The horror always returns
He is thin and mean, treacherous, deceitful and brings death; he drives his victims to madness and suicide. But then - finally! - mortal menace lurks and with it justice also for him. That's what you thought until now.
His name is Paragordius tricuspidatus, and he lives his wild life, regardless of loss. Birth and death overtake the Gordian parasite worm in the open waters of ponds, streams and lakes, where adult specimens end up thinking of only one thing. At the first opportunity, males and females of the long, whip-shaped "Nematomorpha" gather en masse in twitching, wild knots, mate - and die. Just before that, the females bring out the spawning seeds of the next generation. With her, misfortune will take its course again.
At least from the point of view of insect passers-by carelessly strolling along the bank - the target group of young stringworms. While some of the worms get tired of waiting dry on land and wait for better times under a cyst protective cap, others are alive and kicking: they board passing beetles or grasshoppers and use the hard penetrating spur on their front side to bore through flexible synovium into the insects' interior. Meanwhile, the waiting cysts serve as appetizers and can thus get into a terrestrial insect – also via crawling stopovers eaten by hungry predators.
Here then, finally at the goal of all wishes, the young worms parasitize the nutrient richness of their hosts for a happy adolescence, become fat, plump and finally very, very elongated adults. A comparatively quiet period of prosperity – which, however, came to an abrupt end at the latest when the worm's sexual appetite awoke. Their drive forces males and females back into the water to meet the opposite sex and complete their life cycle. But before that, the worms obviously have a problem.
How they solve it - how a string worm makes it from the inside of water-shy, land-living insects to the next pond - is a parasitological feat in itself, which Frédéric Thomas and his colleagues reported to an astonished group of interested parties three years ago. Researchers at the French National Scientific Research Center CNRS had discovered a perfidious molecular mind manipulation of infested insects, engineered by sex-stimulated stringworms.
Under the influence of certain worm-produced neuroproteins, the water-shy hosts suddenly lose all aversion to damp water - video films show, for example, a bold, unhesitating leap from the edge of a swimming pool that a sober, calculating insect would certainly never have dared. The string worms had the brain and thus the behavior program firmly under control and apparently reprogrammed it according to their purpose - hardly having arrived in the wet element, the worms quickly left their wriggling, internally eaten ex-accommodation. And while the worms that have been released are long on their way to soon having mass orgiastic sexual amusements, the most common sad fate of the abused and used-up insect life, frantically wriggling in the water, is death by drowning.
The second most common fate: Death by being eaten, this time by a frog, fish, or other hungry aquatic creature. And that, Thomas and his colleagues thought, should actually be a problem with string worms. Because if loitering parasites don't leave their drowning six-legged friend in time, they too will end up being prey to a hungry predator.
Or not? Thomas, his colleague Fleur Ponton and colleagues follow up and now report on another problem-solving strategy of the amazing freeloaders. To do this, they brought representatives of all three parties involved into the laboratory: hungry aquatic predators (trout, perch and frogs), unfortunate insects (crickets) and, inside, Paragordius tricuspidatus. The researchers then pushed the predator, prey and parasite into a common pool – and waited a total of 477 times for the things that would inevitably happen there.
In fact, apart from the unsurprisingly quick demise of the insects, something completely unforeseen happened: In around twenty percent of all cases in which fish devoured the insect and the worm living in it, the thread-like parasite soon slithered before the eyes of the astonished observers out of the predator again through the mouth, gills or nose – in the case of frogs, even 35 percent of all Paragordiuses eaten managed to do this.
The worms took an average of 8.6 minutes to complete the feat, the scientists stopped. However, if the worms did not appear visibly from the outside after five minutes at the latest, they never returned – probably dying, decomposed in the "hostile environment of the predator's digestive tract", as the scientists rather coldly put it. However, pity is hardly appropriate for such parasite failures: After all, evolution had prepared them more than well for all eventualities, they too had their chance. The crickets would have been happy about that.
