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Why it's so hard to grab a live fish

Sensory organs on and under their skin work to detect mechanical vibrations in water

Washington PostJanuary 6, 2013 

In McHenrya¢,Ǩ,Ñ¢s experiments, zebra fish working in complete darkness manage to detect and evade predators three-quarters of the time. (Photo by Azul)


Ever noticed how difficult it is to touch a fish in the water? 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. 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.

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. 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.

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