The next time you go for a swim, try this: Close your eyes, paddle in place, and imagine using the feeling of the water on your skin to map the shape of everything nearby—from the contours of the pool to the location of a hapless bug struggling on the surface. That’s kind of what it’s like to be a fish with a marvelous sensory apparatus known as the lateral line system.

Composed of exquisitely sensitive skin-based cells and snaking nerves, lateral lines are as integral to a fish’s perception as their sight or hearing. For a human, trying to wonder what it’s like to have one is an exercise in transcending the boundaries of our own ümwelt, or notion of our place in the world.

Doing so might also help us better understand what our gilled counterparts’ lives are like, and how to better care for them, particularly in captive environments. Waves reflecting off aquarium walls could be a source of distress for animals with such a keen sensory system, while common diseases that degrade lateral lines might deprive them of an important part of their everyday experiences.

“The lateral line basically means their entire outer body is sensitive to vibrations in the water,” says Culum Brown, a behavioral ecologist who studies fish at Australia’s Macquarie University. “[People] just don’t have anything like that. It’s very difficult to describe.”

The first scientific observations of lateral lines in fish were made in the 17th century by Dutch naturalist Nicolas Steno, who believed they helped produce the mucus that typically coats the creatures’ scales. Steno’s mistaken belief stuck around until the 1800s, when a few scientists started to suspect that lateral lines formed a so-called “sixth sense.”

At the time, German zoologist Franz Eilhard Schulze described the system’s anatomy. A fish’s body is covered in tiny structures called neuromasts, each containing a cluster of hairlike sensory cells inside a jelly-looking dome. The neuromasts are embedded on the surface of a fish’s body and in distinct, mucus-filled canals that run along the animal’s sides—giving the lateral line its name. The cells respond to changes in pressure and movement, and when stimulated, send signals through specialized nerves directly to a fish’s brain.

While researchers of the era understood the basic physiology of the system, most of them assumed it was involved in hearing, sort of like a whole-body accessory to the inner ear. Finally, after a set of experiments in 1910 showed that pike could avoid walls when blinded, but not when their lateral lines were severed, biologists started to draw the connection to water flows.

Subsequent research has illuminated how fish use lateral lines. Blind cave fish, for example, produce waves with their mouths as they swim; as the turbulence hits obstacles, it creates patterns of flow and pressure that helps them navigate. In another case, surface-feeders locate prey in complete darkness by homing in on the surface waves generated by fallen insects. Meanwhile, far below the surface, mottled sculpin pinpoint the paddling motion of aquatic fleas suspended in the water column.

Rheotaxis—the movements that allow a fish to intercept odors and food in a current without expending too much energy or being swept downstream—is now known to be mediated by the lateral line system. So is the extraordinary behavior of schools turning in perfect unison as though controlled by a single mind. Using their lateral lines, fish can also perceive the wakes of individuals who passed by minutes ago, helping them hunt (and perhaps do much more).

Depending on their environment, some species might even adapt their lateral lines to send and receive communications. Male betta fish make waves to alert offspring of approaching predators; rainbow trout perform a courtship dance akin to an elaborately synchronized caress.

Putting a fish in a home that stimulates their lateral line is “equivalent to the difference between living in a room with a view, so to speak, and living in the dark.”

—Becca Franks, cognitive psychologist

Those are just some of the functions of lateral lines. But what does the “sixth sense” feel like? “It is nearly impossible to know what fish actually perceive through their lateral line,” wrote Sheryl Coombs and Horst Bleckman, modern pioneering scientists on the topic. Meaning that while we’ve described the facts of lateral line perception, we can only try to do the same with the subjective experience.

In his attempt, Dutch physiologist Sven Dijkgraaf called it “touch at some distance,” a term that resonates with Christopher Braun, who studies fish perception at Hunter College. It’s as if “your skin is three-dimensional” so you can feel nearby objects without making direct contact, he explains. Brown from Macquarie University uses the analogy of placing your hand atop a woofer speaker while wearing noise-muffling headphones. “I think that’s just about as close as you could imagine,” he says. The main difference, of course, is that humans are “not designed to respond to that kind of information, whereas fish are highly attuned.”

There’s even less information on how lateral lines intertwine with the animals’ psychological well-being—a gap that can be chalked up to the delayed realization that fish are no less cognitively sophisticated than mammals or birds, along with a disregard for their welfare in general. With their seemingly inexpressive faces and underestimated minds, the gilled creatures don’t get much attention on their inner lives. In fact, it wasn’t long ago that most scientists thought they were incapable of pain.

Today we know better. That leads to some troubling possibilities, specifically for those animals swimming behind glass walls. For one, waves bouncing off aquariums could produce an echo-chamber effect, leaving fish pummeled by endless reverberations. “It would be like having constant loud noise,” says Mary Power, a river ecologist at the University of California, Berkeley. She even questions whether some of the abnormal behaviors she has observed in captivity—such as extreme aggression in typically easygoing fish and abnormally repetitive cleaning habits—are symptoms of discomfort from overstimulated lateral lines.

“There’s bound to be some kinds of problems,” Brown weighs in. He suggests aquarium owners add habitat-enriching structures like logs and plants to help disrupt those flows. He does speculate, however, that captive fish would eventually adapt to the stimulation, not unlike how an annoying, persistent noise can fade into the background.

John Montgomery, a fish biologist at the University of Auckland, says that if aquarium-wall reflections really are a problem, a fish’s lateral line system might simply degrade, much as how constant loudness leads to deafness in people. “Continuous overstimulation would be more likely to result in sensory loss,” Montgomery explains, “rather than continuing acute physical discomfort.”

A less-speculative problem is lateral line erosion, a collection of conditions better known, with gruesome evocativeness, as hole-in-the-head disease. Caused by parasites, poor water quality, and nutrient deficiencies—and perhaps exacerbated by the stress of captivity itself—the scourge is common in aquarium and aquaculture fish. Afflicted fish might have their lateral lines literally rot away.

The fallout doesn’t seem to cause immediate physical pain, but it raises the question of whether there is longer-term psychological distress when such an integral sense is taken away. It also raises the idea of providing captive fish with a happier, not just healthier, life.

Becca Franks, a cognitive psychologist who specializes in animal behavior and fish welfare at New York University, points to a 2019 study on how zebrafish in tanks prefer habitats with areas of flowing water. Perhaps that’s because in dynamic conditions, they can use their lateral lines, just as we respond to having our own senses engaged. “There is so much more to be done,” says Franks, but she considers it “equivalent to the difference for us between living in a room with a view, so to speak, and living in the dark.”

Of course, zebrafish preferences may not be shared by everyone. After all, there are at least 30,000 fish species swimming the world’s water bodies, each with lateral line systems suited to the environments in which they evolved. A great deal remains to be studied and learned about how fish sense their surroundings and where they feel at home.

We hope you enjoyed “Pet Psychic,” Brandon Keim’s new monthly column. Check back on PopSci+ in September for the next article.