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!]

13 Responses to “Pomacea’s Incredible, Indigestible Eggs”

  1. 1 faruk December 28, 2010 at 10:23 am

    very nice selection

  2. 2 Yulfaizy December 28, 2010 at 1:04 pm

    Thank you for the new info.

  3. 3 Just Asking January 12, 2011 at 12:51 am

    I wonder if mice show the same neurological damage if they ingest eggs containing the toxin (instead of having toxin injected into them IP). That would seem to be an important experiment to do if you want to argue that the toxin plays an ecological role in protecting the eggs from predation.

  4. 4 snailbusters1 January 14, 2011 at 5:12 pm

    You are correct. Just because a substance is toxic upon IP injection does not mean that it deadly when injested (Nicotine in humans!). Plus, it appears that the mice can build up a tolerance to PV2. That’s why the researchers looked for something else that discourages consumption and found it (ovorubin). Maybe, somebody out there has some pet mice and access to Pomacea canaliculata/ insularum eggs. If so, put the mice on a snail egg diet and report back! Jess

  5. 5 Just Asking January 15, 2011 at 8:08 pm

    “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.”

    The neurotoxin defense has not been demonstrated. I would question the protease inhibitor defense side of the story as well. Just showing that the ovorubin acts as a trypsin inhibitor doesn’t mean that it serves a protective function for the eggs.

  6. 6 snailbusters1 January 15, 2011 at 11:20 pm

    O.K., what is your hypothesis as to why these numerous, highly visible clusters of protein and carbohydrates go largely uneaten? The color pink is repulsive in of itself??? <>

  7. 7 Just Asking January 16, 2011 at 2:11 pm

    It’s not that the hypothesis is wrong, it’s that the experiments that were done don’t test the hypothesis. I understand the hypothesis to be this -“Do apple snail eggs contain chemical substances that deter predators from consuming them?” Based on the experiments that were done, we don’t yet know. The experiments that were done produce two results (the eggs contain a protein that is toxic if injected, and the eggs contain a protease inhibitor). Do these substances deter predators from consuming the eggs? We don’t yet know. The experiments that would test this have not been done. The story being told is not yet scientifically complete.

  8. 8 snailbusters1 January 17, 2011 at 11:20 pm

    You are right! Thanks!

  9. 9 Just Commenting April 5, 2011 at 6:38 pm

    I believe that the hypothesis “Do apple snail eggs contain chemical substances that deter predators from consuming them?” is partially answered by the experiments performed on ovorubin, the most abundant apple snail egg protein. It is reported that it is indigestible as it withstands the action of digestive enzymes. As ovorubin also inhibits digestive proteases predators are unable to properly digest their food. These experiments are further confirmed in vivo by feeding rats with ovorubin: Rats show slower growth rate (protease inhibitor plus indigestible protein). The predator will quickly learn that eating apple snail eggs is not profitable. This is not a novel defense, as it is very common in seeds, protecting plant embryos from predators. The conspicuous egg coloration would help predators to learn and remember that this coloured egg clutches are not good food and the result is that embryos are protected. I found a reference to this matter at the following link: http://www.newscientist.com/article/dn19831-zoologger-weaponised-eggs-turn-predators-stomachs.html

  10. 10 christian canchingre May 24, 2012 at 4:18 am

    very good information that helps us every day to try to live with this invasive species would like to know if you have information regarding snail control with microorganismos.

  11. 11 snailbusters1 May 24, 2012 at 2:25 pm

    Thanks for your support, Christian! The ultimate biological control for Pomacea would be a pathogenic microorganism. A search for such a “disease” in Argentina might br fruitful. Of course, the problem would be selectivity. In Florida, we prize the native P. paludosa for its value to the endangered Snail Kite and threatened Limpkin. I will look more deeply into this issue. Jess

  12. 12 MIchael May 20, 2013 at 7:12 pm

    Found a couple of what appear to be these clusters on my lakefront in central Florida this weekend. Is there anything similar out there that might be mistaken for Pomacea clusters?

  13. 13 snailbusters1 May 21, 2013 at 3:55 pm

    Hi Michael, It is likely that Pomacea maculate are in your lake. Send me an email with images and location. Thanks, Jess

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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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