The deep-sea blue antimora has thick, rigid parts of the ear that act as sounding boards.
Credit: NOAA
OREGON, U.S.: Some deep-sea fish have evolved unique, specialised ear structures that allow them to 'hear' the environment around them rather than see it.
"We think that hearing is potentially very important for deep-sea fish because light is very scarce and sound travels great distances," said Xiaohong Deng, a neuroscientist at the University of Maryland in College Park, USA.
In addition, in the depths there is less interference from background noises such as surface waves, so that fish can hear more clearly. Good hearing, for them, is like a second sight, said Deng. "All this information can form an auditory scene [for the fish]."
Just as some species of owls are able to locate prey in complete darkness by their hearing alone, some deep-sea fish may have developed sharper hearing as a result of their evolution in dark ocean depths.
Specialised ear structures
Deng presented early results from comparative studies of the ears of deep-sea and shallow-water fish at a meeting of the Acoustical Society of America in Portland, Oregon, on Tuesday.
Many deep-sea species, she said, appear to have specialised ear structures never before seen in other types of fish.
Parts of the ear, as seen in a type of deep-sea cod known as the blue antimora (Antimora rostrata), for example, are very thick and rigid. "We think this would help the ear to pick up limited vibration from the swim bladder better than a softer ear," she said.
Also unique to deep-sea fish are the shapes of structures known as 'otoliths', seen in various ridgehead species. Otolith literally means 'ear stone'. They may help the fish to hear sounds that would otherwise be undetectable.
New shapes of internal ear pieces
"These are internal pieces of the fish's ear that allow it to hear," said David Zeddies, a scientist at consulting firm Marine Acoustics Inc., in Arlington, Virginia. "I can't tell you that we know exactly what [the structures] are doing, but we know that they're important for hearing."
One of the big questions, according to Zeddies, is how the otolith's shape affects its function. "Is the shape important or not? At this point, we really don't know," he said, noting that many deep-sea fish appear to have specialised otolith shapes "that may, in fact, mean something."
Scientists acknowledge that very little is known about the deep sea and that it is one of the few remaining unexplored frontiers. Deep-sea ecosystems are some of the most fragile on the planet, said Zeddies. "This is one of the few studies where we can start to get some understanding of what's going on down there."
