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2 Morphological Development of the Fish Auditory System
During ontogeny a number of important morphological changes occur in the otolithic end organs and peripheral auditory system of fi shes. This section is meant as
an overview of these structural changes that occur and likely infl uence auditory
sensitivity, hearing perception, and communication and should not be considered a
comprehensive review of the literature on this topic. The following sections briefl y
discuss the structure of the teleost inner ear, embryology, and early development of
the auditory end organs, postembryonic development and changes to the peripheral
auditory system.
2.1 The Teleost Inner Ear
Like other vertebrates, fi sh have ears that detect acoustic stimuli (Weber 1820 ;
Parker 1903 ; von Frisch and Stetter 1932 ). The teleost inner ear is composed of
three otolithic endorgans, the lagena, utricle, and saccule, which include otoliths
and sensory epithelia, as well as three semicircular canals (see Fig. 1 ). All three
otolithic end organs are thought to be capable of detecting both inertial stimuli and
acoustic stimuli; however, it is likely that the three end organs differ in their relative
contribution to motion detection and audition (Popper and Fay 1993 ; Popper et al.
2003 ). The saccule is the primary auditory end organ in most teleost fi shes (Popper
and Schilt 2008 ; Webb et al. 2008 ), whereas the other otolithic end organs seem to
have either a vestibular role (von Frisch 1938 ; Platt 1983 ) or mixed auditory-vestibular functions (Popper et al. 1982 ; Schellart and Popper 1992 ).
The otolithic end organs respond to acoustic particle motion much like an accelerometer (Platt and Popper 1981 ; Popper and Tavolga 1981 ; Fay 1984 ). Here we
describe how the otolithic end organs transduce vibrational energy using the saccule
as our example. The saccule contains a dense otolith known as the sagitta, which is
about three times more dense than the fi sh’s body (de Vries 1950 ; Popper and Lu
2000 ). When sound passes through the fi sh, the sagitta moves at a different phase
and amplitude than the saccular epithelium, which is attached to the sagitta by
means of an otolith membrane (Dijkgraaf 1960 ; Fay and Popper 1975 ). A shearing
motion results as the otolith and sensory epithelium move relative to one another
during sound stimulation, causing the ciliary hair bundles to bend (Fay and Popper
1974 ; Popper and Fay 1993 ). Signal transduction occurs as the hair bundles bend
toward the kinocilium and generate a receptor potential that can depolarize the hair
cell and produce an action potential (Popper 1983 ; Fay and Popper 2000 ). Otolithic
organs are most effective at responding to low frequencies below 1000 Hz (Fay
1988 ; Popper and Fay 1999 ).
Although we have described the most common features of the teleost inner ear,
it is important to note that there is a great deal of variation and diversity of inner ear
structures used for hearing in teleost fi shes (Platt and Popper 1981 ). It is likely that
Development of Structure and Sensitivity of the Fish Inner Ear
2 Morphological Development of the Fish Auditory System
During ontogeny a number of important morphological changes occur in the otolithic end organs and peripheral auditory system of fi shes. This section is meant as
an overview of these structural changes that occur and likely infl uence auditory
sensitivity, hearing perception, and communication and should not be considered a
comprehensive review of the literature on this topic. The following sections briefl y
discuss the structure of the teleost inner ear, embryology, and early development of
the auditory end organs, postembryonic development and changes to the peripheral
auditory system.
2.1 The Teleost Inner Ear
Like other vertebrates, fi sh have ears that detect acoustic stimuli (Weber 1820 ;
Parker 1903 ; von Frisch and Stetter 1932 ). The teleost inner ear is composed of
three otolithic endorgans, the lagena, utricle, and saccule, which include otoliths
and sensory epithelia, as well as three semicircular canals (see Fig. 1 ). All three
otolithic end organs are thought to be capable of detecting both inertial stimuli and
acoustic stimuli; however, it is likely that the three end organs differ in their relative
contribution to motion detection and audition (Popper and Fay 1993 ; Popper et al.
2003 ). The saccule is the primary auditory end organ in most teleost fi shes (Popper
and Schilt 2008 ; Webb et al. 2008 ), whereas the other otolithic end organs seem to
have either a vestibular role (von Frisch 1938 ; Platt 1983 ) or mixed auditory-vestibular functions (Popper et al. 1982 ; Schellart and Popper 1992 ).
The otolithic end organs respond to acoustic particle motion much like an accelerometer (Platt and Popper 1981 ; Popper and Tavolga 1981 ; Fay 1984 ). Here we
describe how the otolithic end organs transduce vibrational energy using the saccule
as our example. The saccule contains a dense otolith known as the sagitta, which is
about three times more dense than the fi sh’s body (de Vries 1950 ; Popper and Lu
2000 ). When sound passes through the fi sh, the sagitta moves at a different phase
and amplitude than the saccular epithelium, which is attached to the sagitta by
means of an otolith membrane (Dijkgraaf 1960 ; Fay and Popper 1975 ). A shearing
motion results as the otolith and sensory epithelium move relative to one another
during sound stimulation, causing the ciliary hair bundles to bend (Fay and Popper
1974 ; Popper and Fay 1993 ). Signal transduction occurs as the hair bundles bend
toward the kinocilium and generate a receptor potential that can depolarize the hair
cell and produce an action potential (Popper 1983 ; Fay and Popper 2000 ). Otolithic
organs are most effective at responding to low frequencies below 1000 Hz (Fay
1988 ; Popper and Fay 1999 ).
Although we have described the most common features of the teleost inner ear,
it is important to note that there is a great deal of variation and diversity of inner ear
structures used for hearing in teleost fi shes (Platt and Popper 1981 ). It is likely that
Development of Structure and Sensitivity of the Fish Inner Ear