Fish Turn on One Another When Attacked by Predators

In the underwater world, self-preservation takes many shapes. Fish, for example, will avoid capture by swimming in a large mass that makes it difficult to single-out one individual. But according to the latest research on a small species of South American fish, the creatures will turn on one another when confronted by a predator on the hunt.

Two-spot astyanax, are seemingly innocuous fish. Often kept as pets, the small fish tend to swim in schools of about 50 and have a diet of plankton, plants and debris.

But when a predator species attacks a school of astayanax, the fish have been shown to mercilessly turn on a member of their group, attempting to isolate one individual as a target so the rest of the group can avoid danger.

The find came after researchers were trying to develop ways to keep the fish from swimming into hydroelectric power station machinery, where they can become a nuisance. One tactic explored included trying to scare the fish away from the machinery. It was then that researchers Robert Young and Vinícius Goulart noticed that the fish would turn on one another if threatened.

"They are essentially a prey species and are very commonly preyed upon by other species of fish," Young said, according to the website io9. "There was nothing in the literature to suggest there was anything exceptional about them."

Young and Goulart devised an experiment which used 64 of the fish obtained from a fish farm (which ensured the fish had no prior experience with predators), and observed their reactions to various scenarios: an active search predator, a sit-and-wait predator, a bird-like predator that pecked at the fish from above the water and a control scenario of gently lowering a plastic container into the water.

For the active predator scenario, the researchers found that the fish would chase and bite each other, forcing the victim fish out of the shoal where it could be more easily preyed upon. The find was consistenat across the test groups and was evidenced upon the very first exposure to the scenario, which indicated the behavior in innate and not learned.

"You will see low levels of aggressive from time to time in normal interactions," Young said of the fish. "But we've never seen this kind of intensity before, where the actual aggressive interactions that occur more than a double."

The other predation scenarios and the control did not reveal increased instanced of aggression, which Young said makes sense because if a predator is chasing the group, isolating one individual would draw the hunter's attention.

Young and Goulart's research is published in the journal Animal Behavior.