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 researchers were looking at fish that lived in waters as deep as 1.6 miles. What they found was interesting:
The inner ear (pictured) of a blue antimora (inset) is exceptionally rigid—a feature never before seen in a deep-sea fish, according to a study to be presented in May 2009.
Photograph courtesy Xiaohong Deng et al.; inset photograph courtesy NOAA
A couple of things stand out in the National Geographic article. First, the ears were compared to those of surface dwelling fish and were found to have long rods attached to their ototliths:
"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.
Second, a second species had rigid inner ears similar to that found in bluefin tuna. The close proximity of the swim bladder - an organ that controls buoyancy and amplifies sound - strengthens the suggestion. What this says to me is that evolution has found multiple ways of dealing with darkness. It also indicates that the affects of selection on the swim bladder, or for better hearing, are likely to have unintended consequences...