Small deep-water marine animals communicate through sound indecipherable to the human ear while traveling up and down from the depths of ocean to feed and protect themselves, a new study has found.
Researchers using sensitive acoustic instruments have found that there is a distinct low-frequency hum associated with the daily upward and downward journeys of these animals, including fish, shrimp and squid.
The study suggests that these deep-water organisms, who inhabit ocean's mesopelagic zone - the part where water level is 200 to 1000 metres below the surface - could be communicating by listening and responding to environmental sounds.
Mesopelagic zone is very dark and without abundant food available to these organisms which together weigh about 10 billion tonnes and are a major link in the food chain between microscopic plankton and top predators like tuna, birds and marine mammals, said Simone Baumann-Pickering from University of California.
Due to their combined mass, these animals also play a major role in the global cycling of carbon from the atmosphere to the seafloor, Simone said.
At dusk, many of these deep-water animals migrate up to the nutrient-abundant surface waters to feed, relying on the darkness to protect them from predators. At dawn, they sink back down to the dark mesopelagic zone for protection.
The communal sound is three to six decibels louder than the background noise of the ocean, making it difficult for the human ear to distinguish.
Using acoustics to monitor these organisms could also help scientists study how these animals could be affected by climate change, and the consequences of potential commercial fishing projects, Simone said.
The new research may also give scientists insight into predators who feed on the animals that live in the mesopelagic zone.
It is well-known that dolphins, whales and other marine mammals use sound to communicate underwater, but acoustic communication among smaller animals, like those living in the mesopelagic area, is more difficult to hear, Simone said.
The sound could be a signal for the mesopelagic zone organisms to start migrating up to the surface or back down to the darker depths of the ocean, she said.
If mesopelagic animals convey information through sound, learning more about who is communicating and what they are communicating about could change scientists' understanding of how the ecosystem fits together, Simone added.
The sound the organisms emit is likely only detectable a few hundred meters to a few kilometres away, but that may be enough to signal predators listening nearby that their prey is on the move.
