Archive for the 'Biology' 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)



















False Hope of the Boom-and-Bust Model

It has been a long time since my last post. The problem has not been “writer’s block” but the lack of an interesting topic. It has been very quiet in the southern U.S. regarding exotic Pomacea. Though the USFWS in Alabama continues to struggle against Pomacea insularum at two locations, officials in the rest of the South seem complacent regarding exotic Pomacea. In Florida, the official dogma is that “they tend to boom-and-bust without causing much harm. Besides, they have already spread everywhere.” I am less sanguine.

Regarding the boom-and-bust rationale for official inaction, Dr. Daniel Simberloff and Leah Gibbons (2004) said it best: “Substantial populations of invasive non-indigenous species occasionally collapse dramatically. Although disease is often invoked, the causes are rarely studied experimentally and/or quantitatively, and some collapses remain quite mysterious. Except for the few species in which spontaneous collapse has been repeatedly observed, the possibility of such an event is unwarranted as a potential rationale for a do-nothing approach to management.”

In an exhaustive study of the impact of alien species in the Mediterranean Sea, Dr. Charles Boudouresque et al. (2005) concluded, “The boom-and-bust model predicts the eventual decline of the invasive species and the recovery of the native ecosystem. In fact, species introductions are irreversible, even at a geological scale, and the natural decline of introduced species is quite uncommon. Data have been misinterpreted, leading to the generalization of the probably rare boom-and-bust model.” The graph below depicts the typical population volatility of an introduced species which is not to be confused with a permanent “bust.”

Often, the interaction between a species’ population size and its habitat is subtle. Populations fluctuate due to density-dependent factors, such as disease,
parasitism, predation, and competition, and due to density independent factors, like the weather. As I was driving my tractor through the smoky haze blanketing my farm while disking fire lanes, I pondered, “What is the primary factor causing the perceived bust in the exotic snail populations? What if it is not subtle in this case but so obvious that it is hidden in plain sight?” Then, it hit me – – the exceptional drought plaguing the Southern U.S. has temporarily stemmed the proliferation of exotic Pomacea and spawned official complacency. It makes sense because current “exceptional drought” map of the U.S. generously overlaps the range of P. insularum.

While flooding is documented to assist the range expansion of exotic Pomacea, if not the eggs, one can logically conclude that extreme drought should be detrimental. To test my hypothesis, I took a walk around Wellman Pond, the test site for our apple snail traps and my favorite place to observe Pomacea insularum. I had not visited the site for months, and while I expected some impact of the recent drought, I was amazed to see the lowest water level ever. As I walked on the dry lake bottom within the periphery of emersed vegetation, I saw numerous dead snails, stranded egg clusters, and an army of foraging fire ants (See “Rematch: Pomacea versus the Red Fire Ant”). Clearly, the exotic Pomacea have had a difficult spring season in 2011 at Wellman Pond . . . and likely elsewhere in the South.

The image above is typical of the entire shoreline of Wellman Pond. Fire Ants are devouring most of the snail eggs, while the adult snails have lost access to the refuge from predation and egg laying substrate provided by the emersed vegetation. These hard times for exotic apple snails will surely end on Wellman Pond and elsewhere in the southern United States. The current rainfall deficit will inevitably swing the other way. In some future wet period, the expansion of the exotic Pomacea will likely resume with a vengeance. The snails are more vulnerable to control efforts now than ever. This is no time for official complacency based on the false hope of the boom-and-bust model. Posted by Jess Van Dyke

Pomacea’s Incredible, Indigestible Eggs

I have often wondered why the eggs of Pomacea canaliculata and P. insularum are not quickly devoured by any number of predators. The pink clusters draped on emergent plant stems around lakes could not be more obvious. However, these numerous clumps of protein and carbohydrates go largely untouched aside from occasional attacks by red fire ants, Solenopsis invicta (see post entitled, “Rematch: Pomacea versus Red Fire Ant”). Finally, a fascinating answer to this mystery has been provided by the outstanding team of Professors Dreon, Ituarte, and Heras of the National University of La Plata in Argentina.

