Leuchochoridium paradoxum infecting a snail. Credit: AfroBrazilian
Leuchochoridium paradoxum infecting a snail. Credit: AfroBrazilian

New Study Upholds Reputation of Classic Parasite

I was ten years old when I first saw Trials of Life, David Attenborough’s landmark series on animal behaviour. Its twelve glorious episodes left an indelible mark in my mind, with unforgettable scenes of killer whales beaching themselves to eat sea-lions and elephant shrews bounding along carefully memorised paths.

But if you asked me to list the most memorable scenes from the series, I would tell you about the snails.

They appear in episode 7, grazing on leaves in a Danish meadow. One of them looks weird. Its slender eyestalks have transformed into swollen sacs, with green and yellow stripes, black spots, and red tips. They look like gangrenous popsicles. Something inside them is pulsating.

During an inauspicious meal, this particular snail had accidentally swallowed the eggs of a fluke—a  parasitic flatworm called Leucochloridium paradoxum. The eggs hatched, and the young flukes started taking over the snail’s body. As they matured, they sent large broodsacs, full of larvae, into the snail’s eyestalks. For some reason, the flukes usually go for the left one, but they’ll corrupt both if two of them share the same snail.

In the light, these sacs start to throb. The effect was revolting and unforgettable to my young eyes. But it’s also compelling to birds, because the sacs look very much like juicy caterpillars or grubs. In the Trials of Life, a flycatcher swoops in and devours the snail. That works for the fluke, which can only complete its life cycle in the guts of a bird.

But Attenborough claimed that the flukes did more than change the snail into a billboard. “For some reason, the presence of the parasite changes the snail’s behaviour,” he said. “As the day wears on, it does not like uninfected snails crawl back into the undergrowth out of harm’s way. Instead, it remains exposed out in the open, dangerously so.”

Attenborough isn’t the only person to make such claims. Leucochloridium has become a textbook example of a parasite that manipulates the behaviour of its host for its own ends. But Wanda Wesolowska and Tomasz Weslowski from Wroclaw University in Poland discovered that this “fact” was based on the shakiest of foundations.

Leucochloridium’s hijacking behaviour was first described in 1835. In the intervening 178 years, the only evidence that it manipulates the snails came from a German paper published in 1922. That author simply suggested that infected snails seek the well-lit upper surfaces of leaves, where they are visible to birds, and never presented any data to back up the claim.

Weslowska and Weslowski were chagrined, and they set out to Białowieza National Park in Poland to observe some of the snails. By carefully recording their behaviour, they confirmed that the infected animals behave very differently to the uninfected ones. They sat higher above the ground, kept to better-lit areas, stayed in more open spaces, moved around more, and oozed along more erratic paths. All of these traits would make them easier to spot and to attack.

Phew! Attenborough stands correct; the textbooks don’t have to be rewritten.

But wait! There’s more work to do. The only evidence that birds are attracted to the broodsacs comes from an 1874 study in which captive birds attacked the sacs. No one has checked that wild birds behave in the same way.

As Weslowska and Weslowski write, “Despite strong prevailing opinions and numerous popular accounts, there is not a single study documenting attacks of definite passerine hosts on snails with broodsacs… We think that such a situation is quite embarrassing, and thus, we would like to encourage the readers to undertake studies of this host–parasite association.”

You might think this represents nitpicking of the highest order, but the world of natural history is heaving with claims that sound right but have never been properly tested. Many of these “facts” deflate in the face of evidence. Cheetahs don’t overheat after they hunt—that’s a myth based on a 1973 experiment in which captive animals ran on treadmills. Komodo dragons don’t kill with a bacteria-laden bite—that’s a myth based entirely on speculation. And honeyguide birds do not lead honey badgers to honey—that’s a myth based on folk tales and some very unscrupulous documentary-makers.

But others check out. Thresher sharks really do slap fish with their huge tails—something that was assumed for around a century but filmed this year. Cheetahs really can run at 60 miles per hour, something that was based on a single measurement and had never been checked in the wild. And I’m personally delighted that Leuchochloridium’s manipulative streak seems to fall into this camp.

Reference: Wesołowska & Wesołowski. 2013. Do Leucochloridium sporocysts manipulate the behaviour of their snail hosts? Journal of Zoology http://dx.doi.org/10.1111/jzo.12094

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