When this sea slug eats, it prefers ‘turducken of the ocean’

You’re camping in the woods, about to tuck into some salmon you’ve caught when a bear appears. She’s waiting for you to eat your fish, so she can swoop in to eat you.

It’s a steal for the bear. For the price of a human, she’s bagged a human-salmon combo meal.

This scenario is hypothetical, but the feeding strategy it illustrates is not, according to a study published Wednesday in Biology Letters. In the report, a team of scientists from Britain and Italy termed the tactic “kleptopredation” and described it not in bears, but in a brilliantly colored sea slug, Cratena peregrina, that’s about the length of a soda-can tab and commonly found in the Mediterranean.

These psychedelic slugs, also called nudibranchs, are known to feed on tentacled marine organisms known as hydroids, which are related to coral and sea anemones. They pop the polyps off the hydroids as one might pick a flower off a stalk. But based on lab experiments, the authors of the new paper suggest the slugs prefer to eat hydroids that have just ensnared plankton, a food nudibranchs aren’t capable of capturing for themselves.

Think of it like wielding a living fishing rod. Or eating a “turducken.” Whichever your preferred analogy, kleptopredation — using one prey item to obtain another prey item — falls outside ecologists’ traditional classifications of feeding behavior. There’s predation, and there’s so-called kleptoparasitism (when one animal takes food from another animal, like a pack of hyenas stealing a fresh kill from a lion). But kleptopredation is something new.

Patrick Krug, a nudibranch expert at California State University, Los Angeles, who was not involved in the study, said this “steal your meal, and eat you, too” strategy potentially rewrites how ecologists understand food chains. “You usually think of a predator eating a prey item, not eating what the prey item is eating,” he said.

To test whether Cratena nudibranchs had a penchant for hydroids that had freshly eaten, the authors did lab experiments on 25 slugs, presenting each with four mesh bags. These bags held nothing, empty hydroids, hydroids that had just been fed brine shrimp or brine shrimp only.

In 14 of 25 cases, the nudibranchs “went straight to the bags that contained fed hydroids,” said Trevor Willis, a marine ecologist at the University of Portsmouth in Britain and lead author of the study. The slugs also ate about double the number of fed polyps compared to unfed ones.

Next, the researchers analyzed the diet of wild nudibranchs by measuring an isotope called nitrogen-15. When an animal eats another organism, this heavier form of nitrogen becomes more concentrated in its tissues than it was in its prey’s tissues, so the higher up the food chain you go, the more nitrogen-15 you should find.

Willis and his colleagues found that nitrogen-15 levels in their nudibranchs were lower than they would expect if the animals were eating hydroids only. This suggests that the slugs might be getting a significant amount of nutrition directly from small plankton, which contain lower quantities of nitrogen-15 than hydroids. Using statistical models, the researchers estimated that these plankton possibly contribute as much, or more, to these slugs’ diets as the hydroids themselves.

As for why sea slugs prefer hydroids who’ve had their fill, Willis speculates that it’s a way for the animals to get calories from plankton while not overeating hydroids, which they depend on for shelter in addition to nourishment. “It seems counterintuitive that an animal would destroy its own habitat,” he said.

This is purely conjecture for now, Willis said, adding that the research is a “first step” that raises more questions than it answers. One of these is whether other animals, like nudibranchs that eat coral, perform kleptopredation. He added that unraveling these detailed feeding interactions will help scientists understand how energy flows through ecosystems, and how such dynamics might shift with climate change.

“Once we have an idea of what’s going on at the species level, we can put these little stories together and look at whole communities,” he said.