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Lateral Line: A “Sixth Sense” for Fish (And Other Cool Tricks)

David Coppedge
Photo credit: Pogrebnoj-Alexandroff, CC BY-SA 3.0 , via Wikimedia Commons.

Have you ever heard of the “lateral line”? No, it’s not a football play. It’s a sense organ shared by most fish, from sharks to salmon to goldfish.

From Gills to Tail

The lateral line has been called a “sixth sense” for fish. It runs from gills to tail along the sides of the fish, right in the middle. You can see it when you catch a trout. Or in the case of a goldfish, it’s visible in the photo above. A page from the University of Minnesota Sea Grant describes this amazing organ:

Sometimes referred to as the “sense of distant touch,” lateral lines convert subtle changes in water pressure into electrical pulses similar to the way our inner ear responds to sound waves. Running lengthwise down each side of the body and over the head, these pressure-sensing organs help their owners avoid collisions, participate in schooling behavior, orient to water currents, elude predators, and detect prey.

Lateral lines are composed of neuromasts (hair cells surrounded by a protruding jelly-like cup) that usually lie at the bottom of a visible pit or groove. These hair cells — the same sensory cells found in all vertebrate ears — convert mechanical energy into electrical energy when moved. Presumably, auditory and lateral line pathways evolved in close association since they share many features. [Emphasis added.]

You can shrug off that fish story about how it evolved. What’s remarkable is that this organ constitutes an analog-to-digital converter, as pressure waves (analog) are converted to electrical signals. Actually, we all have that capability in our skin, as Science reports — a fact that has inspired Stanford engineers to create “electronic skin”:

Human skin relies on cutaneous receptors that output digital signals for tactile sensingin which the intensity of stimulation is converted to a series of voltage pulses. We present a power-efficient skin-inspired mechanoreceptor with a flexible organic transistor circuit that transduces pressure into digital frequency signals directly.

Back to fish and the lateral line. News from the University of Florida showed two researchers using lasers to try to understand how the lateral line works. One of them, Dr. James Liao, calls it a “hydrodynamic antenna” that is “configured to receive flow signals,” according to Physical Review Letters. A diagram at Wikipedia shows how many components are involved in this complex sensory apparatus.

Swim Muscles

Another little-known fact about fish is how their muscles are arranged. Have you ever cooked salmon and wondered about those stripes in the muscle? Those are called myomeres. See the good diagrams and photos on Earthlife.net, which points out that “Gram for gram fish have more muscle than any other vertebrate, a male salmon or tuna can be nearly 70 percent muscle, which is one reason why fish are so good to eat.” In cross section, the myomeres (also called myotomes) have a 3-dimensional “W” shape, and they fit together like puzzle pieces (see an illustration from Moorpark College). This arrangement allows waves of contraction to propagate on one side and then the other, causing the familiar back-and-forth swimming motion (explained at MarineBiology.org). It works pretty well because tuna can swim up to fifty miles per hour!

Fish News

Speaking of muscle, the Australian Research Council asks, “Are fish the greatest athletes on the planet?” This article is willing to pit a salmon against sprinter Usain Bolt. Here’s why:

It turns out that fish are far more effective at delivering oxygen throughout their body than almost any other animal, giving them the athletic edge over other species.

“Fish exploit a mechanism that is up to 50-times more effective in releasing oxygen to their tissues than that found in humans,” says study lead author, Dr Jodie Rummer from the ARC Centre of Excellence for Coral Reef Studies at James Cook University.

“This is because their haemoglobin, the protein in blood that transports oxygen, is more sensitive to changes in pH than ours and more than the haemoglobins in other animals.”

It’s hard to top that. But here’s another fish trick to make humans envious: they can regrow their teeth. Scientists at Georgia Tech have studied fish in Lake Malawi to learn how they do it in hopes that, one day, humans will learn how to grow true teeth instead of getting false teeth. The right kind of signals might “coax the epithelium to form one type of structure or the other.”

How about the fish that can stun a horse with electricity? A paper in Nature Communications reported that electric eels not only use their powers to stun prey by making their muscles go into uncontrollable spasms; they also have the ability to locate the prey with their electrical sense. 

Electric eels (Electrophorus electricus) are legendary for their ability to incapacitate fish, humans, and horses with hundreds of volts of electricity. The function of this output as a weapon has been obvious for centuries but its potential role for electroreception has been overlooked. Here it is shown that electric eels use high-voltage simultaneously as a weapon and for precise and rapid electrolocation of fast-moving prey and conductors. Their speed, accuracy, and high-frequency pulse rate are reminiscent of bats using a ‘terminal feeding buzz’ to track insects.

The author, Kenneth C. Catania from Vanderbilt, remarks that electric eels are “far more sophisticated than previously described.” The summary of this paper on PhysOrg explains that the dual processing allows the eel to find its prey after it stuns it in the murky Amazon waters where it lives. Video clips in this article allow you to see the eel react in real time and in slow motion in Catania’s lab setup. It’s a good thing he slowed it down or you might miss how fast the eel responds if you blink.

Conservation

Let’s end with news about conservation. The bad news is that Coho salmon are being poisoned by urban runoff. The good news from NOAA fisheries in Seattle is that they found a simple filtration system that can eliminate the toxins and save the fish. A three-foot filter with sand, gravel, compost, and bark is very effective at cleaning the water. “It’s remarkable that we could take runoff that killed all of the adult Coho in less than 24 hours — sometimes less than four hours — and render it non-toxic, even after putting several storms worth of water through the same soil mixture.”

The Aquarium of the Pacific in Long Beach, California, has an exhibit illustrating the plight of Steelhead, large salmonid fish related to rainbow trout. Pollution, drought, and physical barriers like dams and concrete channels have drastically reduced the numbers of these fish that, like northern salmon, swim from the ocean up freshwater rivers to spawn. The exhibit raises awareness that in humans’ efforts to improve their own lives with dams and storm channels, they may be causing devastating effects on fish and other animals. 

Sockeye salmon of the Fraser River, among the heroes of the Illustra documentary Living Waters, are not doing as well as predicted, according to the Seattle Times (see details in the Washington Department of Fish and Wildlife report). It’s only relatively recently that humans have been making efforts to help the fish get around our dams with fish ladders. One of the most innovative technologies, Live Science reports, is the new Fish Cannon from Whooshh Innovations. It can launch a large salmon over a 100′ dam through a flexible tube. Need some inspiration? Watch comedian John Oliver describe it on YouTube and put it to other practical uses:

This article was originally published in 2015.