There's nothing ruder than taking someone's food without asking—except maybe waiting until they're finished eating and consuming them whole so you can get those extra calories. As Gizmodo reports, this way of feeding, dubbed "kleptopredation," may be how at least one group of mollusks prefers to take its meals.
In their recent study published in the journal Biology Letters, European scientists explored the eating habits of nudibranchs. Nudibranchs are soft-bodied sea slugs that are sometimes called "butterflies of the sea" for their colorful appearances. The striking sea creatures are also effective carnivores, preying on sponges, anemones, and occasionally other nudibranchs.
For their experiment, the researchers placed members of a nudibranch species known as the pilgrim hervia (Cratena peregrina) in a tank with one of their favorite treats: hydroids, a relative of jellyfish. Plankton were also present for the hydroids to eat. When given a choice between the empty-stomached hydroids and hydroids that had just consumed or were in the middle of consuming plankton, the nudibranchs chose the well-nourished prey in 14 out of the 25 trials. According to the study authors, plankton can make up at least half of the nudibranch diet this way even though it's being preyed upon indirectly. This means that a nudibranch's interactions with a plankton-loving cnidarian could go beyond the basic predator-prey relationship.
Though the nudibranchs' preference for hydroids that had eaten plankton wasn't random, that doesn't automatically mean it's a case of kleptopredation. It's possible that the sea slugs simply chose the prey that looked larger, or they went after the hydroids that had just fed because the stinging cells the cnidarians use to hunt were worn out and therefore less harmful.
If more research does support the theory that the animals target full prey before giving them a chance to digest, that would add the unique predation method to the already impressive roster of nudibranch abilities.