Pomacea spp (Blume, 1957)
Common names: Apple snail; Spike-topped apple snail, Mystery snail (English)
Synonyms: Pomacea bridgesii diffusa
Pomacea diffusa was originally described as a subspecies of Pomacea bridgesii. However, the validity of the two taxa (P. diffusa and P. bridgesii) has been confirmed through genetic analyses (Cowie et al., 2006).
Pomacea diffusa is a medium sized snail with a globular shell. The shell bears a raised spire and an operculum, but lacks a channelled suture. Shell colour is generally brownish or golden-brown to yellow. Body colour varies from yellow to off white (Mordan et al., 2003).
Total length: Length of the shell ranges between 4-5 cm
Morphologically similar species:
Pomacea diffusa looks very similar to other species of its genus. It is often confused with P. canaliculata, a major invasive pest species in Southeast Asia. In P. diffusa, the top of the whorl is flat and step-like. However, in P. canalliculata, the line or suture, where the whorls meet, cuts down to form a distinctly v-shaped depression (Mordan et al., 2003). Pomacea diffusa is also, superficially, similar to the native snail species Pila globosa. But they can be readily distinguished from each other by the golden-brown shell of P. diffusa.
History of introduction: Pomacea diffusa was introduced to Sri Lanka through the aquarium industry probably in the late 1970s.
Pomacea diffusa is native to the Amazon basin in South America. Introduced populations have been recorded from India and the southeastern US (Cowie, et al., 2006; Devi et al., 2001).
Established populations of P. diffusa were first observed in water bodies in Colombo and Galle in the early 1980s (Epa, 2006; Mendis, 2012). Currently, it is recorded in Kalutara, Kandy, Ratnapura, Gampaha, Matara and Galle (Mordan, et al., 2003).
Dispersal and reproduction:
Apple snails lay eggs above the water surface. The egg masses have an irregular honeycombed appearance, and have a tan similar to that of a salmon white colour when freshly laid (Mordan, et al., 2003).
Impact on native species and habitats:
The possible effects of P. diffusa on natural habitats and native species are not well documented. However, there is a report from India revealing that in addition to macrophytes, P. diffusa feeds on animal carcasses, live worms, and the eggs of planorbid snails (Devi et al., 2001). Thus, it may have direct effects on both aquatic vegetation and native snails. It could also compete for food with native scavengers such as crabs, shrimps and fish.
Direct exploitation/ destruction of native species:
Direct exploitation of native species by P. diffusa is not known. But, it may have a significant impact on the native plants, snails and other native scavengers as stated above.
Current uses: Apple snail is commonly used in the aquarium industry.
Natural threats (predators): None reported. The diet of the native Asian open-bill stork (Anastomus oscitans) consists mainly of snails (Pila sp.). Therefore it is possible that these birds may also prey on P. diffusa.
Prevention and control: Control methods that are being used for P. canliculata can be employed to control P. diffusa. However, currently used methods (e.g. pesticides) to control P. canaliculata can be harmful to natural ecosystems. Manual removal of all life stages from agricultural and natural ecosystems is a more ecofriendly method. However, this is extremely labour and therefore not a feasible contral method.