In a newly described predation strategy, sea slugs were observed to attack their prey that have just eaten so they no longer have to hunt down a variety of small preys.

Kleptopredation

In a paper published in the journal Biology Letters, researchers reported the feeding behavior called kleptopredation that they observed in colorful sea slugs known as nudibranch off the coast of Sicily.

Cratena peregrina, a type of nudibranch, feed on hydroids that prey on plankton and small crustaceans. Study researcher Trevor Willis, from the University of Portsmouth, and colleagues found that the cratena peregrina preferred to eat hydroids that recently dined. They also observed that nudibranchs were twice as likely to attack hydroids that had just eaten zooplankton compared with their hungry counterparts. It also took the slugs twice as long to consume the newly fattened prey.

Willis said that the sea slug, which lives near the bottom of the ocean, effectively uses another species to get access to plankton. A similar behavior also characterizes some mammals.

"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," Willis said.

Opportunistic Feeding

The findings show that a large proportion of the sea slug's diet is composed of tiny plankton captured secondhand, a strategy known as opportunistic feeding.

"Nudibranchs will, by preference, consume hydranths that have captured and are handling prey. This supports the explanation that C. peregrina is an opportunistic predator that uses the hydroid as a means of obtaining prey from the water column, and ingestion of the hydranth provides just a fraction of the diet by volume," the researchers wrote in their study.

How Sea Slugs Distinguish Full And Hungry Hydroids

The researchers are not sure how the sea slugs distinguish hungry hydroids from those that are full, as well as captured plankton from the free ones. James Newcomb, from New England College, who is not part of the study, offered a possible explanation that involves the hydroid's stingers that they use to catch their prey.

"An alternative hypothesis to explain why the predator (nudibranch) selectively chose hydroid polyps that had recently fed is because those polyps had expended their stinging cells to capture prey and thus could not use them on the nudibranch," Newcomb said.