Posts Tagged 'Dispersal'

Steady Invasion of Florida’s Public Waters by Pomacea maculata

2014 Graph_edited-1

Each year aquatic biologists from Florida Fish and Wildlife Conservation Commission survey all of Florida’s public water bodies – – lakes and rivers with both state sovereignty and public boat ramps. This effort by the Invasive Plant Management Section focuses on aquatic vegetation, especially invasive, exotic plants, such as hydrilla and water hyacinth. In 2006, however, Pomacea maculata was added. Rob Kipker, my former supervisor there, is kind enough each year to provide me with the current data. His chart above shows a steady increase in the number of water bodies invaded by the South American snail. As of 2014, 36% of Florida’s public 450 waters are affected. Even more alarming is that Pomacea maculata can be found in 72% of Florida’s lakes and rivers by area! Jess Van Dyke


How is the Island Apple Snail spreading so rapidly in Florida?

According to FWC’s data presented at the end of my last post, the Island Apple Snail (Pomacea insularum) was found in less than 1% of Florida’s public waterbodies in 2006. Five years later, its range has exploded to 22% the state’s lakes and rivers. How is that possible for an animal that moves an average of only 14 meters/week (Darby et al., 2002)? None other than Charles Darwin (1859) was also “perplexed much” when contemplating wide distribution of certain freshwater snail species among the distant Pacific Islands.

There are two modes of range expansion for organisms: active and passive. The active mode for mollusks is at a proverbial “snail’s pace.” Nevertheless, snails are common globally, including desert oases and newly formed volcanic islands. Clearly, mollusks possess an extraordinary capacity for passive means of dispersal. In the case of P. insularum, I have witnessed them using temporary buoyancy to move easily with the waves across lakes or with the current of creeks to rapidly float downstream. Such passive mobility can easily explain dispersal within a given watershed. However, I have also seen this primarily-aquatic species quickly and inexplicably appear in newly-constructed, isolated ponds. Passive dispersal by human activities is well-documented and, therefore, usually blamed, but I wonder. Range expansion in such far flung areas just seems too commonplace. There cannot be that many hobbyists recklessly dumping aquaria!

Charles Darwin (1859) conducted an experiment on a theory proposed by Lyell (1832) that external transport by birds is the most likely passive dispersal mechanism of freshwater snails: “I suspended the feet of a duck in an aquarium; where many ova of freshwater shells were hatching; and I found that numbers of the extremely minute and just-hatched shells crawled on their feet, and clung to them so firmly that when taken out of the water, they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just-hatched mollusks, though aquatic in nature, survived on the duck’s feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and if blown across the sea to an oceanic island, or to any distant point, would sure to alight on a pool or rivulet.”

Many researchers after Darwin have been equally “perplexed” by the rapid and/or long distance dispersal of slow-moving mollusks, but a scientific consensus is developing. Vagvolgyi (1975) concluded that small body size facilitated the dispersal of land snails across broad expanses of ocean: “Support to the hypothesis is provided by the facts that land snails have been recovered from the plumage of birds [and] that recently formed volcanic islands have been colonized predominantly by minute land snails.”

In a study of 50 springs widely scattered across the arid regions of Australia, Wilmer et al. (2008) determined that “short range dispersal of aquatic snails occurs via active movement facilitated by aquatic connections among springs while long-range dispersal is likely facilitated by an animal vector (phoresy).” Aubry et al (2006) stated that passive dispersal of an invasive snail in France relied on “a behavior, called the ‘climbing reflex’ – – one of the main and most efficient features in the process of passive dispersal.”

Clearly, the theory that birds transport aquatic snails is not new.  In fact, it is no longer a theory but has been demonstrated convincingly, both experimentally and by field observations. “The pulmonate land snail Balea [has]even managed to travel over thousands of kilometers of open ocean, from Europe to the Azores and the Tristan da Cunha islands, and back again” (Gittenberger et al., 2006). In the case of the Island Apple Snail, I wonder if the “climbing reflex” is innate behavior for newly-hatched juveniles. It would certainly be easy for such small snails to attach to the legs of wading birds frozen in their common fishing stance.

