The Otolith

What exactly is it and what is its purpose?

FSF Staff March 18, 2010

After a recent reef fishing trip, we were unexpectedly met at the dock by an FWC biologist. He promptly removed the otoliths from several of our prized mutton snapper and we wanted a better understanding of exactly why. To learn more about the function of otoliths and what scientists can uncover by dissecting them we turned to Dr. David Mann of the University of South Florida. Dr. Mann is a marine bioacoustics expert with a Ph.D. from the Woods Hole Oceanographic Institute. He focuses his scientific studies on hearing and sound production in fish and marine mammals and is a veritable wealth of knowledge.

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A biologist carefully removes an otolith from a redfish for further investigation. Photo: FWC

FSF: What exactly is an otolith?
DR. MANN: An otolith is a form of calcium carbonate and often referred to as a fish’s ear bone, however, it’s not classified as a bone. Bones are living matter and otoliths are accreted, meaning they grow by accumulation.

For game fish, finding prey and avoiding becoming prey are the two most important functions of this system.

FSF: Do all fish have otoliths?
DR. MANN: Yes, however, the shape and size varies greatly and is highly dependent on the species.

FSF: What is an otolith’s primary function?
DR. MANN: Like the vestibular system in a human’s inner ear, otoliths play an essential role in hearing and balance. In fact, a fish’s otolith and the inner ear of a human are homologous structures, meaning they share the same basic blueprint and embryonic origin, although they don’t necessarily share the exact same function.

FSF: So, can fish actually hear?
DR. MANN: While it’s highly dependent on the species, most fish are only capable of hearing low frequencies below 1,000Hz, which is the typical range of most natural aquatic sounds. Tiny hairs (cilia) are located near the otolith and stimulated by the fish’s movement and also by sound. The signals are transmitted to the brain. It’s important to understand that some fish have a greater hearing sensitivity than other species. This is determined by the proximity of the swim bladder to the otolith. Drum, croaker, grunts and grouper are vocal species that utilize a drumming muscle on their swim bladder for communication related to aggression and courtship. Otoliths can only measure vibration, which is why a fish’s swim bladder is essential in converting acoustic pressure into a motion that can be detected by the otolith. Flounder are one such species that lack a swim bladder, and because of this fact they respond better to stimulation of smell and sight rather than sound.

FSF: If an otolith also plays an essential role in balance, would a species such as a sailfish that has a tendency to jump have a more developed otolith for greater out-of-the-water balance and control?
DR. MANN: Unfortunately, little is known about open ocean pelagic hearing. There have been some minor tests, although pelagic species are very hard to study. The availability of inshore game fish such as snook, seatrout, snapper, and grouper make them much more suitable for scientific research.

FSF: Does a fish’s otolith work in conjunction with its lateral line to understand and adapt to its surroundings?
DR. MANN: Absolutely. Lateral lines help fish sense hydrodynamic flows and when used in conjunction with its other sensory organs, a fish can learn a lot about its surroundings. Together the otolith and lateral line make up the octavo-lateralis system. For game fish, finding prey and avoiding becoming prey are the two most important functions of this system. The octavo-lateralis system is also the focus of many lure manufacturers, as effective fishing lures create not only visual stimuli but hydrodynamic stimuli that excite the lateral line, including vortexes and disturbances in the water that simulate wounded prey.

FSF: What can scientists uncover by otolith research?
DR. MANN: There are two ways biologists use otoliths for research. One is for stomach contents studies. Lets say you catch a bull dolphin. You can sift through the fish’s stomach contents and pick through partially digested matter. In larger game fish otoliths from forage fish can be identified in the stomach contents, which can in turn reveal feeding habits and migratory patterns. Another purpose for studying otoliths is to determine age. Otoliths grow and lay down visible rings similar to a tree. In larval fish scientists can actually determine a fish’s age by days, not years. Understanding the relationship between age and size is important for stock management. Weighing, measuring the length, and determining the age are fundamental ways to look at a population structure. Information on a fish’s age, combined with other data, allows biologists to model growth and mortality rates, timing and magnitude of spawning, recruitment and habitat use, larval and juvenile duration, structure of present populations, as well as predict future population trends. There are also new cutting-edge tests that examine chemical components from otoliths, which can to some extent uncover where a fish came from. Through a process called isotope microchemistry, a scientist can tell if a fish was born or bred in Tampa Bay or Charlotte Harbour—two essential nurseries that are only approximately 60-miles apart. Otoliths are natural tags with the accumulation of calcium carbonate and trace metals from the surrounding water reflecting the fish’s larval environment.

So there you have it. It seems that fish can, in fact, hear. Depending on the particular species, they do have the ability to detect low frequencies. What’s also amazing is that an otolith no larger than a fingernail can reveal so much information about a fish.

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