Synonym(s): island applesnail, apple snail; previously known as Pomacea insularum
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Apple snails (Pomacea maculata) can grow to up to six inches high—about the size of a baseball or human fist—although a size of approximately 1.5 inches is more common. The shell opening or aperture is large and oval or round and covered by a bony operculum with concentric rings that can be retracted into the aperture. Shell color varies from yellow to brownish black with or without dark spiral bands. Their presence is often first detected by observation of their bright pink egg masses, which are laid just above the water line on emergent vegetation, woody debris, and manmade structures. Remnants of these egg masses can remain long after the snails have hatched and fade to nearly white over time.
Ecological Threat: Negative ecological effects of this invasive snail species include habitat degradation and competition with native snails. Apple snails are voracious herbivores and feed on a wide variety of both native and introduced submerged, floating, and emergent aquatic plants. These invasive snails reproduce prolifically and can attain high population density, decimating the aquatic vegetation. In addition to their harmful effects on existing habitat, herbivory by these snails can threaten the success of wetland restoration efforts.
Invasive apple snails cause significant damage to rice crops in Asia and potential for similar economic impacts in Texas is a major concern. Although damage to rice crops in Texas has not yet been documented, some rice farmers have experienced increased levee maintenance costs due to the constant burrowing by snails.
Apple snails also pose a risk to human health as they are an intermediate host to the rat lungworm, Angiostrongylus cantonesis, a parasite that can infect mammals, including humans. Humans can become infected by this parasite by eating raw or undercooked snails or by eating raw produce contaminated by rat lungworm larvae. Humans with mild infections usually recover fully. However, severe infections of the parasite can cause eosinophilic meningitis. Rat lungworm has been found in apple snails collected from New Orleans, Louisiana. Thankfully, they have remained isolated, and there have been no cases of human Angiostongylus cantonensis infections.
Biology: Apple snails are a subtropical to tropical species that prefers oxygenated, slow-moving waters. The sexes are separate and fertilization is internal. Egg-laying occurs during cooler times of day, and only during warmer months in subtropical areas. Females lay clutches of 500-700 bright pink eggs just above the water line on vegetation and other hard surfaces; each egg is approximately 1-2 mm in diameter. Female apple snails may lay clutches of eggs every 5-14 days. When juveniles hatch, they fall or crawl into the water.
Apple snails tolerate salinities up to 7-8 parts per thousand (ppt) well and are not likely to invade estuarine habitats. Lower temperature tolerance of this species has been determined to be 50℉ and may limit the northern distribution of the species in the U.S. Apple snails have a broad pH tolerance of 4.0-10.5, which includes all bodies of freshwater in North America. During periods of extreme drought when habitats lose water, these snails may survive by burrowing and aestivating.
History: Historically, P. maculata (also sometimes known as the island apple snail) has been confused with the channeled apple snail, P. canaliculata, in the United States, which is nearly identical in appearance. The first specimens reported in Texas were believed to be channeled apple snails but DNA testing has since confirmed that P. maculata apple snails are the species established in Texas. The primary introduction vector appears to be dumping of aquaria although spread during flooding is likely to occur and overland movement may be possible.
The snail was introduced into Taiwan from Argentina for commercial production in the 1980s. From Taiwan, the snail was distributed to developing countries to help the rural poor earn additional income through backyard rearing and to supplement protein in their diets. The snail was introduced without prior studies on market demands or its impact on the ecosystem. The snail has spread from South America to Southeast Asia and reached Hawaii in the 1990s. In Hawaii, the apple snail has caused significant damage to taro crops.
The earliest reports of apple snails in Texas were in the late 1970s and mid 1980s and are believed to be isolated introductions that did not lead to established populations. In 2000, extensive surveys in the southeast Houston area found that these invasive snails were established in Mustang Bayou and the nearby American Canal and anecdotal reports suggested they were also found in two private ponds in the area. Since that time, they have spread to numerous other watersheds in southeast Texas.
U.S. Habitat: Apple snails are found in shallow freshwater habitats including streams, bayous, ponds, irrigation canals and rice fields.
Native Origin: South America
U.S. Present: AL, AZ, CA, FL, GA, HI, LA, NC, SC, TX and VA. Records of this snail are not consistent and it may be far more established within the United States.
Distribution in Texas: Apple snails have established populations in the following sub-watersheds: Austin-Oyster, Buffalo-San Jacinto, Lower Angelina, Lower Brazos, Lower Trinity, Lower West Fork Trinity, South Laguna Madre, Spring, West Galveston Bay and were recently found in Lake Sam Rayburn. These snails are one of many species that may have been spread as the result of flooding associated with Hurricane Harvey in late summer of 2017.
