Tuesday, June 14, 2016

Specialized Hearing in the Pacific Herring, by Jeff Abersold

Drop a stone into the water and it makes an audible “kerplop”.  Below the surface however, the same sound is much different.  Sounds travel faster underwater and must deal with dissolved particulates that reflect and scatter the wave causing pressure differences and vibrations.  Tap on the glass of an aquarium and the fish will scatter as if they were startled.  That is because they were startled!  The sound traveled through the water, was received by special organs inside the fish, and the fish responded.  This is done with a unique lateral-line and inner ear system composed of neuromast hair cells and the otolith, or ear bone, and the swim bladder (Higgs and Radford 2013).  This information is then used by the fish in order to determine the appropriate reaction to the sound.  Now you know why most aquariums have that little “Please do not tap on the glass” signs.
Pacific Herring Clupea pullasi  source
            Some species of fish, including the Pacific herring, have developed an even more advanced method of hearing.  Herring have three pairs of organs called Weberian ossicles which directly connect the swim bladder to the inner ear.  This structure forms two vesicles close to the otolith with one vesicle sending sensations directly into the utricle, which acts as a functional ear.  These sensations are then converted into vibrations which are interpreted by the fish (Enger 1967).  This advanced system allows herring to hear very well and increases survivability.  So, how would having excellent hearing directly benefit Pacific herring?

What a fish hears directly affects how a fish responds.  Herring, which are a schooling fish, use their increased sound reception to precisely locate a sound and then decide whether to adjust its movement speed, seek out other individuals to increase school size, or change direction.  Since sound travels much faster underwater and comes from all directions the fish must be able to detect exactly where a sound originates.  Pacific herring use their excellent hearing to precisely determine a sound’s source allowing them to determine if there is danger.  Since fish are almost constantly picking up various sounds they must have a way to filter out the important sounds.  Sounds must increase generally by 20-30 dB before any response is seen, in effect cancelling out surrounding “white” noise (Schwarz and Greer, 1984).  This ability to determine sound volume allows them to adjust to constant sounds even at higher decibels.  Once the initial startle response has occurred if the herring sense no danger, but the sound persists, they will continue to ignore it.   
Audiograms depict faintest sounds of differing frequencies detected by different fish.  source
Herring response to sound has generally been categorized as a startle or avoidance response.  Short or low frequency sound will cause a brief directional response (a startle) while high or continuing sound will cause schooling, sinking in the water column, or fleeing the area (avoidance response).  Herring response to differences in sound pitch aid survival by helping the fish avoid predators, particularly whales.  Wilson and Dill (2002) experimented with how sounds affect herring shoals and showed feeding fish would cease feeding and begin to school when threatened and that already schooling fish would increase speed while dropping in the water column.  This ability to hear amazingly well allow herring to adjust their behavior in response to potential predation thereby increasing survivability.  

It is easy to look at a fish and believe they have poor hearing, or lack hearing all together.  They have no ears or other openings that make sound recognition apparent.  All fish can hear the only variation is how hearing occurs.  Different species have developed functional hearing that is appropriate to their survival and surroundings.  Herring have developed advanced hearing that allows them to filter through an abundance of sound, know where the sound is coming from and respond.   They react to their surroundings, avoid predators and change directions in a school without colliding into one another.  They are able to detect subtle changes in pressure and ionic disturbance as well as adjust to a constant non-threatening sounds.  Pretty remarkable for an animal that typically only reaches a foot in length and has a relatively small brain.  So remember the next time you are at an aquarium and the sign says “Please don’t tap on the glass” that it’s because the fish can hear that tapping.  In fact some of them hear it very, very well.

References

Enger, S. 1967. Hearing in Herring. Comparative Biochemistry and Physiology 22:527-538.
Higgs, D. M., and C. A. Radford. 2013. The contribution of the lateral line to ‘hearing’ in Fish. Journal of Experimental Biology 216:1484-490.
Schwarz, A.L., and G.L. Greer. 1984. Responses of Pacific herring, Clupea harengus pallasi, to some underwater sounds. Canadian Journal of Fish and Aquatic Sciences 41:1183-1192.
Wilson, Ben, and Lawrence M. Dill. 2002. Pacific Herring respond to simulated Odontocete echolocation sounds. Canadian Journal of Fisheries and Aquatic Sciences 59.3:542-53.

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