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The Luminescent Underwater Slug Pirates of the Mediterranean Sea

Crafty nudibranchs get two meals in one.

<em>Cratena peregrina</em> grows up to two inches in length and prefers to live on the rocky sea bottom.
Cratena peregrina grows up to two inches in length and prefers to live on the rocky sea bottom. Javier Gomez/CC BY-ND 2.0

Sicilian sea slugs are ferocious hunters. Their prey of choice? Spiky hydroids, and lots of them. Each sea slug, or nudibranch, can snack on as many as 500 of the much smaller predators a day. All this hunting can presumably get pretty tiring, but scientists at the University of Portsmouth in the United Kingdom, have found that nudibranchs are careful to maximize their meals. They prefer, the scientists observed, to hunt hydroids that have recently had a meal of their own, like pirates claiming the booty of their prey.

“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,” marine ecologist Trevor Willis, lead author of the paper in Biology Letters, said in a statement.

Hydroids, distant cousins to coral, feed on tiny plankton and other small crustaceans. Scientists offered sea slugs a variety of dining options: plankton, hungry hydroids, and hydroids that had recently consumed a whole lot of plankton. The brightly colored nudibranch Cratena peregrina, with its luminescent blue tips, picked off the well-fed hydroids twice as often as those that hadn’t recently eaten (though no one’s sure how the sea slugs were able to tell the difference).

Researchers have given this newly observed behavior a name—“kleptopredation,” a combination of “predation” and “kleptoparasitism,” or the practice of stealing food from another species. Rather than simply taking the food from another predator, the sea slug is eating that predator as well as the prey it expended energy to catch. In short, that’s two meals for half the effort and a more diverse diet in the process. Sometimes crime really does pay.