How Parasites Can Drive Ecological Relationships

18 06 2010

Good tidings and well-wishes!

As I’ve mentioned earlier, I read and thoroughly enjoyed Carl Zimmer’s excellent “Parasite Rex: Inside The Bizarre World Of Nature’s Most Dangerous Creatures” last February and have been meaning to post an article about one of the book’s most fascinating points ever since (I’ve just been procrastinating): namely, how the very life-and-death struggles between predator and prey as are often, little more than puppet shows.

More often than you might think, both parties involved are, in fact, driven by parasitic pilots.

Zimmer cites several examples of this most engaging phenomenon. However, by far the most interesting hails from the great American West.

Speaking of the wild west, this story comes with it’s own version of the good, the bad, and the ugly (though precisely who’s who is debatable as we shall see…): the curtain to the tale of the California horn snail (Cerithidea californica), the California killifish (Fundulus parvipinnis), and the various shorebirds of the Golden State’s ample coastlines.

What do these seemingly random animals have in common? A fluke by the name of Euhaplorchis californiensis.

The fluke in question.

Both the snails and the kilifish inhabit salty coastal marshes, and the latter of which naturally attract hordes of piscivorous birds as well. Avian feces inadvertently transport the eggs of E. californiensis, which serve as the primary dietary component of the California horn snail. When these eggs hatch, the flukes initially castrate the unfortunate mollusks before creating several generations of their own until, eventually, larvae (“cercariae”) burst from their initial host.

The Snail In Question

Following the 0nset of this exodus, the cercariae patrol their native salt marsh looking for killifish. Once they manage to happen upon these scaly critters, the parasites latch onto their gills and crawl ever deeper into the host’s body. Eventually, the cercariae follow a certain nerve which leads them directly into the unfortunate fish’s brain. Once there, rather than penetrating this most vital organ, these worm-like creatures merely congregate to form a thin, caviar-like layer atop it.

At this stage, the parasites must now await the consumption of their host by a predatory shorebird. Once this happens, the cercariae erupt from the fish’s stomach and flock to the fowl’s gut. In this new environment, E. californiensis steals nourishment from the new host’s digestive tract and deposits its eggs within the animal’s intestines, to be deposited whenever the bird defecates and, thus, recycling the process.

However, assuming that these parasitic organisms passively leave their lives and reproductive futures entirely up to chance would be a fatal mistake.

In an experiment conducted by ecologist Kevin Lafferty and his then-student Kimo Morris during the early nineties, the behavioral tendencies of 42 captive killifish were individually observed for days on end prior to the animals’ dissection in which the presence of E. californiensis would be confirmed or denied. According to Zimmer:

“What was hidden to the naked eye came leaping out of the data. As killifish search for prey, they alternate between hovering and darting around. But every now and then, Morris would spot a fish shimmying, jerking, flashing its belly as it swam on one side, or darting close to the surface. These might be risky things for a fish to do if a bird was scanning the water. And Morris’s vigil had revealed that fish with parasites inside them were four times more likely to shimmy, jerk, flash, and surface than their healthy counterparts. Since then, Lafferty has been working with a molecular biologist to figure out how the parasites make their hosts dance. They’ve found that the flukes can pump out powerful molecular signals, known as fibroblast growth factors, which can interfere with the growth of nerves. They could turn out to be the parasite’s Prozac[: an antidepressant drug which contains a molecule that acts as a neurotransmitter].”

(For those interested in reading an entire paper on the subject of parasitic brain manipulation in this instance, do go here!)

A Great Egret: one of the many bird species in question.

Some three weeks following Lafferty and Morris’ initial experiment, the pair decided to investigate the effects of this most curious relationship upon the local environment as a whole. Through a series of field tests, they discovered a fascinating unexpected result.

These shorebirds weren’t four times more likely to devour  infected killifish, but thirty times!

Although these flukes do take a slight physiological toll upon their avian hosts, the birds would have to exert a costly amount of energy to steer clear of infected fish (assuming they’d have any way of recognizing them); wasted energy that might prove fatal. Ergo, to these feathered beasts, the benefits of ingesting fluke-filled killifish vastly outweigh the costs.

The aforementioned fantastically large percentage thus begs a very intriguing question: “If this parasite didn’t exist, could populations of these birds survive if their food was thirty times more difficult to obtain?”

For those still unconvinced by the idea that parasites are more than mere hitchhikers and instead wield an enormous amount of influence upon their residential environments as a whole, let’s return to the horn snails for a moment. Prior to his investigations concerning the effects of E. californiensis upon killifish and birds. To once again reference “Parasite rex”:

“[These flukes] don’t quite kill their snails. In a genetic sense, the snails are indeed dead, because they can no longer reproduce. But they live on, grazing on algae to feed the flukes inside them. If the snails were truly dead, the algae they ate would be left for surviving snails to graze on. Instead, the flukes-as-snails are in direct competition with the uninfected snails…

Lafferty measured how the uninfected snails performed without parasitized snails competing with them. They grew faster, released far more eggs, and could thrive in far more crowded conditions. The results showed Lafferty that in nature, the parasites were competing so intensely that the healthy snails couldn’t reproduce fast enough to take full advantage of the salt marsh. In fact, if you were to get rid of the fluke, the snail’s overall numbers would nearly double. And this being the real world rather than a lab, that explosion would ripple out through much of the salt marsh ecosystem, thinning out the carpet of algae and making it easier for predators of snails, such as crabs, to thrive.”

If I’ve managed to pique your interest with these excerpts, I’d most heartily recommend checking out “Parasite Of The Day“: an exquisite blog run by some of the world’s leading parasitologists. If you’re anything like me, your mornings won’t be complete without it!


-Mark Mancini




3 responses

18 06 2010
How Parasites Can Drive Ecological Relationships « The Theatrical … - - Your Search Solution

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21 06 2010
Zach Miller

Parasitic brain manipulation, eh? I’m surprised nobody’s turned this concept into a terrible horror movie!

22 06 2010

There’s probably a few of them out there somewhere I’m afraid… 😛

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