307
Recently, Lu and DeSmidt ( 2013 ) recorded evoked potentials from saccular hair
cells (microphonic responses) from zebrafi sh larvae at 2–7 dpf using particle motion
stimulation delivered by a displacement-driven piezoelectric probe placed adjacent
to the inner ear. Saccular potentials increased with stimulus intensity and frequency
while auditory thresholds (at 200 Hz) decreased gradually during fi sh growth with
age/size. Such developmental changes were correlated with the increases in the
number and density of saccular hair cells. The results reported in this study are in
contrast with the previously published data on the same species (Higgs et al. 2001 ,
2003 ), however the latter investigation by Lu and DeSmidt ( 2013 ) used zebrafi sh
larvae during the fi rst week of development, a period of rapid anatomical and physiological changes in the inner ear, which could explain the changes in ontogenetic
auditory sensitivity.
Siluriformes
Within this taxon (also otophysines), two catfi sh species have been investigated,
namely the squeaker catfi sh ( Synodontis schoutedeni ) (Mockokidae) and the African
bullhead catfi sh ( Lophiobagrus cyclurus ) (Bagridae) (Lechner et al. 2010 , 2011 ).
Based on AEP recordings, both species exhibited considerable improvement in
auditory sensitivity and changes in best frequency sensitivity range with increases
in size/age (Lechner et al. 2010 ). According to Lechner et al. ( 2010 ), the smallest
juveniles S. schoutedeni with 22–37 mm standard length (SL) had relatively poor
hearing ability in comparison with larger juveniles and adults that range up to
127 mm SL (tested over a frequency range of 0.5–1 kHz). The authors reported an
ontogenetic increase in auditory sensitivity of 26 dB re 1 μPa and a change in the
range of lowest thresholds from 2–3 kHz in juveniles of 22–37 mm SL to 0.3–1 kHz
in larger fi sh of 62–127 mm SL.
In the bullhead catfi sh ( L. cyclurues ), auditory sensitivity was reported to increase
up to 40 dB re 1 μPa during ontogeny (Lechner et al. 2011 ) (Fig. 4 ). The smallest
juveniles (11–15 mm SL) were unable to detect frequencies higher than 2–3 kHz
while being most sensitive to frequencies of 0.05–2 kHz, whereas larger individuals
(>24 mm SL) showed best sensitivity to higher frequencies of 4–6 kHz. According
to the authors, the increase in auditory sensitivity and maximum detectable frequency was posited to be due to the development of interossicular ligaments between
the Weberian ossicles.
Gadiformes
The single representative species of this order studied so far is the walleye pollock
( Theragra chalcogramma ) (Gadidae). Mann et al. ( 2009 ) showed that there were no
signifi cant differences in AEP sensitivity between three different size groups tested
that ranged from 14 to 26 cm TL. The three size groups of walleye pollock had best
hearing sensitivity from 100 to 200 Hz with thresholds of approximately 75 dB re 1
μPa. Although there were no signifi cant differences in thresholds among the three
Development of Structure and Sensitivity of the Fish Inner Ear
Recently, Lu and DeSmidt ( 2013 ) recorded evoked potentials from saccular hair
cells (microphonic responses) from zebrafi sh larvae at 2–7 dpf using particle motion
stimulation delivered by a displacement-driven piezoelectric probe placed adjacent
to the inner ear. Saccular potentials increased with stimulus intensity and frequency
while auditory thresholds (at 200 Hz) decreased gradually during fi sh growth with
age/size. Such developmental changes were correlated with the increases in the
number and density of saccular hair cells. The results reported in this study are in
contrast with the previously published data on the same species (Higgs et al. 2001 ,
2003 ), however the latter investigation by Lu and DeSmidt ( 2013 ) used zebrafi sh
larvae during the fi rst week of development, a period of rapid anatomical and physiological changes in the inner ear, which could explain the changes in ontogenetic
auditory sensitivity.
Siluriformes
Within this taxon (also otophysines), two catfi sh species have been investigated,
namely the squeaker catfi sh ( Synodontis schoutedeni ) (Mockokidae) and the African
bullhead catfi sh ( Lophiobagrus cyclurus ) (Bagridae) (Lechner et al. 2010 , 2011 ).
Based on AEP recordings, both species exhibited considerable improvement in
auditory sensitivity and changes in best frequency sensitivity range with increases
in size/age (Lechner et al. 2010 ). According to Lechner et al. ( 2010 ), the smallest
juveniles S. schoutedeni with 22–37 mm standard length (SL) had relatively poor
hearing ability in comparison with larger juveniles and adults that range up to
127 mm SL (tested over a frequency range of 0.5–1 kHz). The authors reported an
ontogenetic increase in auditory sensitivity of 26 dB re 1 μPa and a change in the
range of lowest thresholds from 2–3 kHz in juveniles of 22–37 mm SL to 0.3–1 kHz
in larger fi sh of 62–127 mm SL.
In the bullhead catfi sh ( L. cyclurues ), auditory sensitivity was reported to increase
up to 40 dB re 1 μPa during ontogeny (Lechner et al. 2011 ) (Fig. 4 ). The smallest
juveniles (11–15 mm SL) were unable to detect frequencies higher than 2–3 kHz
while being most sensitive to frequencies of 0.05–2 kHz, whereas larger individuals
(>24 mm SL) showed best sensitivity to higher frequencies of 4–6 kHz. According
to the authors, the increase in auditory sensitivity and maximum detectable frequency was posited to be due to the development of interossicular ligaments between
the Weberian ossicles.
Gadiformes
The single representative species of this order studied so far is the walleye pollock
( Theragra chalcogramma ) (Gadidae). Mann et al. ( 2009 ) showed that there were no
signifi cant differences in AEP sensitivity between three different size groups tested
that ranged from 14 to 26 cm TL. The three size groups of walleye pollock had best
hearing sensitivity from 100 to 200 Hz with thresholds of approximately 75 dB re 1
μPa. Although there were no signifi cant differences in thresholds among the three
Development of Structure and Sensitivity of the Fish Inner Ear