Otoliths: Fish Hearing Bones
Fish Hearing and Otoliths
Did you know fish and invertebrates that live under water make sound? Of course we've all heard of dolphin clicks, whale songs, and haunting melodies by marine mammals, but there's a much richer symphony of sound that occurs under water. Water is a perfect medium because sound waves can travel quite far in vast regions, and areas of reduced visibility. Fish can find mates, signal danger, sense vibrations, and find food/prey all by sensing sound under water.
Depending on the species, fish make a wide variety of sounds, from grunts and clicks to honks, burps, purrs, whistles, hums, groans, and growls. I had a particularly aggressive flounder that used to vibrate his 800 gallon aquarium in breeding season, and he made growling sounds (often before leaping at the naturalist). Many of the sounds that fish make are vocalizations like we make, though some are byproducts of feeding or swimming. It's thought that over 150 species of fish on the East coast vocalize. Fish that make sounds are called soniferous (sound-producing). Check out the research by Dr. Rodney Alan Roundtree and New England soniferous fish.
I know you're dying to hear some sounds, so here are a few examples, the first is the vocalization of the oyster toadfish, the next is a the croaker fish croaking, and a gronco or grunting fish from Cuba grunting like a pig.
There are three main ways that fish make sounds. They can use muscles located near their swim bladder (an air sac in their bodies) to make drumming sounds using what are called "sonic muscles". The muscles move very quickly and vibrate, using the swim bladder as a resonance chamber. Fish can also change speed and direction while swimming, The sounds produced this way are low frequency and especially important for predators like sharks. The final way is by stridulation (like crickets). Stridulation is rubbing teeth or skeletal parts together. This can literally be a gnashing of teeth or rubbing a pectoral fin against the dorsal fin. Seahorses can stridulate by rubbing two parts of their skull (the skull and coronet) against each other! Check out this page if you want to learn more and listen to seahorse sounds.
SO HOW DO FISH HEAR THESE SOUNDS?
Boney fish (not skates or rays) have ears like you and I have, but they don't bother with the earlobes, it would be quite a drag and look kind of funny. Instead their ears are covered by a tympanum, or covering that acts like a drum head. Just down from the tympanum, and inside the fish's ears, are three pairs of bones called otoliths (O-toe-lith), this is analogous to the human's three inner-ear bones. The otoliths sit just behind the brain and float in a fluid filled chamber. There is one large pair of bones (one on each side of the head), called sagittae (sag-it-ay), and two pairs of smaller bones, the lapilli (lap-ill-e) and asteriscii (ass-ter-is-see). The sagittae are most often studied because they're larger and easier to work with.
Otoliths are sometimes called "ear stones" because they are made out of layers of calcium carbonate (and protein), much like the material an oyster shell is made of. As the fish grows, a new layer is added to the otolith. You can age a fish by taking a cross section and counting the rings, much like you would date a tree.
Each species of fish has a unique shape of otolith, some are round and wavy while others are ridged and bumpy. this unique shape can help biologists identify the fish as well as archaeologists that are looking at the diets of animals and early peoples.
Here is a great otolith chart from the Florida Fish and Wildlife Research Institute.
Otoliths, like tree rings can provide a great deal of information. Scientists can tell if there were drastic changes in water temperature by looking a the darkness and lightness of the bands. The darker rings indicate a temperature decline and colder temps. This information has been reproduced successfully in a hatchery/lab situation, which confirms the researcher's theories. The size of the rings also help the biologists determine the age and growth rate of the fish. If years were lean then the otolithic bands would be thinner, because of smaller deposits of calcium carbonate. Cool right?
The otolith's job (in the living fish) is not only to detect sound vibrations, and translate them into something their brains can understand, but they also sense motion and direction too, much like the accelerometers in our phones but way more low tech.
It's fascinating to me that fish can make such a wide spectrum of sounds, and hear them. Otolith biology is a fascinating field full of neat science that has applications in our own technology and underwater navigation too.
For your fish listening pleasure I couldn't resist adding one more video to this post...here's one fish making "farting sounds" to communicate. Happy listening!