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.
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 leafy as these new masters of disguise.
“This may be the best example of an animal masquerading as a plant that we have,” biologist Nicholas Paul, an expert on seaweed and algae at Australia’s University of the Sunshine Coast, said in an email. He wasn’t involved with the new study.
The new species exclusively feed on the seaweed genus Caulerpa and are found throughout the Pacific, including Malaysia, Australia, Guam, and the Philippines.
Humans consider the algae’s caviar-like bulbs, called sea grapes, a delicacy but few sea creatures dare eat the stuff, making them highly invasive.
Thanks to the global aquarium trade, the algae has invaded waters from the Mediterranean to Japan.
But some sea slugs can and do eat Caulerpa, none more enthusiastically than the algae mimics.
They sniff the seaweed out, perching on their haunches and “dancing” in the water to catch its aroma.
“Oftentimes, the habitat or food will actually trigger metamorphosis of the slugs’ larvae into adults,” says lead study author Patrick Krug, a marine biologist at California State University, Los Angeles. “There’s like a chemical dependency.”
Once the sea slugs latch onto the algae, they cut holes into its sea grapes and suck them dry, like thirsty kids gulping down juice boxes.
Nearby predators wouldn’t suspect a thing; the slugs‘ bulbous backs appear studded with sea grapes.
And to complete their disguise, the sea slugs store the algae’s green pigments in their skin.