190
based on auditory evoked potentials (AEPs), were similar between the two ecotypes
with greatest sensitivities between 200 and 300 Hz. The authors found no evidence
for intra-specifi c acoustic communication in both cave and surface ecotypes.
Differences in otolith morphology between ecotypes may refl ect metabolic
differences but had minimal infl uence on hearing sensitivity or acoustic behavior
(Schulz- Mirbach et al. 2010 ).
Schulz-Mirbach et al. ( 2011b ) compared otolith morphology of several locally
adapted populations of P. mexicana living in surface and cave habitats that differed
in levels of hydrogen sulfi de (H 2 S) and darkness. Asterisci, lapilli, and sagittae from
a non-sulfi dic cave were larger than those from the sulfi dic cave, and generally
larger than otoliths from surface habitats (sulfi dic and non-sulfi dic). As noted previously, cavefi sh had thicker otoliths with deep furrows housing the sensory epithelium. Schulz-Mirbach et al. ( 2011b ) also examined inner ear size and brain
morphology. The length and width of the optic tectum were smaller in fi sh from the
two cave populations compared to fi sh from surface habitats. However, inner ear
size and length of the sacculus were similar between cave and surface forms.
Although the populations studied showed clear differentiation in otolith morphologies, no clear directional pattern of trait divergence along the two environmental
gradients (darkness and hydrogen sulfi de concentration) was discernible. Similar
hearing sensitivities despite differences in otolith morphology between cave and
surface fi sh may refl ect the role of hearing for orientation (Popper et al. 2005 ;
Popper and Schilt 2008 ) or other structures, such as the sensory epithelia, may have
co-evolved with otolith changes to maintain inner ear function (Schulz-Mirbach
et al. 2011b ).
Our own studies have examined hearing ability between related cave and surface
fi shes in the family Amblyopsidae (Niemiller et al. 2013) (Fig. 2 ). Specifi cally, we
compared hearing sensitivities between the related, surface-dwelling Forbesichthys
agassizii and cave-dwelling Typhlichthys subterraneus and Amblyopsis spelaea.
Forbesichthys is the sister group to a clade of subterranean genera, including
Amblyopsis , Typhlichthys and Speoplatyrhinus (Niemiller et al. 2013). We used
AEPs and showed that all three species exhibited similar hearing sensitivities at
frequencies lower than 800 Hz, consistent with previous studies in other cavefi shes
(Popper 1970 ; Schulz-Mirbach et al. 2010 ). Unexpectedly, the two cave species
were unable to hear above 800 Hz, whereas surface-dwelling F. agassizii exhibited
a response up to 2 kHz, the maximum frequency tested in our experimental design.
Eigenmann and Yoder ( 1899 ) noted no gross anatomical changes in the inner ear of
Amblyopsis ; however, we noted signifi cant differences in saccular hair cell densities. The cave species, Typhlichthys subterraneus and A. spelaea , had lower hair cell
densities compared to surface F. agassizii . The reduction in hair cell density suggests
peripheral involvement in high-frequency hearing loss in the cave species.
Loss of high frequency hearing in Typhlichthys and Amblyopsis to our knowledge represents the fi rst report of regressive evolution of hearing in a subterranean
organism. In addition to testing hearing ability, we characterized aquatic environmental sound profi les in cave and surface habitats inhabited by each amblyopsid
cavefi sh and the surface F. agassizii (Niemiller et al. 2013). Audio recordings from
D. Soares et al.
based on auditory evoked potentials (AEPs), were similar between the two ecotypes
with greatest sensitivities between 200 and 300 Hz. The authors found no evidence
for intra-specifi c acoustic communication in both cave and surface ecotypes.
Differences in otolith morphology between ecotypes may refl ect metabolic
differences but had minimal infl uence on hearing sensitivity or acoustic behavior
(Schulz- Mirbach et al. 2010 ).
Schulz-Mirbach et al. ( 2011b ) compared otolith morphology of several locally
adapted populations of P. mexicana living in surface and cave habitats that differed
in levels of hydrogen sulfi de (H 2 S) and darkness. Asterisci, lapilli, and sagittae from
a non-sulfi dic cave were larger than those from the sulfi dic cave, and generally
larger than otoliths from surface habitats (sulfi dic and non-sulfi dic). As noted previously, cavefi sh had thicker otoliths with deep furrows housing the sensory epithelium. Schulz-Mirbach et al. ( 2011b ) also examined inner ear size and brain
morphology. The length and width of the optic tectum were smaller in fi sh from the
two cave populations compared to fi sh from surface habitats. However, inner ear
size and length of the sacculus were similar between cave and surface forms.
Although the populations studied showed clear differentiation in otolith morphologies, no clear directional pattern of trait divergence along the two environmental
gradients (darkness and hydrogen sulfi de concentration) was discernible. Similar
hearing sensitivities despite differences in otolith morphology between cave and
surface fi sh may refl ect the role of hearing for orientation (Popper et al. 2005 ;
Popper and Schilt 2008 ) or other structures, such as the sensory epithelia, may have
co-evolved with otolith changes to maintain inner ear function (Schulz-Mirbach
et al. 2011b ).
Our own studies have examined hearing ability between related cave and surface
fi shes in the family Amblyopsidae (Niemiller et al. 2013) (Fig. 2 ). Specifi cally, we
compared hearing sensitivities between the related, surface-dwelling Forbesichthys
agassizii and cave-dwelling Typhlichthys subterraneus and Amblyopsis spelaea.
Forbesichthys is the sister group to a clade of subterranean genera, including
Amblyopsis , Typhlichthys and Speoplatyrhinus (Niemiller et al. 2013). We used
AEPs and showed that all three species exhibited similar hearing sensitivities at
frequencies lower than 800 Hz, consistent with previous studies in other cavefi shes
(Popper 1970 ; Schulz-Mirbach et al. 2010 ). Unexpectedly, the two cave species
were unable to hear above 800 Hz, whereas surface-dwelling F. agassizii exhibited
a response up to 2 kHz, the maximum frequency tested in our experimental design.
Eigenmann and Yoder ( 1899 ) noted no gross anatomical changes in the inner ear of
Amblyopsis ; however, we noted signifi cant differences in saccular hair cell densities. The cave species, Typhlichthys subterraneus and A. spelaea , had lower hair cell
densities compared to surface F. agassizii . The reduction in hair cell density suggests
peripheral involvement in high-frequency hearing loss in the cave species.
Loss of high frequency hearing in Typhlichthys and Amblyopsis to our knowledge represents the fi rst report of regressive evolution of hearing in a subterranean
organism. In addition to testing hearing ability, we characterized aquatic environmental sound profi les in cave and surface habitats inhabited by each amblyopsid
cavefi sh and the surface F. agassizii (Niemiller et al. 2013). Audio recordings from
D. Soares et al.