[♪ INTRO] You’re probably familiar with pufferfish, who can fill their bodies with water to make themselves appear larger to predators.

And while these fish are probably some of the fastest at this, they definitely aren’t the only ones. Researchers have found another fish that bulks up when predators lurk, the Gilthead sea bream.

And not only that, but they can also return back to their original size, kind of like a fish Hulk, if you will. Gilthead sea breams are silver-colored fish frequently found in the Mediterranean and Eastern North Atlantic oceans. And researchers have recently found that these fish can change their body shape when they sense chemicals from nearby predators, say, for example, their pee.

Now, these chemicals don’t cause an instant change, like what happens to Bruce Banner. Instead, it’s a more gradual change. If they’re around these substances, they take about a month to reach a larger size.

Their bodies become deeper. The area from the bottom of their belly to the top of their back gets bigger, which makes it harder for a predator to eat. And sea bream aren’t the only ones.

Researchers have known for a while that other fish species, like the freshwater Crucian carp, can beef up when predators are near. So, it makes sense that the sea bream could have this ability, too, although it hadn’t been observed in a marine species before. And that’s because it’s pretty hard to measure how fish bulk up in the open ocean, because, well, the ocean is huge and monitoring individual fish is nearly impossible.

So the authors of this 2020 study watched how these fish bulk up in a more controlled environment: their labs. They exposed juvenile sea bream to the chemical cues from a common predator, the black scorpionfish. The researchers did this by extracting water from tanks that contained scorpionfish that had been feeding on gilthead.

Then, they mixed it with some skin from dead gilthead and froze it into cubes. The ice cubes were then put into the sea bream’s tanks. Over time, researchers observed that the fish changed their bodies to become larger and deeper.

Now, some fish can’t bulk up forever because growing bigger can be energetically costly. However, the Gilthead sea bream in this study appeared to be unfazed by the extra energy it takes to grow bigger. And when researchers filtered the tank water so that all trace of the scorpionfish was gone, the sea bream returned to their pre-Hulk size in about a month.

While the sea bream isn’t the only fish on the planet with the ability to become a pseudo Hulk, it’s one of the first marine species to be recorded. Scientists are still researching how fitness is affected for individual fish, but they think it might impact on commercial stock levels of this species. But if they’re given time to bulk up before they get released, juveniles of this species could have the chance to evade predation.

But more research is needed to understand how long these fish species can stay in their Hulk state and the large-scale impacts of that behavior. I mean, who knows what other superhero-like fish there are lurking around in the sea. [Outro] Thanks for watching this episode of SciShow! If you would like to help support the channel, check out Patreon.com/SciShow to learn more. [♪ OUTRO]