Water fleas adapt to life with deadly carnivorous plants

Bladderworts are carnivorous plants that are found in freshwater lakes and marshes, and in waterlogged soil on all continents except Antarctica. Their sophisticated “bladders” are highly adapted to suck in any prey that swim past and disturb the trigger “hairs” connected to the trapdoor that opens into the bladder. Species with larger bladders can feed on aquatic prey such as water fleas, nematodes, mosquitolarvae and even fish fry and young tadpoles. 

In a study led by researchers at Ruhr-Universität Bochum (RUB), water fleas were kept in the laboratory in the presence of aquatic Utricularia plants (bladderworts) to investigate whether they have any defenses against this carnivorous plant. 

The team, headed by Dr. Sebastian Kruppert, Dr. Martin Horstmann and Professor Ralph Tollrian from RUB, in collaboration with Professor Thomas Speck from the Freiburg Botanical Garden and Simon Poppinga from the Technical University of Darmstadt, describe their findings in the International Journal of Molecular Sciences.

The researchers began by collecting water fleas, a type of small crustacean, from a site where they naturally occurred alongside bladderwort (Utricularia x neglecta) plants. The water fleas were kept in the laboratory, where they reproduced by parthenogenesis, forming offspring that were all genetically identical. Some of the water fleas were then placed in conditions with no Utricularia present, while others were cultured alongside the carnivorous plants, but separated by a fine mesh so that they would not be eaten.

Although the water fleas did not come into contact with the plants, they could detect the presence of the plants by virtue of chemical signals in the water. Under these conditions, water fleas were found to grow longer lateral spines on their carapace, an adaptation that may have made them more likely to get stuck in the entrance of a bladder rather than being sucked inside it and digested.

In addition, the researchers found that water fleas living in the presence of the plants swam along more slowly than those that grew up without the plant. Since the trigger hairs are set off by the movement of water currents produced by passing prey, swimming more slowly would be less likely to trigger the trapdoor and the suction action of the bladder. 

Since the water fleas were all genetically identical, these changes in morphology and behavior were induced by the presence of the plants, and not by genetic differences. “This shows that the otherwise genetically identical animals only activate defenses when they need them because they grow up side by side with the plants,” said Dr. Kruppert. 

The effectiveness of these changes was tested by the researchers, who counted how often water fleas were sucked into bladders when the two species were kept together in the laboratory. They found that animals that had grown up without the plant were captured more frequently than those that had been previously exposed to the plant and had developed defenses. The adaptations therefore increase the survival rate of water fleas raised in the presence of the plant. 

“This indicates that the activatable adaptations are actually defenses against the plant,” said Dr. Kruppert.

“We assume that the appendages let the water fleas grow wider than the diameter of the suction trap entrances,” explained Dr. Horstmann. “The traps are different sizes, but the smaller traps at least can no longer ingest the animals.” Since the water fleas with defenses are also slimmer, the water current can probably flow past them more easily. Moreover, the slower swimming movements probably trigger the traps less often.

“We hadn’t been aware of any other case where animals can defend themselves against attacks from plants. The fact that various defenses such as behavioral adaptations and changes in body structure can simultaneously be observed shows how adaptable and fascinating these tiny animals are,” said Tollrian.

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By Alison Bosman, Earth.com Staff Writer