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Apple snails

Pomacea bridgesii
Pomacea diffusa (old name: Pomacea bridgesii)

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Apple snails are tropical and sub-tropical freshwater snails from the family Ampullariidae (sometimes referred to as Pilidae). The Ampullariidae are divided in several genera. The genera Asolene, Felipponea, Marisa, and Pomacea are the New World genera (South America, Central America, the West Indies and the Southern U.S.A.), while the genera Afropomus, Lanistes and Saulea are found in Africa. The genus Pila is native in both Africa and Asia.
For more info about the apple snail species: take a look at the species section of this site.
How to recognise an apple snail: basic guide to discriminate between apple snails and other freshwater snails.

Well adapted
Apple snails are exceptionally well adapted to tropical regions where periods of drought are alternated with periods with excessive rainfall.
This is reflected in their partly amphibious life style and their anatomy. Apple snail have, for example, a shell door that enables them to close the shell and thus prevent dessication (while hibernating in the mud during dry periods).
Another typical adaptation of apple snails is the combination of a branchial respiration system comparable with the gills of a fish (at the right side of the snail body) and a lung (at the left side of the body). This lung/gill combination expands the action radius of the snail in search for food, allowing them to leave the water for short periods of time.
Many apple snail species deposit the eggs above the waterline in a calcareous clutch. This remarkably strategy protects their eggs against predation by fish and other water inhabitants. Another predator specific adaptation in the apple snail genera Pomacea and Pila, is the tubular siphon at their left side, used to breathe air while they stay submerged, thus making them less vulnerable to snail eating birds.
Apple snails inhabit various ecosystems: ponds, swamps and rivers. Although they occasionally leave the water, they remain mainly submerged.
Despite the fact that many snail species are hermaphrodite (being male and female at the same time) apple snails are not: they have separated sexes (gonochoristic) and a male and a female are needed for reproduction
For more info about the anatomy of apple snails: take a look at the anatomy section of this site.

A common aquarium pet
The apple snails are popular aquarium-pets because of their attractive appearance and size. When taken good care of some apple snail species can reach a large size (15 cm / 6 inch diameter in case of Pomacea maculata, sometimes faulty referred to as Ampullarius gigas). Apple snails are in fact the biggest living freshwater snails on earth. The most common apple snail in aquarium shops is Pomacea diffusa (spike-topped apple snail). This species comes in different colours from brown to albino or yellow and even blue, with or without banding. The body of these snails also shows great variation from black to yellow and grey. Another common apple snail is Pomacea canaliculata, this snail is bigger, rounder and is more likely to eat your plants, which makes it less suitable for most aquaria. These snails also come in different shell and body colours. The Giant ramshorn snail, Marisa cornuarietis, although not always recognized as an apple snail due to its discoidal shape, also ranks the popular aquatic pets.
Occasionally, the Florida apple snail (Pomacea paludosa) is found in the aquarium trade and are often wild collected from ditches and ponds in Florida. The giant Pomacea maculata, raremy makes its way into aquaria. With tropical fish expeditions, sometimes other apple snail species are collected and offered for sale.
Apple snails are often sold under the name Golden mystery snail and are given incorrect names like Ampullarius for the genus instead of Pomacea and species names like gigas instead of maculata. More info about this can be read in the species section of this website.
Since 2000, there are restrictions around the globe regarding the trade in apple snail. Many species have turned out to devastating when release in the wild. Unfortunately people do release these snails, forcing governments to ban the trade thes esnails and even outright prohibit the possession. So many of the apple snail species listed on this site are no longer easily to obtain through pet shop (for a good reason).
For more info about how to keep and breed apple snails: take a look at the care section of this site. 

A blessing or a pest? 
In the 1980's, the genus Pomacea (like Pomacea canaliculata) was introduced in Taiwan to start an escargot industry. Such food culture can provided protein for the local population, especially useful for the farmers, who primary live on a rice diet, low in proteins. However, the snails didn't become a culinary success. It also became quickly clear that the imported species were able to transfer the Angiostrongylus cantonensis (rat lungworm) parasite just like the native apple snail population (Pila). This parasite spends a part of its life cycle in apple snails and can infect humans when the snail isn't cooked long enough before consumption
Instead of becoming a food source the apple snails escaped, and became a serious pest, posing a real threat to the rice production and the environment. During the 1980's the introduced snails rapidly spread to Indonesia, Thailand, Cambodia, Hong Kong, southern China, Japan and the Philippines and there are indications that they are currently invading Australia. Nevertheless, apple snails are considered a delicacy in several regions and they are often sold in Oriental markets for consumption. In 1989 Pomacea canaliculata was introduced in Hawaii to serve as a food source and aquarium pet. Some snails escaped to the wild and turned into a serious pest in the taro and rice fields. Although a few restaurants serve them, the apple snail didn't become a great gastronomic success here either.
Recent research reveals the catastrophic nature of the Pomacea invasion in new habitats: the snail herbivory drastically alters the state and function of invaded natural wetlands. When the plants are consumed, nutrients in the system are shunted to phytoplankton instead of the plants which creates dense algal blooms (Carlsson et al in press. 2004).
Pomacea and Marisa species have been introduced in Africa and Asia to control snails (Planorbidae: Bulinus sp. and Biophalaria sp.), which serves as intermediate host for trematoda parasites. These parasites can cause swimmers itch and schistosomiasis, a disease that affects over 200 million people in tropical regions. Despite the fact these tremadote parasites do not complete their life cycle in apple snails, apple snails themselves can carry these parasites and nematodes of the genus Angiostrongylus. Angiostrongylus cantonensis can afflict humans and cause eosinophilic meningoenchephalitis. One of the species was introduced as bio-agent is Marisa cornuarietis . This apple snail competes with other snails and predates on them. Hopefully Marisa is less likely to become a pest for the food production.

Update: In late July 2009 an incipient invasion by Pomacea canaliculata was detected in the Ebro Delta, on the Mediterranean shore of Catalonia, Spain. By early September the spread of this applesnail was tremendous, with huge economic losses to rice cultivation. Research on this population's biology and possible control methods is ongoing.
Some reading about this:

Conclusion: The recent spread of apple snails over the world and their ability to become a pest illustrates the dangers that come along the introduction of non-native species. It should be clear that this is the main reason why many countries have very strict rules when it comes to importing foreign animals like apple snails (and other animals).
"It should be clear: never release your apple snails in local ponds or rivers!"

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