Archive for the 'Impact' Category

Does Pomacea’s Aerial Respiration Requirement Determine Environmental Impact?

Aerial respiration of Pomacea via siphon (Jess Van Dyke)

Though Apple Snails are freshwater inhabitants, they are clearly amphibious. Pomacea possess combed, gill-like structures (ctenidium) for aquatic respiration and lung-like pulmonary sacks for aerial respiration, as well as buoyancy regulation. When dissolved oxygen levels are high (5-6 ppm), the snails remain mostly underwater, but when low (1-2 ppm), they rely on their siphons and “lungs” to breathe fresh air (San Martins et al., 2009). In any case, all Pomacea regularly come to the surface to ventilate their “lungs.” Such behavior is obligatory.  Preventing aerial respiration negatively affects activity, feeding, and survival (Seuffert and Martín, 2009). They must occasionally take a breather!

Darby et al. (2002) reported that Apple Snails prefer to inhabit shallow water areas ( < 50 cm) because of the need to breathe atmospheric air without expending a large amount of energy to move to the surface. Darby (1998) also suggested that unconsolidated organic material may restrict movement into deep water. However, recent observations of healthy Pomacea as deep as 14.6 m (48’) in Apopka Spring indicate that depth alone is not a deterrent to the snails (Bernatis, 2010). Seuffert and Martín (2009) have concluded that Pomacea are unevenly distributed relative to the access to air – – concentrated less than 2–4 m from the nearest emergent substrate. Simply put, Apple Snails need easy access to emergent plants (or other structure), so they can crawl up and catch a breath of fresh air.

According to the annual survey by FWCC, Pomacea insularum is now present in 22% of Florida’s public lakes and rivers (see below). The mystery in Florida is why this invasive, exotic snail will strip one lake and leave the vegetation relatively unaffected in another. Perhaps, three physical features of lakes play important roles: shoreline development, average depth, and mean slope. Shoreline development (SD) is simply the ratio of the length of the shoreline of a lake to the circumference of a circle with the same area as that lake. The higher the SD ratio is the more complex the shape of the lake. Average depth is self-explanatory, and mean slope is the proximity of bathymetric contours to one another. Taken together, these parameters determine the extent of the littoral zone and the abundance of emergent vegetation.

A relatively small, shallow lake with a gradual bathymetry but a complicated shoreline would seem to provide the best habitat for Pomacea insularum. In such a lake, on a per area basis, food would be more abundant; emergent vegetation would always be near for aerial respiration (and egg deposition); and dissolved oxygen would tend to be high, unlike in a deeper lake with an anaerobic hypolimnion. In short, a greater percentage of the lake would be within 2-4 m of substrate that Seuffert and Martín (2009) suggest is Pomacea’s preferred habitat. The greater the density of Pomacea insularum, the more likely the snail population could consume all of the aquatic vegetation in a given lake. Let’s see how this theory plays out. Posted by Jess Van Dyke

Annual Survey of Florida’s Public Waters  for Pomacea insularum by the FWC:


        Area (ac)



















Ebro Delta: The Exotic Apple Snail’s Bridgehead in Europe

The Ebro Delta (350 km2), located in Tarragona at the southern end of Costa Dorada, is the largest wetland in Catalonia and is the second most important natural wetland habitat in Spain. About 8,000 ha of the delta are natural park sites, part of the Natura 2000 network of the European Union. The delta has high biodiversity and productivity, excellent fish and wildlife habitat, and important water purification and storage functions. Plant diversity is extremely rich with 515 species catalogued. Large pools surrounded by giant reed beds occupy the coastal portions. No wonder the Ebro Delta is the second most important bird habitat in Spain. Economic activities include agriculture, fisheries, aquaculture and tourism. Rice cultivation, however, is the dominant use.

