by Bob Thomas
New Orleans has a new resident, and we shall see if it is a welcome addition.
Our new citizen is the Island Apple Snail, Pomacea insularum, a large freshwater snail that is almost two inches in diameter. It has taken up residence in canals on the West Bank, and is well enough established to be actively breeding.
There are five or so species of apple snails in the U.S., and only one Florida species is native. Florida hosts all the species, but they appear to be spreading and are expected to be found in most southern states. Being tropical species, they don’t tolerate cold temperatures well. All the non-native species found in our country are native to South America and are believed to have been accidentally released by aquarists.
At home in their preferred freshwater environs, they have steadily spread, and are believed to have been further distributed via floods and being swept down streams.
Oxygen exchange takes place with gills when underwater. If the water becomes low in oxygen, apple snails simply extend a siphon above the surface that is connected to a lung-like air sac. During drought, they may simply close the opening to their shell by withdrawing their foot and the operculum (a corneous structure that acts as a door), thus sealing their living quarters and hopefully enduring a period of dormancy.
Apple snails feed on vegetation, and the Island Apple Snail seems to prefer rooted plants that are abundant in their habitat. They may consume periphyton, the mats of algae that are common in wetlands. Unfortunately, they don’t eat water hyacinths and salvinia.
They are fed upon by most animals that routinely eat aquatic invertebrates such as alligators, turtles, predatory fish, raccoons, otters, and the like.
Apple snails reproduce throughout the warmer months. They are hermaphroditic, each snail having both sexes. Mating, however, requires two snails. They crawl out of the water at night and lay pink, calcareous masses of about 1000 eggs above the waterline on firm objects in or near the water. The eggs whiten a bit as they near hatching, and the young leave the egg clusters and immediately enter the water.
There are two possible reasons for laying eggs out of the water: to avoid low oxygen, as periodically happens in many wetlands, and to reduce predation by aquatic critters.
Though we always fear the impacts of invasive species, there has been little evidence in Florida that they have hurt the native species, other mollusks, or wetland vegetation. If they become a problem, we can market them as a new ingredient for gumbo!