Scientists Used a Magic Mushroom Compound to Calm an Extremely Aggressive Fish

Scientists discovered that psilocybin can make one of nature's most aggressive fish surprisingly mellow.

The mangrove rivulus is a remarkable fish with a big attitude problem.

Put two of these fish near each other, and things can get ugly fast. They square off, size each other up, and launch sudden, aggressive charges. Despite measuring barely two centimeters long, they’re notoriously territorial and always seem ready to throw hands—or fins.

So scientists in Canada decided to try something rather unconventional. They gave these angry little fish a bit of psilocybin, the “magic” ingredient in magic mushrooms.

And something remarkable happened: the fish chilled out. They stopped charging at their rivals so frequently, became less active, and generally seemed much less interested in starting trouble.

Beyond the amusing spectacle of fish apparently mellowing out on psychedelics, the experiment raises an intriguing question: could studying these tiny troublemakers reveal something about how drugs influence aggression in the human brain?

The Extraordinary “Angerfish”

K. marmoratus is an extraordinary creature even without the psychedelics.

Native to mangrove forests around Florida, the Caribbean, and parts of Central and South America, it can survive outside water for weeks. Under suitable moist conditions, it has survived for at least 66 days on land. It breathes through its skin, hides inside rotting logs, and can even propel itself across muddy ground by flipping its body.

But its strangest trick is reproductive.

×

Get smarter every day...

Stay ahead with ZME Science and subscribe.

Daily Newsletter
The science you need to know, every weekday.

Weekly Newsletter
A week in science, all in one place. Sends every Sunday.
No spam, ever. Unsubscribe anytime. Review our Privacy Policy.

Thank you! One more thing...

Please check your inbox and confirm your subscription.

Unlike almost every other vertebrate, the mangrove rivulus can fertilize its own eggs. Most individuals are hermaphrodites, possessing both male and female reproductive tissues. They can produce offspring without ever encountering a mate. In fact, it is one of only two known vertebrate species capable of self-fertilization—the other being Kryptolebias hermaphroditus, a close relative.

Some individuals develop into males, allowing occasional mating between fish and introducing new genetic combinations. But many generations of self-fertilization can produce remarkably uniform family lines, with individuals sharing almost identical genetic makeups.

For scientists, that’s extremely useful. It allows them to investigate how fish respond to drugs without having to account for as much genetic variation between individuals.

Combine this genetic uniformity with the fish’s notoriously aggressive temperament, and you have an unusually useful laboratory animal for investigating the biology of aggression.

Take It Easy

The researchers tested 32 pairs of fish, dividing the experimental subjects into a psilocybin group and a control group, with 16 fish in each. Each fish faced a similarly sized rival in a tank, separated by a mesh partition. They could see and smell each other, but couldn’t physically fight.

The scientists recorded their behavior for 15 minutes, establishing how aggressively each fish normally responded to its opponent.

The results were striking.

Fish exposed to psilocybin launched significantly fewer aggressive swimming bursts toward their rivals. These rapid darts are the aquatic equivalent of suddenly lunging at an opponent.

RelatedPosts

Technique Images Brain Activity When We Think Of Others
Scientists Found Your Eyes Blink In Sync With Your Favorite Songs
Fish-like robot might reveal the secret life of ocean wildlife
Neuralink Patient Went from Brain Surgery to Playing World of Warcraft Solely With His Mind in Just 80 days

But the drug didn’t affect every behavior in the same way. The fish still performed less strenuous displays, such as facing their rivals head-on. These displays became less frequent on the second day in both groups, and psilocybin produced no statistically significant additional reduction.

“Psilocybin’s calming effect appears to selectively reduce energetically costly, escalated behaviors while lower-energy social display behaviors remained largely unchanged,” said first author Dayna Forsyth in a statement.

There was, however, an important complication. The fish also moved considerably less overall. That leaves open whether psilocybin specifically reduced their willingness to fight or simply made energetic behavior less likely.

In other words, were the fish feeling less aggressive, or were they simply less inclined to move? The experiment couldn’t fully separate the two possibilities.

Still, the findings suggest that psychedelics can alter aggressive encounters in ways that aren’t limited to completely suppressing social behavior.

What About the Human Brain?

Psilocybin is attracting growing scientific interest for its possible therapeutic effects on psychiatric conditions, particularly depression.

The compound works through the brain’s serotonin system, which helps regulate mood, stress, and social behavior. Humans and fish share elements of this ancient chemical signaling machinery, making fish useful for investigating how psychoactive drugs affect the nervous system.

Previous experiments have produced intriguing results. A 2024 study in Molecular Psychiatry found that psilocybin increased exploration and reversed certain stress-related behaviors in zebrafish. Earlier research also suggested that psychedelic compounds could reduce aggression in rodents.

The new findings add another piece to this puzzle by suggesting that some aspects of aggressive behavior may be more sensitive to psilocybin than others.

But there’s a long way to go before anyone can draw conclusions about aggression in humans. This was a small experiment involving a single, brief exposure. The researchers didn’t establish how long the effects lasted, which serotonin receptors were involved, or whether reduced movement could account for much of the apparent calming effect.

Understanding why could eventually help researchers investigate the brain mechanisms underlying pathological aggression and whether psychedelic compounds have therapeutic potential beyond their better-studied effects on mood.

But that possibility remains speculative. The researchers tested only a single, brief exposure in fish. They did not establish how long the effects persisted or which serotonin receptors were responsible. And the results offer no evidence that psilocybin treats aggression in humans.

“Future studies can build on this work to explore how psilocybin alters neural signaling, which serotonin pathways are involved, and why some aspects of social behavior are affected while others are not,” said senior author Suzie Currie in a statement. “These are questions that are difficult or impossible to answer directly in humans.”

For now, the experiment demonstrates something both amusing and scientifically intriguing: a compound from magic mushrooms can make one of nature’s most combative little fish significantly less likely to launch an attack. Whether that’s because the fish became more peaceful, less active, or some combination of both is a question for future research.

The study published in the journal Frontiers in Behavioral Neuroscience.