Author: Natasha Hynes
If you have had an Intro to Sharks presentation from one of our shark educators, you might remember us talking about shark senses. Sharks use many systems to sense their environment. This can include their ampullae of Lorenzini, touch, smell, and even hearing. Scientists in Canada have recently taken a closer look at the research that has been done on elasmobranch hearing and what research can focus on in the future.
Studies on elasmobranchs (the group of animals that contains sharks, skates, and rays) have shown that they can hear sounds ranging from 25 to 1500 Hz. Additionally, the hearing sensitivities of different species can depend on their habitat. For example, species that feed at the surface might be more sensitive to frequencies made by splashing prey, while bottom feeders might be more sensitive to frequencies made by prey at the ocean floor.
In the ocean, sound can travel up to 4 times faster than in air. Sound has two ways of traveling: (1) as particle motion and (2) as a pressure wave. Particle motion sends sound information by moving the particles next to it back and forth. This motion gives information about the direction that a sound is coming from. A pressure wave sends sound information by causing changes in pressure as it moves over particles. For fish, particle motion matters more than pressure waves for getting information about a sound.
Elasmobranchs have hair cells in their ears that are covered with a hard structure called a otoconia (this structure in bony fish is called an otolith). If something makes a sound near a shark, the particle motion from that sound moves the shark and the water. But, the otoconia doesn’t move at the same rate with the shark and the water. This is because the shark’s tissue is the same density as the water around it, so the particle motion can move them together. The otoconia are denser than the water and so they do not move as fast. This causes movement in the hair cells that sends information to the shark about the sound.
What does this mean for the elasmobranchs? Elasmobranch populations have decreased by over 70% since 1970. Such a large decline means that they're a high conservation concern. Sharks overlap with human activity in many areas that may expose them to vessel noise. Vessel noise is detectible by elasmobranchs, but more research needs to be done on how noise pollution may affect them.
The authors of this review suggest some studies that will give us more information about elasmobranch hearing. These include measuring elasmobranch hearing sensitivity in terms of particle motion, as well as studies that explore the use of sound as deterrents to reduce bycatch and unwanted shark interactions.
If you liked learning about elasmobranch hearing and are interested in learning more about shark senses, read more here: https://www.sharks4kids.com/shark-senses
References:
Mickle MF and Higgs DM. 2022. Towards a new understanding of elasmobranch hearing. Mar. Biol. 169:12. doi: 10.1007/s00227-021-03996-8
Nedelec SL, Campbell J, Radford AN, Simpson SD, and Merchant ND.2016. Particle motion: the missing link in underwater acoustic ecology. Methods Ecol. Evol. 7:836-842. doi: 10.1111/2041-210X.12544.