for National Geographic News
Examining several fish species collected from waters as deep as 1.6 miles (2.5 kilometers), researchers discovered ear structures never seen before in other fish.
The strange structures may enhance the animals' hearing, researchers speculate.
Can't Stand the (Lack of) Pressure
Roughly 90 percent of the ocean is completely dark, beyond the reach of the sun's rays.
The researchers wondered if the fish had evolved to have sharper hearing, which might help them catch prey, find mates, or elude predators in the darkness.
The question might not be as easy to answer as it seems.
With current technology, it's impossible to give deep-sea fish hearing tests—the surface world's much lower pressure kills the animals shortly after they're hauled up.
Instead, researchers compared the deep-sea fish's ear structures with those of surface-dwelling fish and found entirely unique adaptations in the creatures of the dark deep.
For instance, the ears of a number of Pacific ridgehead species—collected from waters as deep as 3,300 feet (1,000 meters) off Central America—have long, rigid, calcium-carbonate stalks projecting from hard "stones" within the ear known as otoliths.
"We do not quite know what [the stalks'] function is yet," said study co-author Xiaohong Deng, a marine biologist at the University of Maryland.
But Deng suspects these stalks allow the otoliths to register sounds that would otherwise be too faint to hear.
The ridgehead ears' also had very long hair bundles—features also seen in the ears of shallow-water species known to have great hearing, such as the pinecone soldierfish and clown knife fish. The bundles may enhance the fish's sensitivity to sound and to their own head motion, which would improve the fish equivalent of balance, Deng said.
Next: Deep-Sea Hearing Tests?
Another fish, the blue antimora—a type of cod gathered from 8,200 feet (2,500 meters) deep near Ireland—has rigid inner ears, the team found.
Known only in one other fish, the shallower-swimming bluefin tuna, stiff inner ears might help the antimora's ear respond to vibrations from the nearby swim bladder, a gas-filled organ that helps control buoyancy but can also serve as a sound amplifier.
Such unexpected adaptations suggest extraordinary hearing abilities, but scientists will never know for sure unless hearing tests are conducted at depth.
"Landers equipped with cameras and acoustic devices can be dropped down to the ocean and see if these fish can response to sounds," Deng said. "This is my dream. ... "
Findings to be presented May 18 at the Acoustical Society of America meeting in Portland, Oregon.
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