A Blue View: Fishes' Sixth Sense

A system of sense organs called the lateral line gives fishes an underwater advantage.

Published November 10, 2015

The lateral line is found along the head and body of fish, just under the skin. It helps to detect movement, vibrations and changes in pressure in the surrounding water. The -like structure is scattered with neuromasts—clusters of sensory cells with hair-like filaments, or cilia, that can detect mechanical changes in the water.

Water passes through the lateral line, stimulating the cilia of the neuromasts and signaling the fish of any changes in its environment. These signals can help a fish understand its own movement through the water. A variation in the water could indicate which direction a fish should school, that there is potential prey nearby or even that it’s time to perform a quick escape maneuver to flee from a predator.

Schooling

schooling-fish-sardines

The synchronicity with which some groups of fish swim can be attributed to the lateral line. By allowing them to recognize and respond to the movements of other individuals in the group, the lateral line helps fish school. A massive shoal of sardines can effortlessly navigate together. And even without sight, blind cave fish are able to school by virtue of this additional sensory system.

Robotics

A team of European scientists, part of the FILOSE research project, took note of this “sixth sense” that fish possess and are applying it to modern technology. By mimicking the lateral line, they’re working to create an energy-saving underwater robot. The FILOSE team has been testing their robotic fish in a flow tank to see just what this technology can do.

Like a fish, the robot’s ability to sense changes in water pressure would allow it to evade disruptive currents that would otherwise slow its pace and navigate toward weaker currents, avoiding unnecessary energy outputs.

In addition, the man-made lateral line would enable the robot to detect the speed and direction of a current and adjust its rate accordingly, so it wouldn’t lose ground by drifting downstream. It could even take advantage of strong currents by aligning itself in a way that might help propel it in the right direction.


Episode Transcript

As every schoolchild knows, like the fingers of one hand, we humans have five senses: hearing, taste, smell, touch and sight.

But fishes go one step beyond the senses we know—for they have something called a lateral line, a sensory system that allows them to detect minute water currents like vibrations and pressure gradients. This extraordinary, highly versatile organ allows some species to send vibrational signals to mates. Others use it to detect predators or prey. And all use it to enable the exquisite water ballet of fish schooling, in which vast shoals of individual fish change direction so dramatically and uniformly that they look like one animal being pulled by an invisible string. Google “sardine school” and you’ll know exactly what I mean.

The lateral line runs down both sides of a fish's body and is easy to see. Often it’s visible as a faint line of pores along the flanks of the fish. You might also notice similar lines on an animal’s head and in small pits on the surface of its body. All are part of this remarkable sensory system.

These tiny pores allow water into an internal tube-like structure where it flows over specialized cells, called neuromasts. With hair-like protrusions connected to sensory cells, neuromasts are stimulated by a change in water flow across the fish’s body. In a millisecond, the fish responds to the possible presence of a predator by fleeing, changing direction or schooling.

And just as sight is our dominant sense, the lateral line is the "it" sense in the lives of fishes, their sixth sense, if you will.

The South American blind cave fish, a relative of the rainbow tetra—which is common to home aquarists—has one of the most sensitive lateral lines of any species. Not only blind, the blind cave fish has no eyes; it is born with eyes that quickly degenerate and are overgrown by skin tissue. Weird.

Yet, blind cave fish can school. They even have laterality—that is, they have a dominant lateral line side that they use to seek out information in an unfamiliar habitat. You could almost compare this to right- or left-handedness in humans.

There are parallels to the lateral line in other parts of the animal kingdom. For example, the tactile hairs or whiskers of manatees and harbor seals function as sensory transmission systems, too. But none are as finely tuned as that of the fishes.

Taking a page from nature, a consortium of European scientists recently began researching the idea of developing a lateral line for underwater robot submarines. Since their inceptions, such remotely operated vehicles have clunkily and noisily navigated using bright lights and sonar, disrupting everything in their path. What if such a vehicle could move easily and quietly through the deep ocean? In 2013, they created just such a robot, and it’s now undergoing testing. Stay tuned.

Over millions of years of evolution, the blind cave fish has adapted perfectly to its environment. Little surprise then that this unassuming little character may be the prototype for one of the biggest advances in the history of robotic technology.

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