282
still responded robustly to acoustic stimuli, however, did not display clear directionality. Several additional neurons were acceleration sensitive and responded to fi sh
movement, but were relatively insensitive to sound frequencies tested suggesting
dichotomy in utricular hair cells with some hair cells functioning primarily as low
frequency vestibular and not auditory sensors. Whether the converse is true is
unknown, as only candidate fi bers that responded to horizontal movement of the
vibration isolation table during the implant were selected for sound tests.
Alternatively, as these cells were not tested for sound sensitivity between 5 and 80
Hz, these may be representative of the lower frequency fi bers found in the sleeper
goby (Lu et al. 2004 ).
The ability of fi sh to localize sound sources is complicated by small interaural
distances and the high speed of sound underwater. The saccule has been implicated
as the main endorgan of hearing and is certainly the largest otolith in toadfi sh.
However, the caudal ends of the bilaterally positioned saccules are in close proximity, and even in adult fi sh, sound arrives at the posterior of each endorgan virtually
simultaneously. The smaller utricles, on the other hand, are rostral to the saccules
and in large, adult toadfi sh, are separated by distances of 1–3 cm. Whether this spacing provides a suffi cient delay to localize sounds based on interaural time differences remains to be determined.
What is clear, however, is that body movements and normal ventilation can also
stimulate the utricle, and while these latter cyclic movements may be fi ltered in
higher order processing centers (Montgomery and Bodznick 1994 ), the ability to
hear and/or fi nd the sound source may be compromised by self-generated movement. While male toadfi sh remain relatively stationary during advertisement calling, female fi sh must swim to fi nd suitable males. Swimming movements can cause
maximal excitation of utricle afferents and the ability to pin point sound sources
during these forays may be compromised. Observations of female fi sh movement in
the fi eld are complicated by poor water visibility, cryptic coloration, and/or nocturnal movements. However, if the utricle is important in localizing sound, the female
may need to alternate swimming with stationary pauses. Spontaneous toadfi sh
movements in outdoor ponds and large tanks suggests that a typical toadfi sh “swim”
consist of short legs, typically less than 1 m interspersed with pauses rather than
long distance sustained bursts. While this behavior is more likely to have evolved to
minimize alerting prey or predators outside of their protective habitats, it may also
allow the fi sh to sample its acoustic environment without the added complications
of self-generated movement.
The sensitivity of the utricle in the horizontal plane suggests it may function in
detecting particle motion in azimuth, while the more vertically oriented saccule and
lagena better detect particle motion in elevation. For the benthic-dwelling toadfi sh,
sound detection in the horizontal plane is likely extremely important for detecting
sounds generated by conspecifi cs, predators, and prey. Further studies are needed,
however, to determine the relative role of each of the different otolithic endorgans
and how they contribute to sound localization in fi shes.
A.F. Mensinger
still responded robustly to acoustic stimuli, however, did not display clear directionality. Several additional neurons were acceleration sensitive and responded to fi sh
movement, but were relatively insensitive to sound frequencies tested suggesting
dichotomy in utricular hair cells with some hair cells functioning primarily as low
frequency vestibular and not auditory sensors. Whether the converse is true is
unknown, as only candidate fi bers that responded to horizontal movement of the
vibration isolation table during the implant were selected for sound tests.
Alternatively, as these cells were not tested for sound sensitivity between 5 and 80
Hz, these may be representative of the lower frequency fi bers found in the sleeper
goby (Lu et al. 2004 ).
The ability of fi sh to localize sound sources is complicated by small interaural
distances and the high speed of sound underwater. The saccule has been implicated
as the main endorgan of hearing and is certainly the largest otolith in toadfi sh.
However, the caudal ends of the bilaterally positioned saccules are in close proximity, and even in adult fi sh, sound arrives at the posterior of each endorgan virtually
simultaneously. The smaller utricles, on the other hand, are rostral to the saccules
and in large, adult toadfi sh, are separated by distances of 1–3 cm. Whether this spacing provides a suffi cient delay to localize sounds based on interaural time differences remains to be determined.
What is clear, however, is that body movements and normal ventilation can also
stimulate the utricle, and while these latter cyclic movements may be fi ltered in
higher order processing centers (Montgomery and Bodznick 1994 ), the ability to
hear and/or fi nd the sound source may be compromised by self-generated movement. While male toadfi sh remain relatively stationary during advertisement calling, female fi sh must swim to fi nd suitable males. Swimming movements can cause
maximal excitation of utricle afferents and the ability to pin point sound sources
during these forays may be compromised. Observations of female fi sh movement in
the fi eld are complicated by poor water visibility, cryptic coloration, and/or nocturnal movements. However, if the utricle is important in localizing sound, the female
may need to alternate swimming with stationary pauses. Spontaneous toadfi sh
movements in outdoor ponds and large tanks suggests that a typical toadfi sh “swim”
consist of short legs, typically less than 1 m interspersed with pauses rather than
long distance sustained bursts. While this behavior is more likely to have evolved to
minimize alerting prey or predators outside of their protective habitats, it may also
allow the fi sh to sample its acoustic environment without the added complications
of self-generated movement.
The sensitivity of the utricle in the horizontal plane suggests it may function in
detecting particle motion in azimuth, while the more vertically oriented saccule and
lagena better detect particle motion in elevation. For the benthic-dwelling toadfi sh,
sound detection in the horizontal plane is likely extremely important for detecting
sounds generated by conspecifi cs, predators, and prey. Further studies are needed,
however, to determine the relative role of each of the different otolithic endorgans
and how they contribute to sound localization in fi shes.
A.F. Mensinger