Sea slugs are the pirates of the underwater world when it comes to feeding time – targeting prey that have just eaten to “supersize” their own meal.
Scientists who identified the behaviour have named it kleptopredation.
Researchers from the University of Portsmouth are the first to have observed the brutal feeding strategy in the natural world.
Senior lecturer Dr Trevor Willis led the research into the behaviour of nudibranchs, a family of sea slugs, off the coast of Sicily.
He said: “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.”
The tiny brightly coloured sea slugs live and feed on hydroid colonies – a super organism, distantly related to corals, which consists of individual polyps that capture and feed on plankton and small crustaceans.
But Dr Willis studying the nudibranch Cratena peregrina found the creature preferred to eat polyps that had recently dined.
More than half of the nudibranch’s diet was made up of zooplankton – the prey of the polyp they were consuming.
This suggested the nudibranchs doubled their attack rate on prey that had gorged on zooplankton when compared with hungry counterparts.
Dr Willis said: “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.
“People may have heard of kleptoparasitic behaviour – when one species takes food killed by another, like a pack of hyenas driving a lion from its kill for example.
“This is something else, where the predator consumes both its own prey and that which the prey has captured.”
The sea slugs behaviour was a combination of kleptoparasitic competition and direct predation.
The study came about after Dr Willis pondered how a species of nudibranch in his native New Zealand seemed to have evolved to live and feed on hydroid colonies at the risk of depleting them.
He said: “There would always be the risk that the food would run out before the nudibranchs could reproduce, which didn’t strike me as a particularly intuitive strategy.
“A colleague in Sicily had data to indicate that there we something going on other than a simple predator-prey relationship.”
His research into Cratena peregrina aimed to investigate how the species balanced energy intake with the preservation of habitat.
It looked at nitrogen stable isotope levels in the nudibranchs, hydroid polyps and zooplankton, and discovered the nudibranchs had a significantly lower level expected if the polyps were their only prey.
This indicated the hydroid polyp represented a relatively low percentage of the total prey ingested.
The reseatchers said this implied that by consuming fewer hydroid polyps and increasing energy intake from their prey’s plankton diet, nudibranchs may extend the life of the hydroid colony on which they live, feed and shelter.
It is not known how widespread this behaviour might be but it is hoped this and future research could contribute to a better understanding of the marine environment.
Dr Willias added: “Our ability to understand and predict ecosystems in the face of environmental change is impeded by a lack of understanding of trophic linkages.
“While we have some great results, like any science worth its salt, it raises more questions than it answers.”
The findings were published in the journal Royal Society journal Biology Letters.
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