What happens underwater when we can’t hear it? Meet the ocean’s secret sound world

New Delhi: Beneath the surface of oceans, rivers, lakes and ponds is a world filled with sounds that most people on land never hear. Fish snap, shrimp create explosive bursts, whales send low-frequency calls across vast distances and tiny river-dwelling insects can produce noise comparable to that heard at an orchestra concert.

Physicist Helen Czerski, presenter of BBC Radio’s Rare Earth and author of ‘Blue Machine’, which explores how the ocean influences our world, says the surface of the sea plays an important role in keeping much of this underwater soundtrack out of human ears.

She explains that the ocean surface works like a two-way mirror. Sound from the air can travel down into the water, hit the surface and travel back up, while sounds produced underwater reach the surface and are sent back down. As a result, much of the sound produced beneath the water never reaches people above it.

Humans depend on vision to understand their surroundings. However, the situation underwater is very different. Light does not travel far into the sea. Very little light reaches depths beyond about 200 metres. Sound therefore becomes an important way for marine animals to find their way, communicate and locate other animals, especially over long distances.

Professor Steve Simpson, a marine biologist at the University of Bristol in Britain, says water carries sound extremely well. Sound travels through water around four-and-a-half to five times faster than it travels through air.

Research by his team found that fish use sound to locate coral reefs before relying on other signals. Once they get closer, they can detect the smell of particular habitats and eventually see members of their own group.

The underwater world has developed an extraordinary range of ways to produce sound. Baleen whales, including blue whales and fin whales, push air through specialised structures in their bodies to create very loud and low-frequency calls. These sounds can travel enormous distances, with some whale species potentially able to communicate across the length of the Pacific or Atlantic oceans.

Toothed whales and dolphins use echolocation nearby. They produce clicks and buzzing sounds and use the returning signals to work out what is around them.

Fish have their own methods. Some vibrate their swim bladders, the air-filled organs in their bodies, while others grind their teeth or snap their jaws.

Spiny lobsters produce a rasping sound by rubbing part of their antenna against a rough surface below their eyes. Simpson compares the action to the way a fiddler plays a violin.

Male Ambon damselfish in the western Pacific have developed a particularly sharp sound to attract females to their nests. The call has some resemblance to a bird sound and uses a frequency that few other animals appear to use, helping the fish stand out in a noisy environment.

Simpson compares it to recognising the familiar laugh of a friend in a crowded party.

Underwater sounds also reveal interactions between different species. Simpson’s team recently recorded a snapper and an angelfish fighting for access to a cleaning station under a small rock.

Even animals that seem silent from above contribute to the underwater soundscape. In New Zealand, sea urchins use their shells as natural amplifiers. When large numbers feed together on rocky reefs, they produce an impressive chorus. Other animals may use these sounds to locate a reef from offshore.

One of the most unusual examples comes from snapping shrimp. The tiny crustacean closes its claw at extraordinary speed and create a bubble that collapses under pressure. The process produces an explosive sound, with temperatures in the collapsing bubble reaching around 3,000°C and a flash of light appearing at the same time. The resulting blast can stun prey.

Simpson describes the snapping shrimp’s ability as one of the most remarkable examples of a sonic weapon found in nature.

The underwater soundscape is not limited to the sea. Freshwater ponds, lakes and rivers can also be surprisingly noisy.

Dr Jack Greenhalgh, an ecologist and acoustician at McGill University in Canada, says aquatic insects can produce sounds through stridulation, a process in which two hard parts of the body are rubbed together.

Among them is the pygmy water boatman, a freshwater insect measuring only around 2mm. Greenhalgh says it is capable of producing one of the loudest sounds in the animal world relative to its body size.

The insect produces a sound of around 100 decibels by rubbing its penis against its abdomen. Greenhalgh compares that level to sitting in the front row of an orchestra concert.

Even plants and algae may contribute to the underwater soundtrack. During photosynthesis, they appear to produce sounds, although scientists are still working to understand exactly how this happens.

The cologist says oxygen bubbles released by the plants could be responsible for strange ticking sounds that have a quality similar to the rhythm of a jazz drummer.

The sounds produced by marine life form only part of the picture. Human activity has also changed what animals hear underwater.

Czerski refers to research that examined cortisol, a stress hormone, in earwax samples from baleen whales collected during the 20th century. Cortisol levels appeared to change along with the rise and fall of industrial whaling.

During the Second World War, when whaling activity fell as humans turned their attention to the war, cortisol levels in whales were still high. Czerski says the animals may have been affected by the sounds of warfare at sea, including battles, guns, bombs, destroyers, aircraft crashes and possibly sonar.

Simpson says shipping, fishing boats, motorboats and offshore oil and gas drilling create noise across different levels of the underwater environment. Such noise can affect how animals detect predators, find prey and reproduce.

His team studied the Great Barrier Reef and found that boat traffic near reefs was associated with an estimated 40% reduction in fish reproduction. When boats were moved away, the reef’s natural soundscape returned.

Researchers are now studying ways to reduce the impact of human noise. One approach involves creating curtains of air bubbles, including bubbles produced naturally by sources such as kelp, to shield marine animals from noise in places such as ports.

Scientists are also testing whether sound can help damaged reefs. Simpson’s team has used underwater speakers to play recordings made at healthy reefs.

When coral reefs suffer bleaching, many of the animals that produce sound leave or die. Simpson says this can reduce the reef’s overall sound level to about one-quarter of what it was before. The pops, clicks, rumbles, trumpet-like calls and croaks disappear, leaving occasional snapping sounds as a reminder of the life that once occupied the reef.

Playing recordings of healthy underwater reefs could help recreate that missing sound environment and attract marine life back to damaged areas.

Most people will never hear the clicks, calls and collective noise beneath the water. Scientists are learning to capture these sounds with underwater microphones and recordings to understand aquatic ecosystems and how human activity is changing them. The same sounds may also help researchers find better ways to restore marine habitats.