The pirate perch, Aphredoderus sayanus, is a very strange creature: it’s a small fish, only 5.5 inches long at most, that’s the only species in its family. Its cloaca (sort of a combination waste and reproduction opening) is right under its chin, it breeds by secretly dropping eggs into a mass of tangled wood, it’ll eat anything in your fish tank (hence its name), and now it’s exhibiting some exceedingly odd predatory behavior.

The pirate perch, native to freshwater environments in North America, was the subject of a study published in The American Naturalist. It’s not a particularly well-understood fish, being solitary and nocturnal and all-around mysterious, so two researchers decided to try to see how exactly it preys. The experiment had frogs and aquatic beetles lay eggs in an environment populated by different types of potential predators that might want to chomp down on some eggs.

The frogs and beetles laid much fewer eggs in most of these environments; the predators were placed in a trash can outfitted with netting, so their presence could be made known. Frogs and beetles see, smell, or otherwise sense the predators, they don’t lay their eggs. Pretty simple. But the pirate perch had no such effect at all.

According to the paper, this is likely due to some kind of chemical crypsis, or cloaking. Crypsis is a method taken by animals to conceal themselves; the most common method is visual crypsis, better known as camouflage, though some practice olfactory crypsis, disguising themselves with scent.

The pirate perch, though, isn’t doing either of those. In fact, the researchers weren’t really sure what the pirate perch is doing. The researchers suggest that it could be producing some chemical that blocks the frogs and beetles from sensing it, which would make it the only known animal to use chemical crypsis.

[via Inside Science]