Talk like a marine bioacoustician

Acoustic niche — the frequency range that a species has evolved to use, often unique within an ecosystem to avoid interference with other sounds 

Anthropophony — sounds generated by humans and our technology 

Bioacoustics — the study of how living organisms produce, transmit and respond to sound 

Elasmobranchs — a group of fish, including sharks, rays and skates, that have skeletons made of cartilage instead of bone 

Hydrophone — a microphone that is used underwater 

Soundscape — the collection of sounds in an environment, including both natural and human-made sounds 

As you stand waist-deep in the cold, salty surf, you listen to the sounds of the beach around you: children laughing and splashing in the waves, parents chatting on their deckchairs, seagulls squawking as they divebomb for chips. Plucking up the courage, you dive headfirst below the surface, and instantly the world becomes muted, muffled and quiet.

Your ears are designed to make sense of sound travelling through air, so, when you submerge them in water, they are of little use. If, however, you were a fish, your hearing system would have evolved to suit your underwater home. “Fish ears differ significantly from human ears in both structure and function, largely due to the contrasting physical properties of air and water,” says Dr Lucille Chapuis from La Trobe University. “Sound travels more efficiently in water, making it a vital source of information for aquatic species in marine environments, where visibility is often limited.” 

Lucille is studying the hearing systems of elasmobranchs, a group of fish that includes sharks, skates and rays. “Elasmobranchs are particularly sensitive to low-frequency sounds, and use this sensitivity to detect environmental cues relevant to their survival, such as the sounds of their prey or predators,” explains Lucille. Unfortunately, the widespread introduction of human-made noise in marine environments — generated by activities such as shipping, seismic exploration and underwater construction — can interfere with elasmobranch hearing and put these fish at risk. 

The impact of anthropophony 

“Human-made noise, or anthropophony, has become a pervasive and increasingly disruptive force in marine ecosystems, fundamentally altering the underwater soundscape that many animals rely on,” explains Lucille. “The result is often a disruption of normal behaviours such as mating, foraging and predator evasion.” This is particularly true for elasmobranchs, as a lot of marine anthropophony, such as noise from ship engines, occupies the low-frequency range, masking important natural sounds that elasmobranchs rely on. “Experimental studies have shown that exposing elasmobranchs to artificial sounds can alter their swimming behaviour and increase stress indicators,” says Lucille. 

In order to fully understand the impact of anthropophony on elasmobranchs and other fish, we need a better understanding of fish hearing systems. “Elasmobranchs are ideal for studying fish hearing because they represent an early evolutionary branch of jawed vertebrates, meaning their auditory systems have had over 400 million years to diversify and adapt,” explains Lucille. “They occupy nearly all marine environments, from shallow coastal nurseries to the deep sea, providing a wide ecological gradient to test hypotheses such as the eco-acoustical constraints hypothesis.” 

The eco-acoustical constraints hypothesis 

Compared to other animal groups, fish have a remarkable diversity of inner ear structures. This diversity has puzzled scientists for years, but Lucille is hoping to solve the mystery. “One compelling explanation for this variation is the eco-acoustical constraints hypothesis which proposes that fish auditory systems have evolved in response to the specific acoustic properties of their environments,” explains Lucille. “According to this hypothesis, fish inhabiting quiet environments, where background noise levels are low, may evolve enhanced auditory structures to increase sensitivity to faint or distant sounds. In contrast, species living in noisy habitats, such as turbulent coastal zones or reef environments, may not benefit from such specialisations because background noise would likely mask any subtle auditory cues.” 

The eco-acoustical constraints hypothesis predicts that fishes living in noisy environments would gain less advantage from evolving sophisticated hearing structures, and so are more likely to have simple auditory systems. “Historically, testing this hypothesis has been challenging due to the complex and modular nature of the fish inner ear and limited physiological data,” says Lucille. However, recent technological advances are allowing researchers like Lucille to study this problem in depth for the first time. 

Solving marine mysteries with technology 

Lucille plans to combine innovative new tools such as 3D bioimaging, artificial intelligence (AI) and biomechanical modelling to test the eco-acoustical constraints hypothesis and explore the relationship between ear structure and function in elasmobranchs. For example, advanced, high-resolution 3D bioimaging techniques will allow her to reconstruct and compare the inner ear anatomy of different elasmobranch species living in different habitats. She will then use hydrophones to record the soundscapes of each species’ habitat and use AI to identify each habitat’s key acoustic features, before analysing whether these features can be used to predict the inner ear structure of the fishes living in each habitat. 

“We will then develop the first biomechanical model of a fish inner ear,” says Lucille. “This model will simulate how sound-induced forces cause displacement and stress within inner ear structures, allowing us to identify which anatomical elements are most functionally significant.” The last step in Lucille’s research project will involve raising shark embryos in tanks with different soundscapes to see if this affects the development of their inner ear structure. “This not only offers a novel test of the eco-acoustical constraints hypothesis in living animals, but also explores how flexible or vulnerable elasmobranch hearing systems are in the face of changing acoustic environments,” says Lucille. 

From research to conservation 

“By illuminating how elasmobranchs hear and how their auditory systems are shaped by their habitats, this research provides a much-needed foundation for protecting these vulnerable species (and other fish) from the growing threat of underwater noise pollution,” says Lucille. “With the right regulatory frameworks and adoption of quieter technologies, we can restore healthier ocean soundscapes and allow marine life to regain their acoustic niches. Ultimately, this project creates a robust scientific basis for understanding and managing the auditory ecology of elasmobranchs, a critical step toward ensuring their resilience in a rapidly changing world.”

Dr Lucille Chapuis
Department of Ecological, Plant and Animal Sciences, School of Agriculture, Biomedicine and Environment, La Trobe University, Australia

Fields of research: Marine bioacoustics, neuroecology, animal behaviour

Research project: Studying the hearing systems of fish to understand how they are affected by human-made noise

Funders: Dr Lucille Chapuis is the recipient of a Discovery Early Career Award (project number DE240100188) funded by the Australian Government. The views expressed herein are those of the authors and are not necessarily those of the Australian Government or Australian Research Council. This project has also been partially supported by the Australia and Pacific Science Foundation (project number APSF22040). 

Website: sharkslikejazz.com