What great . . . ears you have
finds that the ocean’s most fearsome predator has a highly tuned ability to match sounds to locations
Sharks are incredible creatures for a seemingly endless list of reasons. They can hear prey nearly a kilometre away — and their razorsharp sense of smell can detect the equivalent of a teaspoon of blood in a swimming pool.
But there’s a vast amount that scientists have yet to learn about them — and what they discover could change the way we think about entire marine ecosystems.
One team of New Zealand researchers are focusing on one remarkable function that remains largely a mystery to science: how do sharks, and fish more generally, match sound to location?
University of Auckland marine scientist Dr Craig Radford and his group have led the world in showing how reef fish larvae can use sound to guide themselves and settle on to reefs.
Better understanding this sound also revealed how reef sound varied with time of day, seasons and even the lunar cycle.
“Furthermore we have shown . . . reef sound can propagate the distances offshore where the animals are returning from,” Radford said.
“But even though we have shown that these animals are able to localise sound, we are not 100 per cent sure how they do it.”
The primary part of the sound stimulus that all fish detect is the particle motion — and they can only detect the sound pressure with some sort of ancillary hearing structure, such as a gas bladder.
While there were several notions about how fish could home in on particle motion as well as sound pressure, current theories about how they could localise sound using particle motion alone were limited.
As it happened, sharks offered an excellent model organism for cracking the puzzle, as they didn’t have any such mechanism for detecting sound pressure.
They also inhabited a range of marine environments, from deep ocean to shallow coastal waters.
With collaborator Professor Peter Rogers from Georgia Tech in the United States, Radford’s team had come up with an alternative model they’d dubbed Time-Integrated Intensity Vector, or TIV.
“Basically . . . we think sharks are able to localise sound using the vertical particle motion that is generated close to the sea surface instead of sound pressure.”
Their new study, supported with a $935,000 grant from the Marsden Fund, had three key goals.
First, they aimed to experimentally characterise the vertical structure of the entire underwater sound field.
Next, they would attempt to describe the structure of the ears of a range of shark species.
Radford expected to find that pelagic sharks, which roamed the open waters of oceans, would be more capable of sound localisation as they spent more time closer to the surface than those benthic species that lived near the sea floor.
Lastly, they would test the directional hearing capacities of these species as a function of distance from the sea surface.
“We have recently purchased a sensor that is capable of detecting particle motion,” Radford said.
“These sensors have US military embargoes, so we are lucky . . . they granted us permission to purchase and use these outside of the US.
“Using these, we will measure the vertical structure of the sound field at different depths and distances from the reef.”
To understand shark hearing anatomy, meanwhile, they’d use MRI technology in collaboration with Dr Kara Yopak, Uni North Carolina Wilmington.
“We will scan a range of species [and look at] what variation exists in terms of the structure of the ears and the number of hair cells present.”
And to achieve their third goal, they’d use sophisticated physiological techniques to reveal their hearing and localisation abilities.
They’d do this partly in a lab — and partly on the water, which would pose their biggest challenge.
“This is relying on the sharks to co-operate and as we will be using a cardiac condition paradigm to train the sharks to specific sounds in tanks — and if we can do this, we can then take them out into the field and use a speaker area to test their localisation ability.”
Radford ultimately hoped the study would tell us, for the first time, how fish lacking a mechanism to detect sound pressure could still localise sound.
“I am sure I can think of real world applications for this, like development of technology based on shark ears, but in reality this will have more scholarly impact,” he said.
“We will also be the first to record the particle motion of reef noise with our new sensors outside of the US.”
He said the project’s techniques might also be widely used on other marine animals.
I am sure I can think of real world applications for this, like development of technology based on shark ears.
Dr Craig Radford