Throughout the natural world, undefended eggs provide easy, nutritious meals. It is common for half of them to be lost to predation. Most animals rely on either hiding their vulnerable ova, guarding them, or producing so many eggs that a future generation is assured. No doubt Pomacea are prodigious breeders, but they also employ “aposematism,” a common characteristic of dangerous prey. Using warning signals, such as color, sound, or odors, certain prey clearly advertize that it is unwise to attack them. Such warnings are beneficial to both predator and prey. Certainly, the blatant display of bright-pink eggs by exotic Pomacea is the form of aposematism, called “warning coloration,” but what could possibly be danger in eating them?

Endowing eggs with chemical defenses in not uncommon in invertebrates, and Dreon, Heras, et al., (2008) already established that Pomacea canaliculata eggs contained such a predator repellant. The authors found that a rare protein neurotoxin was produced by albumen secretory cells in developing Pomacea canaliculata eggs. Further, they demonstrated that injections of this neurotoxin, called Perivitellin-2 or PV2, had lethal effects on rodents (LD50, 96 h @ 2.3 mg/kg) primarily because of damage to their spinal cords. However, this neurotoxin was fragile (heat sensitive), however, and there was evidence of antibody response to sublethal doses. The presence of PV2 did not seem enough to dissuade almost all predators from consuming Pomacea eggs suggesting some complementary defensive mechanism.

The rest of the story is provided in a publication this month by the same team (see Dreon, Ituarte, and Heras (2010) in Recent Publications). It is hard to imagine eggs that are not highly nutritious, and developing apple snail ova are “filled with large amounts of polysaccharides and proteins,” as the authors put it. However, there is another surprise for predators in the perivitellin fluid that surrounds the fertilized Pomacea oocyte, besides PV2. The same brightly-colored, caratenoid  protein, called ovorubin, that warns away predators and blocks damaging solar radiation is also a proteinase inhibitor. Feeding trials revealed that rats fed ovorubin lost weight because it binds to trypsin, a common digestive enzyme that breaks down proteins.

In an elegant defense of her young, the female snail not only adds a neurotoxin to the perivitellin fluid, but for good measure, colors it bright-pink with a compound that impedes digestion of protein. “This [protease inhibitor] role has not been reported in the animal kingdom, but it is similar to plant defenses against herbivory,” state the authors. Only red fire ants are determined enough to ignore the apple snail’s clear warning. A common TV advertisement trumpets that chicken ova are “incredible, edible eggs.” Well, the apple snail’s brightly-colored advertisement to predators is: “These are my incredible, indigestible eggs!” Posted by Jess Van Dyke

[Note: I started this weblog two years ago. Subsequently, there have been 24,000 hits from all over the world. I am grateful to all my old and new friends. Thanks for your help and encouragement!]

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

Rematch: Pomacea versus the Red Fire Ant


While collecting thousands of Island Apple Snail (Pomacea insularum) egg clusters from Wellman Pond, we often speculated on what ferocious predator could have elicited such high reproductive capacity in this South American snail through evolutionary time. Initially, we fantasized about some large reptile, maybe a 20’ Snail Anaconda (Eunectes pomaceoraptor?), feeding so effectively on apple snails in the Argentine rivers and marshes that the remaining few adults had better reproduce prolifically for the species to merely survive. Then, as we began to notice the numerous ant bites received while wading through the shoreline plants, it came to us. Maybe, the ferocious predator wasn’t all that large. This notion was solidified when our 4-ton pile of disposed snails became one giant Fire Ant bed!

The Red Imported Fire Ant (Solenopsis invicta) and the exotic Apple Snails (Pomacea canaliculata & P. insularum) have two common traits: They are both highly invasive in the southern U.S. and their native ranges in South America generously overlap. Surely, they co-evolved in a predator-prey relationship with hydrology as a key component. Fire Ants are, indeed, ferocious predators of snails, having caused the extinction of a native tree snail in the Florida Keys (Forys et al, 2001). Stevens et al (1999) documented that Fire Ants attacked native apple snails (P. paludosa) in Florida that became “exposed during dry down conditions.” Yusa (2001) observed that “when egg masses [of P. canaliculata] were experimentallyplaced on levees [infested with Fire Ants], on average 50% of the eggs were lost within two days in March and 38% were lost within threedays in August.No eggs were lost when ants weresuccessfully excluded by water.” Way et al (2009)noted that “periodic drainage . . . enables [the tropical fire ant] to join the predator complex . . . valuable for ant-based control of pests such as snails” in rice paddies.

