Zoologger: The fish with a chemical invisibility cloak

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Aphredoderus sayanus
Habitat: Clear, warm waters in the central and eastern US, particularly in Mississippi

A squirrel tree frog lurks by a small pond, seeking a safe place to lay its fertilised eggs.

The pond is thick with vegetation: rich with food for the frog's young, but also a good place for a predator to hide. The frog strains its senses to detect the presence of a hungry fish, but it doesn't see or smell anything. Reassured, it hops into the water and lays its eggs, then calmly leaves.

In reality, its young are doomed to become a fish supper. The frog has been tricked by the pirate perch, the only animal that literally cannot be sniffed out. Other animals cannot smell its presence, so they unwittingly put themselves and their offspring in harm's way.

Many animals, both predators and prey, use camouflage to disguise themselves. From mossy coated-millipedes to glow-in-the-dark sharks, nature has a huge range of strategies to hide in plain sight.

Camouflage goes beyond the visible ? chemical camouflage can conceal an animal's smell and taste as well. But whereas visual camouflage hides a creature from any prying eye, chemical camouflage normally blinds a specific species. For instance, the caterpillars of the large blue butterfly mimic the chemical make-up of one type of red ant. This allows them to enter the nest unmolested and slaughter the ants for food. But it doesn't work on other ants.

The chemical camouflage of pirate perch is a different matter. It works on every animal tested so far.

Merciless pirates

Pirate perch are night-hunting predators that will eat almost anything, from insects to fish. They earned their name after people tried keeping them in aquariums: like a merciless pirate, the perch would kill and eat all the other fish. "They are pretty voracious," says William Resetarits of Texas Tech University in Lubbock.

With Christopher Binckley of Arcadia University in Glenside, Pennsylvania, Resetarits ran a series of experiments with aquatic beetles and tree frogs. He wanted to see which ponds they would colonise and which they would avoid. As a general rule, they were expected to avoid ponds containing fish because fish are likely to eat their young.

Resetarits and Binckley set up small pools in a field in Virginia, some containing predatory fish. The fish were trapped beneath screens, so the animals could only detect them by smell. The researchers tested 11 distantly related fish species, including the pirate perch. The colonisers were 14 types of beetle and three species of tree frog.

The beetles and frogs "consistently avoided the [ponds] that had fish", Resetarits says ? except for the ones with pirate perch. The beetles were quite happy to live in a pond with a pirate perch, and the frogs laid their eggs in them just as often as in unoccupied ponds.

Mind-boggling

"It just boggled our minds," says Resetarits. "Two different groups have evolved the same avoidance of fish, but they both missed the same species."

Nobody knows how the pirate perch does it. "Any organism produces a whole lot of chemicals," Resetarits says. The pirate perch might have evolved not to release some of them, or might mask them with another chemical.

Pirate perch are the first animal found to have this global chemical camouflage, and Resetarits thinks there could be many more. "There's no reason to believe that this isn't quite common," he says. "It's just hard to home in on, because we humans don't perceive things that way."

Resetarits hopes to find other animals that use chemical camouflage, and he already knows where to look. Scent is more important in water, where chemical signals travel widely and visibility can be limited. It may also be more advantageous to nocturnal animals like the pirate perch, because smell is more important in the dark. Finally, animals that live in still water would reap more benefits from chemical camouflage, because in a fast-moving stream a fish's smell would be dispersed quickly anyway.

"Cavefish seem like a likely candidate," Resetarits says. Many cavefish are sit-and-wait predators that must be good at hiding themselves.

Animals that use visual camouflage, like chameleons, might have evolved chemical camouflage as well, says Resetarits. "It doesn't do a lot of good [to be] visually cryptic if you're smelly."

Journal reference: The American Naturalist, doi.org/ks8

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