These Long, Skinny Fish Hide Behind Bigger Fish to Sneak Up on Their Prey

In the waters of the western Atlantic Ocean, long and thin trumpetfish have evolved a unique hunting strategy, researchers say. The predatory creatures will sometimes swim pressed up against the sides of larger, more colorful parrotfish, a strange behavior that serves as a kind of disguise, masking the trumpetfish from their unsuspecting prey.

In a new study published Monday in the journal Current Biology, scientists 3D-printed models of trumpetfish and parrotfish and tested how living prey in the ocean responded to them. Their experiments suggest that trumpetfish can swim under the radar, evading detection by prey, when they employ this “shadowing” tactic.

“This is the first non-human example of an animal using another animal as camouflage to get close to or to conceal themselves from their prey,” Sam Matchette, a co-author of the study and zoologist at the University of Cambridge in England, tells New Scientist’s Chen Ly.

“Although fish biologists have known about this for a long time, this is the first time someone actually made an experiment that demonstrated how much of an advantage this strategy gives the trumpetfish,” Luiz Rocha, a fish expert at the California Academy of Sciences who wasn’t involved in the study, tells the New York Times’ Annie Roth.

Atlantic trumpetfish are elongated creatures with large snouts, which they use to suck in the small fish and shrimp that they eat. They hunt with a variety of tactics, including changing their color to blend into their surroundings and hiding amongst the similarly tube-shaped, long branches of soft corals, according to the conservation organization Oceana.

Scientists have long suspected that shadowing other animals is another hunting strategy for the trumpetfish, allowing them to approach small fish without scaring them off. In Caribbean coral reefs, researchers have observed the creatures hiding alongside parrotfish, which are herbivores and don’t pose a threat to prey species. But nobody had tested whether this helped the trumpetfish hunt, the study authors write.

For the new experiment, the researchers brought their 3D-printed fish models to 36 colonies of a prey species called bicolor damselfish off the island of Curaçao, just north of Venezuela. At each underwater habitat, they attached nylon lines to tripods, essentially creating a pulley system that passed over the damselfish colonies. The scientists affixed their fake fish to the lines and reeled them over the living prey.

Stealth swimmers: the fish that hide behind others to hunt

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They conducted three versions of the experiment. In one, they pulled just the parrotfish model. In another, they only pulled the trumpetfish model. And in the third, they attached the trumpetfish model to the side of the parrotfish model and made them move over the habitat together, representing the “shadowing” technique.

When only the model parrotfish “swam” overhead, the damselfish did not swim away. But when the proxy trumpetfish passed by the colony, the prey fish inspected it for a moment, then fled. Finally, as the researchers tugged the makeshift trumpetfish-parrotfish pair across the wire, the damselfish did not bolt immediately—their response was similar to how they reacted to the lone parrotfish.

Compared to the solo trumpetfish, the shadowing one was able to get closer to the colony before the damselfish swam away, and fewer damselfish avoided it. The results suggest that shadowing makes it harder for damselfish to detect trumpetfish predators, the study authors write.

“This is an excellent example of where a fascinating piece of observational natural history is turned into hard science by focused experimentation,” Innes Cuthill, a behavioral ecologist at the University of Bristol in England who did not contribute to the work, tells New Scientist. The findings suggest that trumpetfish shadow other fish to hunt, rather than to avoid their own predators, he adds to the publication.

“We see a lot of amazing strategies from animals across the animal kingdom in camouflage and concealment and the ways in which they disguise themselves and they disguise their intent,” Matchette tells the Guardian’s Nicola Davis.

Although the trumpetfish is the first non-human species documented to hunt by shadowing, both Rocha and Matchette tell the New York Times that other animals out there likely use this same strategy.

As coral reefs continue to decline, shadowing may become more popular, the researchers hypothesize in the paper. In a study last year, the team worked with divers to track the occurrence of trumpetfish shadowing behavior in the Caribbean Sea—and they found that the most common sites for shadowing were flat or degraded coral reefs, per Forbes’ Amanda Kooser.

If trumpetfish no longer have reefs to hide in, using “other organisms around you for cover might become more commonplace in the future,” Matchette tells New Scientist.

Will Sullivan | | READ MORE

Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.