In the waters of the western Pacific drift odd bits of seaweed that look like grapes. But look carefully: Impeccably hidden sea slugs lie among this algae, cloaked in what look like green capes.

For decades, these incredible mimics were thought to be one species. But a new study shatters that preconception. Not only are these sea slugs more genetically unique than first thought, they're actually five species, not just one.

The new species, recently unveiled in Zoologica Scripta, provide a powerful example of mimicry, which is fairly uncommon among sea slugs. Those that are mimics generally impersonate other animals, and some have evolved bright colors to ward off predators. Few, if any, are as invisible—or as green and