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1 Sound travels very differently in water than air.
Like most humans, I assumed that sound didn’t work well in water. After all, Jacques Cousteau himself called the ocean the “silent world.” I thought, beyond whales, aquatic animals must not use sound much.
How wonderfully wrong I was.
In water a sound wave travels four and a half times faster, and loses less energy, than in air. It moves farther and faster and carries information better.
In the ocean, water exists in layers and swirling masses of slightly different densities, depending on depth, temperature, and saltiness. The physics-astute reader will know that the density of the medium in which sound travels influences its speed. So, as sound waves spread through the sea, their speed changes, causing complex reflection or refraction and bending of the sound waves into “ducts” and “channels.” Under the right circumstances, these ducts and channels can carry sound waves hundreds and even thousands of kilometers.
What about other sensory phenomena? Touch and taste work about the same in water as in air. But the chemicals that tend to carry scent move slower in water than in air. And water absorbs light very easily, greatly diminishing visibility. Even away from murky coastal waters, in the clearest seas, light vanishes below several hundred meters and visibility below several dozen.
So sound is often the best, if not only, way for ocean and freshwater creatures to signal friends, detect enemies, and monitor the world underwater. And there is much to monitor: Earthquakes, mudslides, and volcanic activity rumble through the oceans, beyond a human’s hearing range. Ice cracks, booms, and scrapes the seafloor. Waves hiss and roar. Raindrops plink. If you listen carefully, you can tell wind speed, rainfall, even drop size, by listening to the ocean as a storm passes. Even snowfall makes a sound.
Not only has the ocean never been silent, but within its depths, sound may in fact be the sense of choice.
2 There are many (many) more ways to listen than with an ear.
If an animal detects sound, it must have an ear, right? Well, no. Far back in evolutionary history, and certainly in the water, early animals evolved something called a hair cell. Hair cells can get quite complex but, put simply, they’re structures with a tiny “hair” or bundle of hairs which, when physically bent, send a signal down an attached neuron.
Hair cells feature prominently in the inner ears of mammals—including whales—specifically, in the spiral-shaped cochlea, where the tiny vibrations of a sound wave are translated into the nerve impulses that provide our brains with acoustic data. But hair cells show up in other places and organs in marine creatures, too. To name just a few: Fish have them along the lateral line that runs the length of their bodies. Many invertebrates such as squid have hair cells in organs called statocysts, which they use for balance and orientation. Crustacean antennae also have hair cells, as do mollusk abdominal sense organs.
Sound is often the best way for marine creatures to signal friends, detect enemies, and monitor the world.
Wherever “hairs” can be bent, there’s the possibility of detecting sound. Sound is a pressure wave, after all, and the molecules of water pushed by the sound wave can bend that tiny hair. That’s how many earless animals detect sounds.
Of course, fish also have ears (which, for sticklers, is an organ specifically to detect sound). In fact, fish evolved the first ears, but they’re a bit different from those of mammals. They don’t have an outer or middle ear, or a cochlea.
We’re still learning the implications of this. For example, many invertebrates and fish sense the changes in motion of particles created by a passing sound wave, not changes in pressure, the way human ears and most hydrophones do. Particle motion and pressure can sometimes uncouple, such as when sound waves interfere with one another in confined tanks or shallow water, the very places we often study these animals. Scientists are realizing we may have been measuring aspects of sound these animals don’t sense, and missing some aspects they do.
3 Noise is noxious to ocean critters because it silences other critical sounds.
When I began this project, I admit I was uncertain about how sound harms animals. Oil spills and warming waters, those impacts I could intuit. But noise? Except for extremely loud noises that burst sensitive structures, there was a disconnect when I tried to conceive of the dangers. But the more I learned how fundamentally sea life relies on sound, the more I could fathom the risks.
“Noise” is a technical term. It’s an unwanted sound that interferes with a signal. A noise doesn’t have to physically harm an animal directly, just obscure that signal. Noise can, for example, shrink the distance at which an animal can sense its world. It can mask the sound of an approaching predator until it’s too late, or conceal the telltale signals that critical nourishing prey is nearby. It can prevent amorous fish calls from reaching potential mates. Chronic shipping noise may harm or even break invertebrates’ hair cells, leaving them not only deafened but numb or disoriented.
Lacking arms to hold their babies, many whale mothers use sound to help keep their dependent newborn calves close. If noise masks their quiet contact calls, they can get separated with no way to find each other again.
Once we begin to ask how sound works underwater, and the myriad, incredible ways it mediates even the smallest lives, we hear just how much of the world eludes our human senses—and just how profoundly we can trespass unaware. 
Lead image: Titima Ongkantong / Shutterstock
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