To test that theory, I went back to my favorite experimental site, Wellman Pond, and placed 30 bamboo stakes with diameters similar to those of the legs of wading birds near hatching egg clusters of P. insularum. Upon return, I carefully inspected each stake and found only one juvenile snail. Quoting Gittenberger (2012) again, “Long-distance dispersal implies a series of unlikely events. However, time is available and a single snail may be sufficient for a successful range extension.” In Florida, the distance from lake to lake is relatively short. It seems reasonable to conclude that the rapid range expansion of the Island Apple Snail is via passive dispersal on the legs wading birds. Posted by Jess Van Dyke

2008: It was a very good year!

Live Island Apple Snails in the incurrent stream of Wellman Pond

live-snails-070830bYes, 2008 was a VERY good year . . . for exotic snails in Florida, that is. My former supervisor, Rob Kipker (FWCC), just emailed me fresh 2008 data on the presence of Island Apple Snail in the public waters of Florida. Each year, his team of top-notch, aquatic biologists inspects the Sunshine State’s 1.25 million acres of lakes and rivers with public boat ramps. Their primary mission is to survey aquatic plants, but for the past three years, Pomacea insularum has been on their list of species to mark “present” or “absent.” Rob wrote, “This is not a snail survey in the sense that we are documenting range, rather it is an indication of which of the 460 public waters in Florida currently have populations of the Island Apple Snail. Use at your own discretion.” O.K. and thanks, Rob!

The results of the 2008 survey reveal that 50 public waters (11%) are now are infested with the exotic Island Apple Snail. That’s bad enough, but the situation is worse when one takes into consideration the size of the affected waters. Some of Florida’s largest are on the list. The “800 lb. gorilla,” of course, is the Lake Okeechobee (448,000 ac), largest lake in the southern U.S., but Lakes Tsala Apopka (19,000 ac), Toho (31,500 ac), Kissimmee (35,000 ac), and the St. John’s River (96,000 ac) also have the exotic snails. Therefore, in terms of surface area, most of Florida’s public waters (55%) have populations of Pomacea insularum.

What about the future? There is every reason to believe that the Island Apple Snail will also have a very good 2009. There is a growing awareness that tropical cyclones play a major role in the dispersal of exotic species. The classic example occurred in 1992, when Hurricane Andrew destroyed much of the Miami Zoo, releasing parrots, baboons, orangutans, wallabies, capybaras, and more. One reptile dealer lost 10,000 geckoes, and a private research institution, 2000 monkeys. The hurricane facilitated a population explosion of large feral iguanas. South Florida will never be the same.

The dispersal of aquatic snails by tropical cyclones though more subtle is no less dramatic. In July 2000, exotic apple snails were discovered in the American Canal near Alvin, Texas (between Houston & Galveston). In June 2001, severe flooding from Tropical Storm Allison dispersed the snails throughout the entire region. In 2008, Cyclone Nargis hit Myanmar (formerly “Burma”) and spread the Golden Apple Snail (Pomacea canaliculata) into the vast rice production areas. “We have never seen so many. They have destroyed our fields,” said one rice farmer (see News Page).

In August of 2008, Tropical Storm Fay made a record four landfalls in Florida, zigzagging from land to water, and causing major, often record, flooding in 27 counties. Rainfall amounts included 27” in Melbourne, 23” at Cape Canaveral and 21” at Palm Shores. The St. Johns River overflowed in Jacksonville. Where I live east of Tallahassee, we received over 20” in a few hours. I’ve never seen flooding like that here. How can such a weather event not exacerbate the dispersal of the invasive apple snails in Florida? I have to believe that Pomacea insularum will put on quite a show in 2009. Happy New Year!! Posted by Jess Van Dyke

For more information on the survey, contact:

Invasive Plant Management Section (FWCC):


About Snail Busters

The Snail Busters Blog was created to facilitate communication between aquatic resource managers who are fighting the spread of invasive, South American apple snails, specifically Pomacea maculata (formerly P. insularum) and P. canaliculata, in the U.S.

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