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Fish play to have fun, according to a new study.

Researchers from the University of Tennessee in Knoxville have been able to define "play" in a way that helps people identify it in species like wasps, reptiles and invertebrates, which have previously been thought to be incapable of play.

"Play is repeated behavior that is incompletely functional in the context or at the age in which it is performed and is initiated voluntarily when the animal or person is in a relaxed or low-stress setting," said Burghardt.

Burghardt and his team are the first to discover play with objects in a cichlid fish species.

After filming three male fish individually over the course of two years, researchers found that the fish seemed to repeatedly "attack" a bottom-weighted thermometer. Researchers noted that the presence or absence of food, or other fish within the aquarium or visible in an adjacent aquarium, had no effect on the frequency fish struck the bottom-weighted thermometer.

Researchers said that this type of behavior satisfies their criteria for "play".

"The quick righting response seemed the primary stimulus factor that maintained the behavior," said Burghardt. "We have observed octopus doing this with balls by pulling them underwater and watching them pop back up again. This reactive feature is common in toys used for children and companion animals."

"Play is an integral part of life and may make a life worth living," said Burghardt.

The findings were published in the journal Ethology.