Japanese scientists working with nematodes (C. elegans) noticed one day that several cultivated worms in the lab mysteriously kept ending up attached to the lids of Petri dishes instead of the dog food agar where they were initially placed. Intrigued, they conducted experiments to figure out how the worms were getting from one point to the other in less than a second.
The researchers found that, rather than crawling up the walls of the dish, the worms were leaping from the bottom of the plate to the lid—and they were using electric fields to do so. They could even leap from the Petri dish onto a bumblebee, both individually and in large clusters. The team described their work in a new paper published in the journal Current Biology.
“Pollinators, such as insects and hummingbirds, are known to be electrically charged, and it is believed that pollen is attracted by the electric field formed by the pollinator and the plant,” said co-author Takuma Sugi, a biophysicist at Hiroshima University in Japan. “However, it was not completely clear whether electric fields are utilized for interactions between different terrestrial animals.”
Interactions between different living organisms “strongly shape the structure and function of ecological communities and ecosystems,” the authors wrote. Different species can rely on different environmental stimuli: chemical interactions between insects and plants, for instance; visual perception to detect prey and select preferred foods; and the ability to detect mechanical energy from other animals. Then there are electrostatic interactions. In addition to pollinators, the authors note that several species of fish use electric fields to sense prey and predators. And ballooning spiders—immortalized in E.B. White’s children’s classic Charlotte’s Web—shoot out silk threads to form a parachute and float off into the air.
How do the spiders do this? One hypothesis holds that the spider threads have a static electric charge that interacts with the weak vertical electric field in the atmosphere. A competing hypothesis is that, as the air warms with the rising sun, the silk threads the spiders emit to spin their “parachutes” catch the rising convection currents (the updraft) that are caused by thermal gradients. A 2018 study found that spiders seem able to detect electric fields under natural atmospheric conditions. This triggers ballooning behavior, and the electric fields provide sufficient force to generate lift. Last year, physicists demonstrated via 3D numerical simulations that, at least for small spiders, electric fields are indeed sufficient to generate enough lift without an assist from air currents.
One reason interspecies interactions have such a strong influence on ecosystems is that they assist with animal dispersal, which Charles Darwin deemed critical to the evolution of a species and the expansion of its range. When one species relies on another for such dispersal, it’s known as phoresy. Smaller animals that are wingless and legless, like worms, frequently attach to passing larger animals like insects and birds to traverse large distances.
C. elegans is found on a wide range of species and relies on a type of phoresy to achieve that range. Prior research suggested that, in some cases, like snails and certain bugs, the dispersal mechanism is fairly simple. Nematodes engage in a behavior known as “nictation,” in which the worms stand on their tails, thereby decreasing the surface tension of the water in which nematodes are often found, making it easier for the worms to attach to their dispersal hosts. Per Sugi et al., this also increases the frequency of direct contact with other animals.
However, unlike snails and bugs, flying insects like bumblebees naturally accumulate charge during flight, producing an electric field. Sugi and his colleagues thought that electrostatic interactions might explain why their lab-grown nematodes kept ending up on the lid of the Petri dish. The first experiments confirmed that the worms weren’t crawling up the walls of the Petri dish. Switching to high-speed video enabled the team to catch the leaping motion on camera and confirm that the worms nictate before making the leaps. And the worms did not appear to be generating the leaping force, suggesting an external force was at work.
To find out whether that external force was electric fields, Sugi et al. conducted another experiment. They embedded a square array of micro-posts on the surface of the agar, mimicking a natural soil environment. They placed about 1,500 worms on that agar substrate and then placed it atop a glass electrode. They placed a second glass electrode parallel to the first but separated by a small distance. Then they applied voltage to see what happened. The worms only lept to the other electrode when the charge was applied and moved at an average speed of 0.86 meters per second. That’s close to the speed of human walking, and their speed increased as the electric field intensified.
Finally, the team rubbed flower pollen on bumblebees to create a natural electric charge and placed the bees near the worms. When the bees were sufficiently close, the worms stood on their tails and jumped to the bees. This even worked for clusters of worms piled on top of each other, with one poor overladen worm carrying the load during transfer.
The mechanism might now be clear, but Sugi et al. still aren’t sure exactly how all of this works. Fortunately, C. elegans is a model organism and the relationship between its genes, behavior, and neural activity has been widely studied. “Therefore, further studies on the electric field and the behavior of C. elegans are expectated to provide more details on the electrical ethology of microorganisms,” the authors concluded.
Listing image by Current Biology/Chiba et al.