Plants with an appetite for water fleas: Researchers have gained insight into a bizarre predator-prey relationship. In order to defend themselves against the hungry Utricularia plants, which follow them with suction traps, water fleas apparently adapt in a clever way: When the small crustaceans “sniff” the enemy in the water, they develop lateral thorns and swim more slowly. Both make it more difficult for the carnivorous aquatic plants to be sucked into the traps, the scientists explain.
Animals normally eat plants – but about 600 plants have turned the tables. They prey on insects and other prey to make up for the nutrient shortages in their habitats. The Venus flytrap, the sundew and the pitcher plant, which capture flies, beetles and the like with their ingenious strategies, are particularly well known. However, there is also a group of carnivorous plants that have developed the underwater world as a hunting ground: the representatives of the genus Utricularia (water hose) set their traps in lakes and ponds. These are bubble-shaped trapping organs in which negative pressure forms. The slightest touch opens a flap, allowing water to rush into the trap, taking small organisms with it. After that, the door closes again and the prey is dissolved by digestive fluid.
Midgets with green enemies
The voracious plants are particularly interested in water fleas. These are small crabs that often scurry through standing water in large numbers. They form an important basis of the food web, as they serve as food for many organisms. However, the tiny creatures are not completely helpless victims: they arm themselves in dangerous habitats, as previous studies have already shown. When they sense the presence of animal enemies based on certain substances in the water, water fleas strengthen their armor and develop thorns. The scientists led by Sebastian Kruppert from Ruhr-Universität Bochum now wanted to know whether the tiny creatures also show defensive reactions when threatened by the carnivorous aquatic plants.
The researchers first looked for habitats in which Utricularia plants and water fleas coexist. They found what they were looking for in waters near Gelsenkirchen: Utricularia neglecta is hunting the water flea Ceriodaphnia dubia there. The researchers obtained representatives of these two opponents and then multiplied them in the laboratory. As they explain, water fleas are parthenogenetic: they produce genetically identical offspring, i.e. clones of themselves. This is how a defined breeding line for the experiments was created in the laboratory. First, the scientists cultivated the water fleas together with the plants, while they were separated by a fine grid. So the shrimp were not eaten, but there was the possibility that they could “sniff” the enemy via chemical messengers in the water.
New: Animal defense against plants
Comparisons with controls then showed that water fleas that lived in the same water as the plants developed longer extensions on their shells and were also slimmer. The researchers also found that they moved more slowly than the comparison animals that grew up without plants. As they explain, these are obviously defense strategies: “We assume that the extensions make the water fleas wider than the diameter of the suction trap entrances,” says co-author Martin Horstmann from the Ruhr University Bochum. “The traps are of different sizes, but the animals can no longer be eaten, at least by the smaller traps.” The slimmer shape probably also has a positive effect: the water suction when the traps are triggered can flow past the little animals more easily. In addition, there are also the slower – “more cautious” – swimming movements, which probably trigger the traps less frequently, the researchers explain.
In any case, the effect of the adjustments was clearly evident. The team compared how often the different water fleas were eaten. In fact, those that had been co-cultivated with the plants were subsequently less likely to be preyed on by them. “This indicates that the adaptations are actually defenses against the plants,” says Kruppert. Apparently they are slightly disadvantageous for the animals and are therefore only produced when it is worthwhile. “Because the otherwise genetically identical animals only activate these defense mechanisms when they need them because they grow up with the plants,” says Kruppert.
Finally, senior author Ralph Tollrian from the Ruhr University Bochum emphasizes the special nature of the discovery: “We have not previously known of any other case in which animals defend themselves against attacks by plants. The fact that various defenses such as behavioral adaptations and changes in body structure can also be observed at the same time shows how versatile and fascinating these seemingly inconspicuous water fleas are,” says the scientist.
Source: Ruhr-University Bochum, specialist article: International Journal of Molecular Sciences, doi: 10.3390/ijms23126474
Video: The video shows how a Utricularia suction trap captures small aquatic animals. © Ruhr University Bochum