Archive for the 'New Publications' Category

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!]

The Pomacea Project – – A New Website on Florida Apple Snails

Pomacea paludosa laying eggs

Sometimes, it takes the population decline of a beautiful animal like the Snail Kite (Rostrhamus sociabilis) to draw attention to the importance of, as The Pomacea Project puts it, “less charismatic” species like the native Florida apple snail (Pomacea paludosa). Clearly, the fate of the endangered Snail Kite hinges on the density of its apple snail prey. Patty Valentine-Darby and her husband Dr. Phillip C. Darby, Associate Professor at University of West Florida, were early in understanding that the density of the native apple snail could be used to determine the health of Florida’s aquatic habitats, in general: “Given their relative immobility and sensitivity to changes in water level and plant community structure, apple snails are an excellent indicator of local habitat conditions.”  In the late 90’s, while Phil was conducting his PhD research on P. paludosa, Patty was publishing kite-related papers, such as “Seasonal Patterns of Habitat Use by Snail Kites in Florida.” Since then, Patty and Dr. Phil Darby have made quite a team when it comes to wetlands ecology, specifically concerning the role of the Florida apple snail as a proverbial canary in Florida’s wetland “coal mine.”  

In 2005, Patty and Phil conceived the idea of creating a website to support other biologists who share the view that Florida apple snails act “a barometer of Florida wetland ecosystem health.” Incorporated in 2008, the Pomacea Project gained its official not-for-profit status this year. Subsequently, The Pomacea Project received funding the National Park Service to “to summarize available information on apple snails, snail kites, and other snail predators, including management recommendations to improve habitat conditions for these species.” Though this document will not be completed until 2012, a website is now available “to provide conservation and management information pertinent to the immediate needs of natural resource managers and the general public.”     

The Pomacea Project website became available this week and already contains a wealth of information on native apple snails, including basic biology, sampling methods, and management issues. The impacts of dry downs, exotic snails, and predators are discussed. There is an online brochure cleverly entitled “The 60-Second Snail – – a primer on apple snails when you only have a minute.” To me, the page with the greatest potential is the “Status and Trends” page, where observations on apple snail densities from eight of the most important apple snail habitats are recorded and updated. They hope to expand this effort to many other wetlands and lakes in the future. All in all, Patty & Phil have done a great job on this new site. They currently do not have funding to support updating and expanding their website. Check it out and, if you can, help them out by making a tax-deductible donation! Posted by Jess Van Dyke  

The Pomacea Project:  

http://www.pomaceaproject.org

Juvenile Snail Kites in Trouble

snailkite_landing1

Check out this site’s Recent Publication Page for reports published since 2000. Please let me know what is missing. For instance, I just found new research on the effects of Pomacea insularum on the Snail Kites in Florida:

 

For his Master’s Thesis at UF (see Recent Publication page), Christopher Cattau recently found that the exotic snails (Pomacea insularum) that have invaded Lake Tohopekaliga were 69% larger than the native apple snails (P. paludosa). The larger exotic snails require longer handling times for Snail Kites (Rostrhamus sociabilis plumbeus), an endangered raptor in the U.S. that exhibits an extreme form of dietary specialization. The larger size of the prey leads to increased drop rates and depressed capture rates. However, the average exotic snail weighed 3.5 times the native and provided 2.7 times energy than the average native snail. Consequently, the effects of the exotic snail on foraging behavior do not have negative energetic repercussions for adult kites. In fact, he found that adult kites are attracted to Lake Toho and that the relative contribution of the lake to the range-wide nesting effort increased from 6% to 33% after the invasion of the exotic snail.

 

The invasion of exotic snails is a problem for juvenile Snail Kites, however. The exotics were 4 times harder to capture, plus drop rates and handling times for the exotic snails were over 5 times those of the native. These effects on juvenile foraging behavior lead to insufficient daily energy balances and suppress juvenile survival.  Given the critically endangered status of the snail kite and the propensity of the exotic apple snail to spread, serious management and conservation initiatives that address the exotic apple snail are necessary to prevent further deleterious consequences for the kite population in Florida.

Posted by Jess Van Dyke

For more of Jim Neiger’s incredible images of Snail Kites on Lake Toho:

www.naturescapes.net/052006/jn0506.htm

 


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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