When marine researchers began recording sounds in the seagrass beds of the Mediterranean Sea, they picked up a mysterious sound, like the croaking of a frog, echoing through the dense foliage and nowhere else.
“We recorded over 30 herbaria and it was still there and no one knew what species was producing this kwa! kwa! kwa!” said Lucia Di Iorio, ecoacoustics researcher at CEFREM of France.
“It took us three years to discover the species that produced this sound.”
The melodious songs of whales may be familiar music from the world’s underwater habitats, but few will have heard the raspy growl of a ridged gurnard or the rhythmic drumbeat of a red piranha.
Scientists are now calling for these sounds and many thousands more to become more widely available.
They say a global database of sea booms, whistles and chatter will help monitor the diversity of aquatic life and put a name to mysterious sounds like the one Di Iorio and his colleagues have investigated.
Experts from nine countries are working to create what they have dubbed the Global Library of Underwater Biological Sounds or “GLUBS”.
This would bring together records held around the world and open them up to artificial intelligence learning and mobile phone apps used by citizen scientists.
While experts have been listening to underwater life for decades, the team behind GLUBS says audio collections tend to narrowly focus on a specific species or geographic area.
Their initiative is part of a burgeoning work on marine ‘soundscapes’ – the collection of all sounds in a particular area to discern information about species types, their behavior and overall biological diversity.
Scientists say these soundscapes are a non-invasive way to “spy” on underwater life.
In a paper recently published in the journal Frontiers in Ecology and Evolution, the GLUBS team said that many fish and aquatic invertebrates are primarily nocturnal or hard to find, so acoustic monitoring could help conservation efforts.
“With biodiversity declining worldwide and humans relentlessly modifying underwater soundscapes, there is a need to document, quantify and understand the sources of underwater animal sounds before they potentially become extinct. said lead author Miles Parsons of the Australian Institute of Marine Science.
Scientists believe that all 126 species of marine mammals make sounds, as do at least 100 aquatic invertebrates and some 1,000 species of fish.
Sounds can convey a wide range of messages, acting as a defense mechanism, to warn others of danger, in the context of mating and reproduction or simply be the passive noise of an animal chewing a meal.
Di Iorio, co-author of the GLUBS paper, said that while marine mammals, like humans, learn their communication language, the sounds made by invertebrates and fish are “just their anatomy”.
Many fish produce a distinctive drumming sound using a muscle that contracts around their swim bladder.
“This dum-dum-dum-dum-dum, the frequency, rhythm and number of pulses vary from species to species. It’s very specific,” Di Iorio told AFP.
“It’s like a barcode.”
Scientists can recognize families of fish based on these sounds alone. So, with a global library, they might be able to compare, for example, the vibrant calls of different groupers in the Mediterranean to those off the coast of Florida.
But another key use for the library, they say, could be to help identify the many unknown sounds in the world’s seas and freshwater habitats.
After several months of investigating the strange seagrass croaker, Di Iorio and his colleagues were able to point the finger at the scorpionfish.
But they struggled to explain how he was making such an unusual noise and he refused to play for them.
They tried to catch the fish and save it in a carrier. They sank sound equipment on the seabed next to the fish. They even listened to aquariums containing scorpionfish.
“Nothing,” she said.
Eventually, Belgian colleagues took a camera capable of recording in low light and staked out seagrass beds in Corsica.
They were able to capture the kwa! kwa! the sound as well as the video of the fish doing a shimmy movement.
Back at the lab, they dissected a scorpionfish and found it had tendons strung along its body.
Their hypothesis is that the fish contracts these muscles to produce the sound.
“It’s a guitar, an underwater guitar,” Di Iorio said.
But there are many more mysteries where it came from.
Di Iorio said that in the Mediterranean, up to 90% of the noises in a given recording could be unknown.
“Every time we put a hydrophone in water, we discover new sounds,” she added.