Neuromasts help fish sense when a predator is approaching, even if the water is muddy or the fish can’t see. (PICHI CHUANG/REUTERS)

Have you ever noticed how difficult it is to touch a fish in the water? Maybe you’ve stood on the beach with a school of fish swimming around you and attempted to grab one. Or perhaps you’ve been lucky enough to go snorkeling in a place such as Hawaii’s partially submerged Molikini crater, so densely packed with marine life that you feel you can barely turn around without smacking a fish across the face. And yet, you never do. The fish will let you get close, even reach out to touch them. But the instant before your finger can make contact, they dart off. “Only teasing,” they must be thinking. How do they do that?

“Fish have what some people call a ‘sixth sense’ or ‘touch at a distance,’ ” says Allen Mensinger, who studies how fish sense and respond to cues at the University of Minnesota at Duluth and the Marine Biological Laboratory in Woods Hole, Mass. “They can detect mechanical vibrations in the water.”

Here’s how it works:

Fish have sensory organs known as neuromasts located both on their skin and in channels just beneath the skin. When water passes over the neuromast, it pushes hairlike cells that are surrounded by a gel called cupula. The cupula detects that the hair has been displaced and sends a signal to the brain to indicate that something is moving in the water nearby.

“It’s very similar to the system in the human ear,” says Matthew McHenry, who studies animal sensation and movement at the University of California at Irvine. “We have cilia in our ear that are displaced by waves, sending signals to the brain to help with hearing and balance. In invertebrates, the cilia appear on the skin.”

Those are the basics, but there are still several mysteries as to how fish make this system useful. Water is always passing over a fish’s skin, and not always because of a predator or prey. It could be because of currents, detritus or even the movement of the fish itself.

Researchers have some interesting theories about how fish manage to distinguish the good currents from the bad currents. For example, the fish’s brain may suppress its response while swimming, because it knows that water flow is likely to be intense during that time. In experiments, a swimming fish is half as likely to respond to the presence of a predator than one at rest, and some fish stay very still when trying to detect prey. (Note to snorkelers who want to touch a fish: Go for the ones on the move.)

Some species use the currents created during swimming to their advantage. As Mexican blind cave fish swim through darkness, they detect their own waves bouncing off underwater walls around them. The sonarlike system allows them to create a mental map of the environment, keeping them away from stationary hazards.

Fish also integrate visual clues with the information they receive from their sensory system. Some fish can see predators many yards away, which helps them identify what’s causing disruptions in the water as the threat approaches. But sight can’t help all of them. Many fish inhabit muddy waters and have terrible vision, yet they usually avoid predators. In McHenry’s experiments, zebra fish working in complete darkness manage to detect and evade predators three-quarters of the time. The marine toadfish that live in Mensinger’s lab are similarly effective in darkness, snapping up prey in around 75 percent of their strikes, which suggests that they can detect not only the presence of prey but also its precise location.

Some researchers think that, using their “sixth sense” alone, fish can construct a fairly detailed picture of what’s approaching.

Although neuromasts are located in many places along the body, in many fish they form a straight line down the side in what is called a lateral line. “The lateral line is a sensor array, with neuromasts located in many places along the body. Fish may know that predators or prey create a characteristic wave frequency along the array, allowing them to distinguish different types of flow changes,” McHenry says.

Although the lateral line seems borderline magical to humans, it has serious limitations.

“It’s a short-range system,” Mensinger notes. “The conventional wisdom is that fish can only use the lateral line to detect objects two body lengths away, but it’s probably even less than that.”

That partially explains why fish seem to wait until the last minute to flee from divers. The lateral line kicks in only as an object gets very close, triggering a fish’s escape response. But there’s also an element of exposure. Fish in heavily snorkeled areas are more accustomed to humans and seem more comfortable with them than fish in more remote areas. Researchers in Australia recently demonstrated a related phenomenon in a study that showed that fish living in marine reserves are easier to catch by divers using spears than their street-smarter wild cousins, because they’ll let humans approach.