey that is bigger than itself. The gray-foot lancetooth does do something totally unexpected, however. It transports its partially eaten prey to a sheltered location where it finishes off its dinner at leisure. Transport is done by a complex series of actions. First, the snail crawls an inch or two away from its prey. It then turns around to face it, and anchors its tail end to the ground. Next it stretches out to the prey, grasps it with its mouth and the front part of its foot, and then drags the prey by contracting its body. Repeating this sequence again and again, the snail can transport the prey to a sheltered location several feet away. Experiments in the lab indicate that the gray-foot lancetooth marks its prey by covering it with mucous slime. We don't know whether the lancetooth in the wild has a preferred shelter; but we do know by experimental manipulations in the lab that the damaged shells of its prey will accumulate beneath particular rocks or pieces of flower-pot. This feature is not unique to the lancetooth snails. In South Africa, similar shell middens are found in gardens frequented by Natalina cafra, a snail that eats the flesh of its snail prey at one sitting, but which carries the shell around on the posterior tip of its foot as it gradually extracts the calcium.
The carnivores are not the only land snails to exhibit unexpected behaviors. Some snails and slugs, even common ones, have complex mating behaviors. For example, mating between two brown gardensnails (Helix aspersa)-a European species that has the distinction of being the first mollusk to be introduced to the United States and has since become a common garden pest of the western states-includes each stabbing its mate with a sharp calcareous "love dart." The function of these love darts is not well understood. They may play a part in stimulating egg release, as the darts are usually used before copulation, but they may also function to inhibit further matings for a period. Like most land snails and slugs, these snails are hermaphroditic but must exchange sperm during the mating process (only one or two species are known to be able to fertilize their own eggs with their own sperm). Another European species now established in the United States is the great gray or giant garden slug (Limax maximus). Courtship usually begins on a branch, where the slugs pursue one another in circles for two hours or more until they clasp together and fall from the branch, finally exchanging sperm while suspended on a long slime thread extruded as they fell!
Great gray slugs also exhibit other intriguing behaviors, including strong homing capabilities. As nocturnal feeders they return to shelter before the sun rises. There is evidence that they have a favored home shelter that they return to by following slime trails, and yet they can also locate the shelter without the slime trail to follow. In the latter cases they apparently locate the shelter by airborne odors. Experimental investigations of such homing behavior have revealed that some slugs will defend their home from intrusions by slugs of other species. When several different species of slugs are maintained in an enclosure with a single source of food, they will sort themselves out into a hierarchy with the more aggressive species occupying the shelters closest to the food source.
A long-term study in Denmark of the escargot snail (Helix pomatia) in the wild revealed a remarkable ability of these animals to navigate from woods to meadow and back again. These animals overwinter in the woods, buried in the soil below the leaf litter. They emerge in the spring and migrate out of the woods into the meadows and large glades where they feed on some of the green plants growing there. In the fall, responding to undetermined seasonal cues, these snails migrate back into the woods and bury themselves within a few inches of the site from which they had emerged many months earlier. How do they find their way? Do they keep a memory of the odors of objects within the woods? If so, how do they compensate for changes in the objects between the spring when they emerge and the fall when they return?
There are many things that remain to be learned about land slugs and snails. What we do know of them suggests that they have capabilities that exceed our expectations. In our laboratory we are challenging herbivorous land snails with an odor-permeable barrier between them and their food. We find that most of the snails have been able to solve the problem by turning away from the food and slowly crawling around the barrier. This detour behavior is one that many animals with larger and more complex brains are unable to perform. The success of these snails has prompted us to present them with increasingly challenging situations requiring them to navigate through a complicated landscape of odor objects in order to find their food. Rather than being merely attracted to or repelled by a specific object, these deceptively simple creatures may be able to recognize and remember the relative locations of objects and know their position within the landscape. Their world may be richer than we have imagined. Our world is certainly richer for knowing about them.
About the Author: Jim Atkinson is a Professor of Zoology at Michigan State University. He received his AB degree from Kenyon College and his Ph.D. from Emory University. At MSU, he teaches courses in invertebrate zoology, curates the University's collection of preserved, non-insect, invertebrate animals, and pursues his research interests in molluscan development, behavior, and evolution.
Also in this issue:
Jumping-Slugs! by William Leonard and Kristiina Ovaska. Little known residents of Pacific Northwest forests, "Hemphill's slugs" have a remarkable and unexpected way to confuse predators.
Kahuli: The Tree Snails of Hawai'i, photographs and text from David Liitschwager and Susan Middleton's Remains of a Rainbow: Rare Plants and Animals of Hawai'i showcase these spectacular snails, which are threatened by invasive species and habitat loss.
Saving the Tumbling Creek Cavesnail, by Tom Aley and David Ashley. Extensive efforts are being made by private landowners, scientists, and private agencies to save a unique cave-dwelling snail.
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