Posts Tagged 'eggs'

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!]


Slick Eggs

dscf26351Within an egg shell, the rapidly developing embryo needs plenty of oxygen. Using an oil to kill eggs by impeding oxygen transfer through the shell is nothing new. For instance, the USFWS has recommended mineral oil for use on nuisance geese nests. Why not use vegetable oil to kill apple snail eggs?

I spoke my old friend Dr. Bill Haller this morning about his team’s work with the use of oil as an oocide on apple snail eggs. At the University of Florida’s Center for Aquatic Invasive Plants’ laboratory, they have tried castor oil, Crisco oil, peanut oil, Pam Spray, corn oil, and even Mobil oil!

“Just about any oil will work,” Bill said, “but corn oil is the way to go because it is most likely exempt from EPA regulations. Technically, however, we need a company to submit a label to EPA to get the EPA review and an exemption letter. A company hasn’t stepped up because a patented product is doubtful. The ingredients are just too generic.”

Bill discussed the nuances of effectively treating snail eggs. First, the fresher the better – – Recently laid eggs are more vulnerable to the smothering effects of an oil barrier. Secondly, straight oil is the way to go. Diluting the oil and adding surfactants didn’t work as well as 100% oil. Finally, a thorough application is necessary to kill the eggs. This was made apparent in the discrepancy between the efficacy of dipping the eggs in oil versus spraying. Dipping clearly worked better in the lab tests.

Is it practical to load up a 50 gallon sprayer with oil on an airboat and treat the perimeter of an infested lake? “In reality, vegetable oil is tough to use,” Bill noted. And, it’s not cheap. Vegetable oils range from $3.00 to $6.00/gallon.

My bench tests using an emulsion of Crisco oil (50%) and Yucca saponins (3%) provided mixed results (see image above). The idea was to reduce costs via dilution of the oil with water, while adding a saponin with possible molluscicidal properties. Hatching numbers were reduced, but the cost/benefit wasn’t that appealing. I also tried hand-spraying vegetable oil on some egg clusters Wellman Lake and found it to be almost as time consuming as simply hand-picking the eggs, plus some of the “slick eggs” eventually hatched.

This is not to say we should all give up on this avenue. A student project in the Philippines recently revealed the great promise of coconut oil as an oocide on Pomacea canaliculata (See Recent Publications, June, 2008): “Results showed that coconut oil could inhibit the hatching of 82.2% of the golden snail eggs.” So, let’s keep working on it. After all, “the weak point of the invasive apple snails are the egg clutches,” as Dr. Haller put it. Posted by Jess Van Dyke

Dr. Wm. T. Haller:

The Center for Aquatic & Invasive Plants:

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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