Up until now, the loaded predatory fish were considered loners – but now researchers have discovered a common hunting behavior in electric eels: they drive fish together and then use their combined power to shock the surrounded victims with electric shocks. Then the band of robbers attacked the paralyzed prey. Such group behavior is only known from a few fish species – in the case of electric eels, it is literally exciting, according to the scientists.
It was not just humans who made use of electricity – in the course of evolution, fish from different families have already developed potential: They use fields of tension and electrical surges when catching prey, for defense or for navigation and communication. Some representatives of the biovoltaic community have achieved considerable potential. The electric eels from the waters of the Amazon region are considered the undisputed record holders. In these representatives of the knife fish, the tension is generated by modified muscle cells under the skin. The head forms the plus pole and the body end forms the minus pole. They can build up hundreds of volts of voltage and use it to chase violent electrical surges through their prey and enemies. The clout can even paralyze people.
A strange collection of electric eels in sight
For some time now, David de Santana’s team from the National Museum of Natural History in Washington DC has been researching these bizarre fish. They have already shown that three electric eel species create tension in the Amazon basin. The king of biovoltaics is Electrophorus voltai: It strikes with a record voltage of up to 860 volts, according to the scientists’ measurements. These electric eels, which are around one meter long, were now also the focus of the current study. The researchers discovered a cluster of more than 100 adult specimens in a small lake connected to the Iriri River in the Amazon basin. To investigate what this strange gathering might be about, the researchers recorded the behavior of the fish through video recordings.
The evaluations of the film material showed: Most of the day and night the electric eels lay almost motionless in deeper areas of the lake and only rarely came to the surface. But at dusk and dawn there was astonishing activity: the fish seemed to interact with each other and numerous animals began to swim in a large circle. In the center they drove thousands of fish up to about five centimeters in size to form increasingly narrow schools. The researchers observed how the electric eel squadron then drove the concentrated prey from the deeper part of the lake into about one meter deep shallow water.
Combined clout meets surrounded victims
There up to ten electric eels broke away from the group and began to attack the encircled swarm in the center of the circle with synchronized electric shocks. Some of the prey fish were thrown out of the water by their muscle spasms. Then the entire electric eel group went to work on the paralyzed victims. Each dawn or dusk hunting ritual lasted about an hour and included between five and seven high-voltage attacks, the scientists report.
“Hunting in groups is quite common among mammals – wolves and killer whales are known for this. But it is rare with fish, ”emphasizes de Santana. “There are only nine other fish species whose behavior has been documented so far, which is what makes this discovery so special,” says the researcher. Apparently electric eels have an amazing flexibility in their diet. So far it was only known that they sneak up on sleeping fish alone at night and then paralyze them with electric shocks. By hunting in a group, however, they can apparently also use large groups of nimble prey for their food.
So far, however, it remains unclear how often and under what conditions the electric eels show the communal behavior. The researchers now want to pursue this question further. They have started a project that calls on locals to tell them about electric eel gatherings. De Santana and his colleagues are now planning another expedition to the small lake on the Iriri River.
Among other things, they want to answer the question of what electrical power is generated when the fish are hunted together. “An individual of this species can cause a discharge of up to 860 volts – so if ten of them discharge at the same time, they could theoretically fall back on a potential of up to 8,600 volts,” says de Santana. The researchers also want to investigate the question of whether the electric eels communicate with one another in group behavior. They may use fine electrical signals to coordinate – similar to how some whales and dolphins use sounds to coordinate when hunting their prey.
Source: Smithsonian, Article: Ecology & Evolution, doi: 10.1002 / ece3.7121
Video: A group of electric eels jointly give electric shocks to surrounded prey fish. (Credit: Douglas Bastos)