It's sort of like Russian nesting dolls, but more vicious. A tiny animal eats its prey. But when a larger animal eats said tiny animal, it gets two meals for the price of one.

Such premeditated behavior is the subject of a study published in the journal Biology Letters. "Kleptropredation" is when a predator waits for its prey to eat and then attacks.

Eating the eater

Kleptropredation isn't entirely without precedent. A behavior called kleptoparasitism has been widely observed in the animal kingdom. In this instance, an animal waits for another animal to either kill prey or collect food and store it. The first animal then moves in to either scare off the original predator so it can eat the prey instead — think of hyenas — or steal food the outright, which some flies do to ants.

This new behavior is different, however. It'd be like if the hyenas decided to eat the lion after the lion ate the gazelle. So far, this behavior has only been observed in the Cratena peregrina species of nudibranch sea slugs.

These slugs enjoy munching on hydroid polyps, which are tiny sea creatures that live in tree branch- or fan-like colonies. These hydroids, in turn, like eating zooplankton. Researchers gathered up these three organisms from a coastal town in Sicily and then transported to a lab for study.

The slugs were then presented with a few options, including hydroids that had just eaten zooplankton, hydroids that had already digested zooplankton, hydroids that hadn't eaten anything at all, and just zooplankton by themselves. In 14 out of 25 cases, the slugs went after the hydroids that had just eaten. Two meals for the price of one.

A smart diet

That small of a sample doesn't conclusively prove a preference, but the study does seem to indicate that the slugs are more opportunistic than previously believed. Researchers noted that the sea slugs had "elevated feeding rates" in the presence of hydroids that had just eaten, consuming double the amount of fed hydroids compared to unfed. But it's a smart choice, energy-wise, for the slugs. Getting two prey in one meal maximizes their time and energy, and that means slugs can spend more time on other things, like reproduction and general survival.

It isn't clear what triggers this minor feeding frenzy among the sea slugs. The researchers thought it might be a matter of smell, but the sea slugs could differentiate between a plankton that was just floating freely and a plankton that had already been consumed and preferred the already devoured plankton. A mechanical response, like seeing a hydroid's feeding movements or appearing to be in the process of digesting, also could trigger the sea slugs to move in for a snack.

The observation of kleptropredation could indicate that other organisms are engaging in it as well, particularly other "invertebrate specialists," like C. peregrina. More importantly, this behavior complicates not only our understanding of predator-prey relationships, but also our understanding of environments' food webs. Insights like these could teach us more about how organisms respond to shifts in their environment and how we can better protect them.