Abstract
A number of species of clupeid fish, including blueback herring, American shad, and gulf menhaden, can detect and respond to ultrasonic sounds up to at least 180 kHz, whereas other clupeids, including bay anchovies and Spanish sardines, do not appear to detect sounds above about 4 kHz. Although the location for ultrasound detection has not been proven conclusively, there is a growing body of physiological, developmental, and anatomical evidence suggesting that one end organ of the inner ear, the utricle, is likely to be the detector. The utricle is a region of the inner ear that is very similar in all vertebrates studied to date, except for clupeid fish, where it is highly specialized. Behavioural studies of the responses of American shad to ultrasound demonstrate that they show a graded series of responses depending on the sound level and, to a lesser degree, on the frequency of the stimulus. Low-intensity stimuli elicit a non-directional movement of the fish, whereas somewhat higher sound levels elicit a directional movement away from the sound source. Still higher level sounds produce a “wild” chaotic movement of the fish. These responses do not occur until shad have developed the adult utricle that has a three-part sensory epithelium. We speculate that the response of the American shad (and, presumably, other clupeids that can detect ultrasound) to ultrasound evolved to help these species detect and avoid a major predator – echolocating cetaceans. As dolphins echolocate, the fish are able to hear the sound at over 100 m. If the dolphins detect the fish and come closer, the nature of the behavioural response of the fish changes in order to exploit different avoidance strategies and lower the chance of being eaten by the predators.
Next-generation sequencing of small RNAs from inner ear sensory epithelium identifies microRNAs and defines regulatory pathways