The current range of the exotic Red Fire Ant in the U.S. encompasses the projected range of the invasive South American Apple Snails. Certainly, predation by the Fire Ant will provide some friction to the rate of expansion of the exotic apple snails in the U.S., especially in areas with fluctuating water levels. Can we take further advantage of this ancient predator-prey relationship? Based on Yusa (2001), Way (2009), and personal observations, it appears a short period of water level manipulation in the presence of red fire ants can have a deleterious effect on apple snail reproduction. When the bases of shoreline plants are exposed in Wellman Pond, for example, numerous Red Fire Ants begin eagerly scouting for egg clusters. I am inclined to ask the Parks personnel not to treat the fire ant mounds there. Periodic, partial “drawdowns” also seem warranted. Let the rematch of invasive Pomacea and the Red Fire Ant continue! Posted by Jess Van Dyke

UF’s Excellent site on Red Fire Ants:

National Geographic video on Fire Ants:

Collaborative Research with Students


 Though her eclectic interests vary from fine chocolate to “the effects of macrophyte density on the interactions between benthic predators and pelagic prey” (whew!), Professor Romi Burks is keenly focused on training young minds via collaborative research on the invasive apple snail (Pomacea insularum). For five years, her Apple Snail Ecology Lab at Southwestern University, Georgetown, Texas, has enabled students to investigate multiple aspects of basic life history of this new invader. “The diversity of questions that can be explored is endless. I work directly with the student to develop his/her own research question that includes a reasonable degree of personal ownership or investment. To get the most out of the research process, students need to work on questions that pique their own interests.”

 Dr. Romi Burks writes, “When first embarking on a research projects, students review primary literature and then draft a proposal that includes a rationale, hypotheses, and proposed methods for developing experiments. We spend substantial time developing the context of the research and determining the appropriate methods. After executing the experiments, students participate directly in data analysis and dissemination through poster and oral presentations and, hopefully contributions to a scientific paper.” In fact, three of her students presented their research findings on the Island Apple Snail this week at the Texas Academy of Sciences meeting (See Recent Publications Page).  

There are two news items that Dr. Burks would like to share: First, “my South American colleagues and I think we have found a clear, native population of Pomacea insularum in western Uruguay.  I am sending samples to Rob Cowie for genetic confirmation.  This is exciting because it gives me an accessible population with which to compare patterns with the exotic population in Texas.” Second, she visited a newly reported site of exotic apple snails in Texas, where she didn’t find any live snails but numerous shells. “Based on variable shell morphology or shape, I cannot tell if the snails present are only P. insularum or could include P. canaliculata. We found an intact individual to send for genetic identification. In addition, we saw recent egg clutches which strongly suggest P. insularum. We brought back four recently laid clutches, but I’m not sure if they are going to yield any hatchlings. I expect that when it warms up that this site will definitely show productivity.”   

Aside from news, she also had some kind words: “I really appreciate the added awareness that your site brings. As you have observed, the amount of interest and incidents regarding Pomacea has been steadily rising over the past five years.  When I first got into this [snail] business, I definitely felt alone.  Now, the science is almost happening too fast for me to keep up. I also know that we have lots of information to get out to the scientific community, but the process of publication is slow — even slower when you focus on teaching undergraduates how to do research and publish. Your site speaks to the broader need for [scientific] integration. Clear time-pressure exists to protect native biodiversity. Exotic invaders routinely move faster than scientific publication processes.”

Thanks, Romi! “Scientific blogging” is a new and fascinating format. It holds great potential because it is incredibly fast. Dissemination of information on the internet is instantaneous and “viral.” Started in December, this site has been viewed by thousands of people all over the world. The downside of all blogs is that the quality of the information is unknown. Though a slow process, publication in a scientific journal is “refereed” or “peer-reviewed” to assure accuracy. I do my best to stick to the facts or clearly hedge when I’m uncertain, but I need your help. PLEASE let me know if you have any criticisms or corrections by clicking Comments above. Help make this a peer-reviewed, scientific blog! Posted by Jess Van Dyke

For more information, contact: 

Romi L. Burks, Ph.D.
Southwestern University
1001 E. University Avenue

Georgetown, TX 78626E


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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