Florida’s Invaders: The Apple Snail

Many of Florida’s invaders arrived here through the aquarium hobbyist trade, and that includes one of our recent arrivals—the Apple Snail.

There are probably thousands of freshwater snail species throughout the world. Most of them lead drab uneventful lives, munching on plants, and are of interest only to the handful of scientists who study mollusks. One such group are in the genus Pomacea, which contains around 30 species in South America. “Pomacea” means “apple”—the snails were named for their round shape and large size (some are as big as three inches in diameter). The Apple Snails all look the same and have similar dull brownish bands of color, and it takes an expert to tell them apart.

Like nearly all snails, the Apple Snails are vegetarian, living on a diet of water plants and algae. Snails have a raspy tongue with many small teeth, called a “radula”, which they use to scrape food into their mouths. The Pomacea also have their gills located inside a lunglike pouch. During warm weather when the dissolved oxygen levels in their pond are low, the snails use these gilled pouches to breathe air at the surface, and because of this they prefer to stay in shallow water near shore.

Only one of these species is native to Florida. The   Florida Apple Snail, Pomacea paludosa, also found in Cuba and the Caribbean, is one of the smaller members of the genus. Found mostly in the Everglades, it is not very cold-tolerant and is limited to southern Florida, though it is occasionally seen elsewhere in the southeastern US where the water is artificially heated, such as power plants. Fossils show that the Florida Apple Snail hasn’t changed much in several million years.

In the 1980s, pet dealers in the US began importing Apple Snails from South America for the aquarium trade. Hobbyists liked them because they were very efficient at scraping algae off the glass and keeping the tanks clean. Soon they were being bred in a variety of colors, including albino white and a deep golden yellow. Because nobody was sure which species they actually were, they were usually sold under the name “Mystery Snails”.

It wasn’t long before dumped non-native Apple Snails began turning up in the wild. Florida officials first noticed them in 1987, in some canals near Lake Okeechobee. Within a short time they could be found in nearly every urban area in Florida, and across the US. By the 1990s they had reached Europe, the Philippines, Hawaii, and China.

There seemed to be several species involved. One, identified as the Spiketop Apple Snail, Pomacea diffusa, appears to have settled into the Florida ecosystem and is harmlessly slurping up algae. But another species turned out to be not so harmless. Initially identified as the Channeled Apple Snail, P canaliculata, this one was becoming an aggressive invader. Unlike the Spiketop and the native Florida Apple Snail, both of which preferred to graze on algal and bacterial pond scum, these larger Apple Snails targeted aquatic plants. Heavily-infested ponds soon became denuded, and the snails were spreading quickly, with eggs and young snails carried from one waterway to another by sticking to the feet of ducks and the bottoms of boats. By 2000, the invaders could be found in almost three-fourths of the state’s surface waters. Genetic study re-identified the Mystery Snails as the Island Apple Snail, P insularum—and, to add to the confusion, this species was shortly later lumped together taxonomically with another as P maculata.

The invaders had a couple of ecological advantages over the native snails. The Mystery Snail, as well as being larger, is more cold-tolerant and can establish itself in areas where the native Florida Apple Snail cannot survive. The invasive Apple Snails also produce a greater number of eggs, which are deposited in masses on plant stems, where they resemble wads of bubble gum. The bright pink is a warning color—the eggs contain a mild toxin that protects them from predators.

The state of Florida has now taken steps to control the snails, and there are programs to find and destroy egg masses, bait and trap the adults, and treat ponds with chemical toxins that kill the mollusks. But the effect so far has been limited.

One Florida species, however, has welcomed the invaders. The Snail Kite is a small raptor which specializes in a diet of snails—particularly the native Florida Apple Snail. As drought and habitat loss reduced the number of snails, the bird’s population declined to less than 1,000: it was listed as an endangered species. But as the population of invading Apple Snails has grown, so too have the birds which now feed on them. The Snail Kite has doubled its numbers in the past decade, thanks to the bonanza of Mystery Snails.

Advertisements

Post a Comment

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s