Gabriella Luongo

This sea slug has a trick for getting an extra treat

Talk about shortcutting the food chain. The nudibranch Cratena peregrina (pictured), a type of sea slug, often attacks prey that have just eaten, thus getting an extra boost of nutrition from the prey’s undigested meal, a new analysis reveals. Scientists have dubbed this previously unreported style of gaining sustenance “kleptopredation” (a term including the Greek word for thief). In shallow waters of the Mediterranean and along the Atlantic coast of western Africa, C. peregrina lives on and near corallike colonies of polyps called hydroids. The nudibranch, besides using the hydroids for habitat, regularly consume its home colony’s polyps. But in the lab, the marine biologists noticed an odd behavior: When given the chance to consume either a polyp that had just swallowed prey or an empty polyp, the nudibranch strongly preferred polyps that had just eaten, the researchers report today in Biology Letters. It’s not yet clear how the nudibranch identified polyps that contained an undigested meal; they could have been attracted to a chemical signal released by the polyp when or after it captured prey, the researchers suggest, or they might have been drawn to vibrations in the water triggered by struggling prey inside the polyp. In any case, this odd dietary habit may prove beneficial for both C. peregrina and the colony of hydroids it preys on: The nudibranch gets extra nutrition, and as a result it can make ends meet by consuming fewer polyps—thus reducing the overall damage to its hydroid home.