Hakai Magazine

Gilt-head bream (Sparus aurata)
Gilthead seabream are the first marine fish known to physically transform when exposed to a predator. Photo by Franco Banfi/NPL/Minden Pictures

The Scent of Danger Makes These Fish Hulk Out

Some fish go through pronounced, yet reversible, physical changes when they sniff a predator’s trail.

Authored by

by Susan Cosier

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Gilthead seabream swim back and forth in their tank, the yellow markings between their eyes that they’re named for flashing as they glide by. Carlos Díaz-Gil, a graduate student at LIMIA, the marine research and aquaculture laboratory of Mallorca in Spain, adds a few ice cubes to their tank. The ice is made of water from a neighboring tank containing black scorpionfish*, one of the seabream’s feared predators. Díaz-Gil knows that the scent of the scorpionfish, just a trace of its existence, will likely cause the seabream to change in surprising ways.

After a month, Díaz-Gil kills a select few seabream and measures them. The fish’s bodies have gotten deeper—bigger from back to belly—and more difficult for black scorpionfish to swallow, expanding in what appears to be a natural defensive change.

Díaz-Gil expected as much, but what he really wanted to know is whether that alteration came at a cost. Did the fish grow more slowly? Could they reproduce as quickly? And, would they revert to their former shapes when the threat disappeared?

Díaz-Gil’s research, conducted in 2014 and published in a recent study, is the first to show a marine fish experiencing such a pronounced physical change when exposed to a predator’s cue, such as its feces or urine. It would be extremely difficult to test this in the field, so Díaz-Gil conducted his research in a lab. He chose the gilthead seabream, as it is a species commonly produced in aquaculture. The study investigated the consequences of that shift—demonstrating adaptive changes fish can go through to survive.

Most fish species that scientists have seen physically change in response to predators live in fresh water. One notable example is the crucian carp. This fish can be found in two different morphs, either shallow-bodied or deep-bodied; a distinction even Carl Linnaeus, a Swedish botanist and zoologist, observed in the 1700s. More recently, in 1992, Christer Brönmark, a biologist at Sweden’s Lund University, showed why this difference exists when he divided a pond filled with carp into two sections by installing a plastic wall, and then added pike to one side. When he came back 10 weeks later, he compared the carp and found that the individuals that existed with the pike (the ones that were left, anyway) had much deeper bodies.

Brönmark’s work spurred more scientists to begin looking at how fish may change in response to predators, which led to the discovery that other species also have morphing abilities. For instance, the dorsal and rear fin of a roach can get bigger when exposed to predators, a modification that makes it easier for it to escape predators. A Eurasian perch can develop a darker color that helps it camouflage more easily against a pond’s bottom, while pumpkinseed sunfish can develop defensive spines. It’s likely that many more species can adjust their bodies in similarly interesting ways.

“Fishes are well known for being shapeshifters, meaning that they can alter their body shape during their lifetime in response to changes in their environment,” says the University of Western Australia’s Jennifer Kelley, a biologist who studies fish morphology but was not a part of the study.

The costs of these changes, however, are still largely unknown. Crucian carp with deeper bodies have more drag, so they expend more energy; females aren’t found with deeper bodies as often as males, which could be because they need energy to lay eggs. In Díaz-Gil’s study, however, there appeared to be no detectable cost to the gilthead seabream. The deep-bodied seabream grew just as quickly as the fish that weren’t exposed to the scorpionfish smell. That’s particularly interesting, says Díaz-Gil, because it shows “the great plasticity and adaptability fish have without jeopardizing functional traits.”

Not only that, but after another month, Díaz-Gil filtered the water in the seabream tank so that no more black scorpionfish odor existed. Soon after, the seabream returned to their original shape, raising questions about how long a cue has to be around before the change is permanent.

It’s important to consider that not every fish, even within a species, responds to predators in the same way, says Brönmark. “Some express this defense more than others,” he says. “Fish are individuals, they’re not just a group. All of them behave differently to predation,” whether they’re crucian carp in a pond, or gilthead seabream flashing their foreheads in a tank.

Correction: The predatory fish in this study was black scorpionfish, not pike.