On August 1, 2009, the Island Apple Snail (Pomacea insularum) was first documented on the European Continent. Its conspicuous, bright-pink egg clusters were detected on shoreline vegetation along a small irrigation canal, adjacent to a fish farm in the Ebro Delta. The range expansion of the exotic snails was explosive. Within a month, they had colonized half of the interconnected canals in the northern half of the delta (up to 5.1 egg masses m-1) as well as the main river channel (up to 6.2 egg masses m-1). After reviewing the literature, local experts recognized the great potential of widespread economic and  environmental damage.

Government agencies quickly implemented plans to eliminate the pest manually, mechanically and chemically hoping to avoid harmful effects on agriculture and native biodiversity. All emersed vegetation from 16 miles (26 km) of shoreline in the initial area of infestation was harvested and incinerated to no avail. The desiccation of 9,500 hectares of the delta for five months at a cost of 3.5 million € in attempt to eradicate the snails proved fruitless. In spite of these and other extreme measures, “the ability of the Island Apple Snail to thrive and disperse in the Ebro Delta shows that this bridgehead is firmly established,” writes Miguel Angel López of the Catalonian Government Environmental Department.

I have been in communication with a biologist who is currently assisting the frustrating struggle to stem the Island Apple Snail’s invasion of Europe: “The Environmental Department has been hand-collecting adults, putting barrier traps in the irrigation canals, destroying egg clutches, and burning shoreline vegetation in non-sensitive areas to remove egg substrate. They are now using saponins as well. The snail is occupying an area of the delta where there are mostly canals for habitat. The Environmental Department has mapped about 240 km (150 miles) of canals which are infested. We regularly have sustained temperatures of 9 or 10º C during winter here with occasional drops below zero and there has been snow here during two of the last five years, and they seem to be doing just great! Copper is not permitted as there are native bivalve species sharing the irrigation canal habitat. Baits are problematic as the current in the canals rapidly disperses them. Despite the Environmental Department’s efforts with a crew of 30 people, there seems to be no end in sight. The bottom line is more funding is needed to get ahead of this pest. Any suggestions from your experiences would be appreciated.”

O.K., I’ll give it my best shot: Clearly, the situation is dire; the wetlands of an entire continent are at stake. In my opinion, the ultimate solution is biological control via some selective pathogen, though such an organism will not be easy to find (or create). At present, containment is paramount. Accidental transport of the exotic snails to other aquatic sites must be prevented. In my experience, anglers with boats on trailers are most often responsible for the inadvertent dispersal of invasive, aquatic species. In the case of Pomacea, however, aquarium hobbyists have played a major role.

Within the Ebro Delta, I suggest that you focus on Pomacea’s two key vulnerabilities: the conspicuousness of its eggs and its vulnerability to chemical attractants. Continue to collect eggs, as much as manpower will allow, and possibly make some money to subsidize your control efforts. Biotechnology companies in Taiwan are spending huge sums to develop technologies to extract the anti-oxidant Astaxanthin from Pomacea eggs. These companies estimate that the value of the anti-aging, skin care products based on that active ingredient could reach over $400,000,000 annually. Natural Astaxanthin is currently priced at $7000/kg (not a typo). Maybe, a Spanish biotech firm would like to participate and fund the collection of Pomacea eggs in the Ebro Delta!

Regarding baiting, do not underestimate the chemoreception capabilities of the Island Apple Snail. Even with some flow, Pomacea can be drawn to the right attractant. For collection (or poisoning), bait stations make sense. I have shipped Miguel one of our traps with non-toxic bait to test. By doing so, I am not
suggesting that trapping and collecting snails along 240 km of canals is practical. However, with an attractant bait that is also toxic, progress could be made in reducing the population of adult snails. I am in touch with a company that produces an innovative, plant-derived molluscicide, and am eager to incorporate it into the bait we produce for laboratory testing. I will let you know how that goes. Good luck with your battle, Miguel! Posted by Jess Van Dyke

For more information, contact:

Miguel Angel López

Catalonian Government Environmental Department

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:

Lake Munson: A Case Study of the Impact of Exotic Apple Snails on Aquatic Vegetation

