Do you know what kleptoparasitic behaviour is? That's when one species of animal steals prey killed by another, such as hyenas driving a lion away from its kill. Kleptopredation, however, describes what happens when one animal makes a point of eating another that has just fed, essentially eating both that animal and its recently-consumed meal. The behaviour has been observed in nature for the first time, in sea slugs off the coast of Sicily.

The research was led by Dr. Trevor Willis, of Britain's University of Portsmouth. He and his team were studying a nudibranch (sea slug) known as Cratena peregrina, which both inhabits and preys upon hydroid colonies – distantly related to coral, hydroids take the form of individual polyps which consume zooplankton.

Watching the nudibranchs feeding, the scientists discovered that the animals were twice as likely to eat hydroid polyps which had themselves just eaten. In fact, over half of the sea slugs' diet consists of zooplankton, which they can only obtain by eating hydroids which have eaten it.

"Effectively we have a sea slug living near the bottom of the ocean that is using another species as a fishing rod to provide access to plankton that it otherwise wouldn't have," says Willis.

Additionally, though, the slugs' kleptopredation also helps ensure that their food supply will last.

When previously studying nudibranchs in New Zealand, Willis wondered how they avoided depleting hydroid colonies by eating all of the polyps. As it turns out, by concentrating on consuming polyps that provide the maximum amount of energy (i.e: the ones that have recently eaten plankton), the slugs don't need to eat as many of them, leaving more to maintain the colony.

"This is very exciting, we have some great results here that rewrite the text book on the way these creatures forage and interact with their environment," states Willis.