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.