Munson June 30, 2008The old saying about Lake Munson is that “it is a beautiful lake, if you don’t look down.” Lake Munson’s beauty lies in the dense cypress strand, largely in public ownership, that surrounds it. Looking down, however, reveals that nutrient pollution and sedimentation have plagued this lake for over 70 years. This 255-acre system has been then receiving water body for treated waste water from 1934 to 1988, and for most of the City of Tallahassee’s stormwater at present. Though 25% of the watershed is protected, 50% is urbanized and home to 100,000 people. Little wonder that aquatic plants have historically been highly prolific in this fertile lake. In fact, this lake was usually “topped-out” in submersed vegetation during the growing season . . . that is, until a prolific, voracious herbivore arrived.

From 1982 to 2007, I monitored the aquatic plant communities of Northwest Florida, as DEP’s Regional Biologist.  Matt Phillips (FWCC), who filled this responsibility after I retired, recently provided me the vegetation surveys for 2008 and 2009. Lake Munson is one of the sixty public water bodies we have collectively monitored for 27 years. It is also the lake where the Island Apple Snail (Pomacea insularum) put on quite a show in 2002, painting all of the cypress trees along the shoreline with pink egg clusters. I had never seen anything like it!

The resulting population of exotic, apple snails dramatically changed the plant community of Lake Munson. Historically, submersed vegetation blanketed 80% of the lake during our surveys, necessitating the use of an airboat. In the early 90’s, the dominant submersed species were Coontail (Ceratophyllum demersum) and Southern Naiad (Najas guadalupensis).  However, in 1993, the invasive, exotic, plant  Hydrilla (Hydrilla verticillata) found its way into the lake. By 1995, hydrilla reached the surface in 200 acres of Lake Munson and that density became the norm. When the exotic snails first appeared in 2002, I noted 150 acres of Hydrilla. In two years, the Hydrilla was gone – – completely gone. In fact, no submersed, vascular plants have been found in Lake Munson, since 2004.  

Emersed and floating vegetation has fared little better. A sixty-acre stand of the beautiful, native, American Lotus (Nelumbo lutea) vanished between 2003 and 2004. The invasive, exotic, Water Hyacinth (Eichhornia crassipes) has nearly disappeared. From 2002 to 2006, a total of 330 acres of hyacinths were controlled in Lake Munson. Such control is no longer necessary. Only a trace of Water Hyacinths remains in Lake Munson. A favorite food for Pomacea insularum, Wild Taro (Colocasia esculenta) declined but rebounded on the east side of the lake in 2009, while Pomacea insularum egg deposition was concentrated on the west side (Another behavioral mystery!) .

There are some lessons here: First, the Island Apple Snail can completely strip a heavily-vegetated lake of its submersed plants and most of its emersed vegetation. Secondly, some hardy species will remain, such as Soft-Stem Rush (Juncus effusus) and Smartweed (Polygonum densiflorum). These and other apple snail resistant species should be considered first in lake mitigation, though there is still much to be learned on the subject.  Research on the food preferences of Pomacea insularum is badly needed.

So what is left in Lake Munson in terms of plants? There are plenty of blue-green algae (cyanobacteria). The lake is dominated by a dense phytoplankton bloom, consisting mainly of Microcystis spp. and Anabaena spp. The only submersed “plant” is there now Lyngbya – – a black filamentous algae. Sounds inviting, doesn’t it? Leon County continues to make great progress in improving the quality of water entering the lake, but Lake Munson remains “a beautiful lake, if you don’t look down” and, by destroying the vascular plants, the Island Apple Snail certainly isn’t helping any. By Jess Van Dyke

For more information on Lake Munson, contact:

 Matt Phillips, Florida Fish and Wildlife Conservation Commission, Invasive Plant Management Section, 3900 Commonwealth Blvd.  MS 705, Tallahassee, Florida 32399, (850) 245-2831,

“Lake Munson: Past, Present, and Future” by Johnnie Richardson, Water Quality Scientist, Leon County Public Works Munson Update Short 6_25_09.pdf