Posts Tagged 'birds'

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

Bioaccumulation of Cyanotoxins in Apple Snails

Snail Kite (Rostrhamus sociabilis) on Lake Catemaco, Mexico (

               Bioaccumulation is the sequence of processes in an ecosystem by which certain chemicals can accumulate in organisms up the food chain, generally through a series of prey-predator relationships. If the chemical is highly toxic, the results can be devastating in an aquatic ecosystem. Since 1999, 54 bald eagles (Haliaeetus leucocephalus) have died on Lake Thurmond (71,000 acres), the largest such mortality in U.S. history. Dr. Susan Wilde, an assistant professor at University of Georgia’s Warnell School of Forestry, is part of the team that believes they have solved this mystery. She and her associates have concluded that the cause is bioaccumulation of a new neurotoxin produced by a newly described cyanobacterial species in the order Stigonematales.

               Produced by filamentous blue-green algae (cyanobacteria) growing on submerged plants, especially hydrilla verticillata, this neurotoxin is bioaccumulated from the vegetarian American coots (Fulica americana) to their magnificent predators, bald eagles. Consumption of vegetation containing the neurotoxin by coots and the consumption of sickened coots by eagles resulted in the discovery of an emerging neurological disease, called Avian Vacuolar Myelinopathy (AVM). This often fatal disease results from lesions in the brain stem and spinal cord. Coots affected with AVM lose vision and muscle coordination, have difficulty flying and swimming, and become easy prey for the opportunistic bald eagles, who themsleves become victims. The disease agent, a neurotoxin produced by an epiphytic bluegreen alga in the order Stigonematales (Wilde et al., 2005), has recently been extracted from the plant samples from problem lakes (Wiley et al., 2009). Test animals exposed to this extract contracted AVM. The evidence seems clear.

               Unfortunately, this emerging neurotoxin is not the only cyanotoxin in aquatic ecosystems with the potential for bioaccumulation. Microcystin is one that affects the liver long term. Another is Cylindrospermopsin which is rapidly becoming is one of the most important toxins produced by freshwater blue-green algae. The rapid distribution of cyanotoxin producers into temperate zones has heightening concerns that these toxins will create serious environmental and human health risks on a global scale. Importantly, a recent study in Mexico documented the bioaccumulation of cyanotoxins by native apple snails. In eutrophic Lake Catemaco (18,000 acres), Cylindrospermopsin was biomagnified 157 times by endemic Tegogolo snails (Pomacea patula catemacensis) (Berry, J.P., and Owen Lind, 2010, in press). That is not comforting.

               These findings raise serious questions regarding an additional environmental impact of the range expansion of exotic apple snails. It appears that the key ingredients for bioaccumulation of cyanotoxins are nutrient-rich aquatic systems, especially reservoirs, with abundant submersed vegetation covered with filamentous, blue-green algae. Because of nutrient pollution and the introduction of exotic plants, such systems are all too common in the expanding range of Pomacea canaliculata and P. insularum in the United States. Will exotic apple snails play the same role as the American coot and lethally transfer cyanotoxins to their avian predators, such as the Limpkin (Aramus guarauna) and Snail Kites (Rostrhamus sociabilis)? I asked Dr. Wilde:

               “Hydrilla mats provide an enormous substrate for epiphytic cyanobacteria, and many of these species are capable of producing toxins. Because they are voracious consumers of hydrilla, the invasive apple snails may facilitate the transfer of those toxins through the food chain. We have ongoing research funded by the Florida Fish and Wildlife Conservation Commission to determine the levels of toxins in tissues of exotic apple snails and the potential of transferring of those toxins to birds of prey. Our initial feeding trials indicate that concern may be warranted, but it is too early to make any definitive conclusions.”  She will keep us posted on her new, excellent website (below). Thank you, Dr. Wilde! Posted by Jess Van Dyke

For more information on AVM contact:

 Dr. Susan B. Wilde

Warnell School of Forestry and Natural Resources,

University of Georgia

Athens, Georgia 30602


Avian Vacuolar Myelinopathy (AVM) Website:

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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