May be confused with any of the apple snails of the genus Pomacea, particularly Spiketop Apple snails, P. bridgesii, which are still legally sold in the aquarium trade as “mystery snails”. However, the only species of apple snail that is likely to be found in Texas is P. maculata.
Spiketop apple snails have a more prominent spire and the angles between whorls are close to 90 degrees, whereas those of P. maculata are far more obtuse.
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Management options for apple snails are limited, particularly in areas where chemical treatments could negatively impact native aquatic life or drinking water. Physical removal and destruction of egg masses can help to reduce snail abundance and size. However, research shows that reproduction and growth are significantly greater when apple snail populations are at low density. Their only known terrestrial predator in the U.S. is the raccoon but such predation is not an effective control. Management of aquatic vegetation in apple snail-infested areas has the potential to exacerbate their impacts and should be undertaken with care. In lakes in the Kissimmee River chain in Florida, chemical treatment of invasive hydrilla is believed to have had the unintended consequence of causing a dietary shift in apple snails to consumption of Kissimmee Grass with devastating effects on wetland habitat.
When egg masses are found, they should NOT be scraped into the water to drown them. Instead remove and/or crush them.
In Japan, management of apple snails in rice fields has generally achieved good results in fields of transplanted rice. Snails do not feed on transplanted rice in shallow water. Keeping paddy water shallow thus helps to control snail damage and is now the most commonly used management practice. Transplanting older rice is also effective because rice gradually becomes tolerant to the snails as it grows. Using these practices, with occasional pesticide applications, snails can be reasonably well controlled in transplanted rice.
Burlakova, Lyubov, Alexander Karatayev, and Dianna Padilla, Leah D. Cartwright and David N. Hollas. "Wetland Restoration and Invasive Species: Apple snail (Pomacea insularum) Feeding on Native and Invasive Aquatic Plants." Restoration Ecology. 17.3 (2009): 433-440.
Carlsson, N. O. L., and J. O. Lacoursière. 2005. Herbivory on aquatic vascular plants by the introduced golden apple snail (Pomacea canaliculata) in Lao PDR. Biological Invasions 7:233-241.
Cowie, R. H. 2002. Apple snails as agricultural pests: the biology, impacts, and management. Pages 1-28 in G. M.Barker, editor. Molluscs as crop pests. CABI, Wallingford, Connecticut.
Estebenet, A. L., and P. R. Martín. 2002. Pomacea canaliculata (Gastropoda: Ampullariidae): life-history traits and their plasticity. Biocell 26:83-89.
Hayes, K. A., R. H. Cowie, S. C. Thiengo and E. E. Strong. 2012. Comparing apples with apples: clarifying the identities of two highly invasive Neotropical Ampullariidae (Caenogastropoda). Zoological Journal of the Linnean Society. 166:723-763. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1096-3642.2012.00867.x
Naylor, Rosamond. 1996. Invasions in agriculture: assessing the cost of the golden apple snail in Asia. Ambio 443-448.
U.S. Fish & Wildlife Service. Giant Applesnail (Pomacea maculata) Ecological Risk Screening Summary: https://www.fws.gov/fisheries/ANS/erss/highrisk/ERSS-Pomacea-maculata-FINAL-March2018.pdf
Teem, J. L., Y. Qvarnstrom, H. S. Bishop, A. J. da Silva, J. Carter, J. White-Mclean, and T. Smith. 2013. The occurrence of the rat lungworm, Angiostrongylus cantonensis, in nonindigenous snails in the Gulf of Mexico region of the United States. Hawai'i Journal of Medicine & Public Health 72(6 Suppl 2):11-14. Accessible here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3689474/
Teo, Su Sin. 2001. Evaluation of different duck varieties for the control of the golden apple snail (Pomacea canaliculata) in transplanted and direct seeded rice. Crop Protection 20(7):599-604.
Wada, T. 1997. Introduction of the apple snail Pomacea canaliculata and its impact on rice agriculture. In Proceedings of International Workshop on Biological Invasions of Ecosystems by Pests and Beneficial Organisms, ed. National Institute of Agro-Environmental Sciences, Tsukuba, Japan, 170-180.
Yusa, Y. & Wada, T. 1999. Impact of the introduction of apple snail and their control in Japan. Naga, ICLARM Q 22(3):9-13.
Presentation by Dr. Romi Burks, Southwestern University