The Pomacea Project - Non-Native (Exotic) Apple Snails in Florida

The Florida apple snail (Pomacea paludosa) belongs to the Family Ampullariidae; this family includes 7-10 known genera that are found in tropical and sub-tropical regions of Africa, South East Asia, and North, South, and Central America (Cowie 2002). The large size, round or globose shape, and greenish shells of some Ampullariids (of the genera Pomacea and Pila, in particular) are what gave them the common name “apple snail." [See dictionary definitions for 'pome', derived from the french word for apple.] The rapid spread of exotic snails in the U.S. and asia has resulted in many published studies and information available on websites (see website section below).

The Only Native Apple Snail in Florida

The first description of the Florida apple snail (Pomacea paludosa) was by Say in 1824 from the St. Johns River and its tributaries (Clench and Turner 1956). This species is the only native apple snail in the United States, and it ranges as far north as southern Georgia (Thompson 1984) and as far south as the Everglades. [See Florida Apple Snail Basic Biology for the basic biology of the native species].

The Non-native, or Exotic, Apple Snails

Some of the apple snail species are difficult to distinguish from one another. This has led to misidentification in the U.S. and elsewhere. Rawlings et al. (2007) used phylogenetic analyses of mtDNA sequences and study of introduced populations and museum collections to identify non-native apple snail species in the U.S. (especially Florida) and their distributions and geographical origins. Based on their research,, there are four non-native Ampullariid species in Florida; one of these four species (Marisa cornuarietis) has a different life history and appearance (e.g., lays gelatinous eggs in water) and will not be discussed here. The three non-native apple snails discussed here are: 1) Pomacea insularum (previously identified as P. canaliculata in Florida), 2) Pomacea haustrum, and 3) Pomacea diffusa (previously identified as P. bridgeseii in Florida). Currently, the best way to definitively identify these species is through a combination of genetic information, morphological characteristics, and egg mass morphology (Rawlings et al. 2007).

Only a few details on these species, primarily from Rawlings et al. (2007), are presented here. For more information on these species and this work, see Rawlings et al. (2007) and other references and website links provided in this section. Also see the website link listed below for Rawlings et al. (2007) that contains excellent photos comparing egg masses and shells of native and non-native apple snails.

Threats Posed by Non-Native Apple Snails

Where they have invaded elsewhere, some of the non-native apple snails have proven a serious threat to agriculture, native wetland ecosystems, and human health (Rawlings et al. 2007). Pomacea species (e.g., P. canaliculata) have caused the most damage to crops, particularly to rice in Southeast Asia (Cowie 2002). The spread of Pomacea in the Philippines has been rapid, being found in 9,500 ha of rice in 1986 to 500,000 ha in 1989 (Cowie 2002). Rawlings et al. (2007) suggest that due to confusion in identifying P. canaliculata and P. insularum, some of the damage in Asia that has been attributed to P. canaliculata was probably due to P. insularum.

In a study of the effects of P. canaliculata on aquatic plants and ecosystem functioning in wetlands in Thailand, Carlsson et al. (2004) found in a field survey that high snail densities corresponded to a shift in ecosystem state and function (an absence of aquatic plants, high nutrient concentration, and high phytoplankton biomass); in a field enclosure experiment they found that snail grazing caused a loss of aquatic plants that led to dominance by planktonic algae.

On Lake Tohopekaliga in Central Florida, Darby et al. (2007) have observed the non-native P. insularum in wetlands where the team has been sampling for the native apple snail. Darby et al. (2007) observed endangered snail kites foraging on P. insularum individuals, but kites dropped 44% of the non-native snails captured (compared to 0% of native, P. paludosa, apple snails). The high drop rate, likely due to the much larger size of P. insularum (see pictures above that compare shells of these two species), may affect some snail kites (e.g., less experienced juveniles) in their obtaining food. [A thesis that deals wtih this topic in much greater detail, completed by C. Cattau, UF, Gainesville, is being reviewed for publication.]

Rawlings et al. (2007) also suggest concern for the native, P. paludosa, due to: potential hybridization of P. paludosa with non-native species; competitive interactions between native and non-native snails; and potential future efforts to control the non-natives by targeting snails or their eggs (and accidentally confusing them the native species).

Invasion by non-native apple snails is also of concern due to the parasites that the snails may harbor. Such parasites could affect humans or wildlife predators of native apple snails.

Due to increasing concerns with non-native snails, the U.S. Department of Agriculture began requiring permits for importation or interstate movement of aquatic snails in 2006 (Rawlings et al. 2007).