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Alien Snails

By using the molecular signature displayed by the organisms DNA, a team of evolutionary biologists have clarified the identity and distribution of exotic (non-local) freshwater apple snails (Ampullariidae) in the continental U.S.
Dr Tim Rawlings of the Biology Department of CBU was part of an international team trying to establish the identity of these alien snails and to determine where they had come from. In the late 90s, he was interested in assessing local populations of Florida’s native apple snail, Pomacea paludosa, and while doing so began to encounter a number of other species of apple snails that he didn’t recognize - these were the exotic snails.
Human behaviour can play a major role in facilitating the invasion of exotic species, which in turn may threaten biodiversity, ecological function and the very survival of other species. Scientists suggest that about 50,000 exotic species have been introduced into the United States, and that the havoc they wreak upon agricultural crops, wildlife, ranch animals, forests and grasslands costs about $137 billion a year.
This study, recently published in BMC Evolutionary Biology, a scientific journal, clarifies the identities, introduced distributions, geographical origins, and introduction histories of exotic freshwater apple snails in the continental U.S. Such information should help to predict the impact that these exotic species may have on the freshwater ecosystems of Florida and other states.
Pomacea haustrum is a large species of apple snail native to Brazil, Peru and Bolivia. The molecular signature of a specimen from Peru was compared with specimens from Florida, and the match was close enough to indicate that the Florida specimens were probably derived from populations in the Amazon river systems. This species of apple snail appears to have arrived in the Palm Springs area in the 1970s and has not spread appreciably in thirty years.
In contrast, molecular matching suggests that P. insularum was introduced to Florida by the mid to late 1990s. This species is native to Rio Uruguay, near Buenos Aries, and is known for its cold-tolerance, which means that it has the potential of spreading north. Indeed, the spread of P. insularum has been very rapid with many new populations being established across Florida within the last few years.
The introduction of exotic apple snails, many of which grow larger than the Florida’s native apple snail, Pomacea paludosa, may spell trouble for animals reliant on P.paludosa as a primary food source.  The Everglade snail kite, for instance, is an endangered species of bird which relies heavily on the native apple snail as a food source. Rawlings points out that research underway by others is suggesting that the snail kite may not be able to forage as successfully on these larger heavier prey items compared to the smaller lighter native apple snail. Should the exotic species compete successfully against the local species, then the endangered kite may suffer from a reduced food supply.
Studies have shown that invading species are responsible for about half of all native species that go extinct. When one compares the egg masses of P. paludosa (native) with P. insularum (d. versus e. in accompanying picture), one can easily see that P. insularum egg masses win in terms of the number of eggs per egg mass - some 1000+ compared to the 30-40 eggs of the native apple snail.
Invasive species can threaten the ecosystem in a number of ways. One documented example is by introducing new parasites. Rawlings provides another example particular to exotic apple snails, many of which are voracious herbivores,—the rapid consumption of all large aquatic algae within lakes. This may cause a lake to become murky, as nutrients are released into the water column and stimulate the growth of microscopic planktonic algae.  This habitat change can in turn disrupt the environment for many other plant and animal species.
How can South American snails suddenly appear in Florida? Rawlings speculates that many of the exotic apple snails were introduced by humans through importation as pets for aquariums. They then may have dispersed across the state by attaching themselves to fishing vessels or being transported as bait by fishermen.
This study demonstrates the use of molecular techniques to document the invasion and spread of an invasive species. Knowledge of this phenomenon may be important to understanding past invasions and predicting or controlling future ones.