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Evidence for hearing loss in amblyopsid cavefishes
Matthew L. Niemiller, Dennis M. Higgs and Daphne Soares
Biol. Lett. 2013 9, 20130104, published 27 March 2013


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Evidence for hearing loss in amblyopsid
Matthew L. Niemiller1, Dennis M. Higgs2 and Daphne Soares3

Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520, USA
Department of Biological Sciences, University of Windsor, Windsor, Ontario, Canada N9B 3P4
Department of Biology, University of Maryland, College Park, MD 20742, USA

Cite this article: Niemiller ML, Higgs DM,
Soares D. 2013 Evidence for hearing loss in
amblyopsid cavefishes. Biol Lett 9: 20130104.

Received: 1 February 2013
Accepted: 5 March 2013

Subject Areas:
evolution, neuroscience, ecology
auditory, evolution, fish, subterranean

Author for correspondence:
Daphne Soares

The constant darkness of caves and other subterranean habitats imposes sensory constraints that offer a unique opportunity to examine evolution of
sensory modalities. Hearing in cavefishes has not been well explored, and
here we show that cavefishes in the family Amblyopsidae are not only
blind but have also lost a significant portion of their hearing range. Our
results showed that cave and surface amblyopsids shared the same audiogram profile at low frequencies but only surface amblyopsids were able to
hear frequencies higher than 800 Hz and up to 2 kHz. We measured ambient
noise in aquatic cave and surface habitats and found high intensity peaks
near 1 kHz for streams underground, suggesting no adaptive advantage in
hearing in those frequencies. In addition, cave amblyopsids had lower hair
cell densities compared with their surface relative. These traits may have
evolved in response to the loud high-frequency background noise found
in subterranean pools and streams. This study represents the first report of
auditory regression in a subterranean organism.

1. Introduction
Animals that live in continual darkness are faced with unique challenges in
order to locate and identify food, predators and each other [1]. Without
visual information, independent lineages of obligate cave-dwelling organisms
have evolved regressive features, such as the loss or reduction of eyes and pigmentation and constructive traits, such as longer appendages and hypertrophy
of non-visual sensory systems [2]. Aside from darkness being common to all
subterranean habitats, several other abiotic factors influence subterranean
organisms, such as relatively stable temperature, high humidity and hydrological factors (for example, periodic flooding) [2]. However, little to nothing
is known about how the diverse abiotic characteristics of caves affect the sensory ecology of cave animals. Here, we examine the relationship between the
acoustic environment of caves and hearing in amblyopsid cavefishes.
Aquatic cave organisms, such as cavefishes, survive in perpetual darkness.
An important sensory modality in such environments may be the sense of hearing. In above-ground aquatic habitats, hearing is important for many aspects of
fish behaviour (reviewed in [3]) and is effective over relatively long distances
owing to the nature of underwater sound travel. Sound may play an especially
important role in subterranean habitats owing to the lack of visual signals yet
the acoustic properties of these habitats have been largely ignored to date.
Hypertrophy of hearing characteristics could be adaptive in caves for several
reasons, including working in association with other non-visual senses to
detect prey, conspecifics or predators. However, the degree to which hearing
abilities are modified in cavefishes is largely unknown, as behavioural and
neurophysiological studies on the acoustical biology of cavefishes are extremely
limited. Popper [4] showed that the cave and surface forms of the characid
Astyanax mexicanus do not differ in hearing. Similarly, no differences were
found between cave and surface forms of the molly Poecilia mexicana [5].

& 2013 The Author(s) Published by the Royal Society. All rights reserved.

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(b) 50

hair cell density (units 100 mm–2)



Biol Lett 9: 20130104




Chologaster Troglichthys (i) Speoplatyrhinus (ii)



A. spelaea

T. subterraneus

F. agassizii

Figure 1. (a) The phylogenetic relationships of the two obligate cave species (white) (i) Typhlichthys and (ii) Amblyopsis and one surface species (black) (iii)
Forbesichthys. (b) Cell density counts for the three species show fewer hair cells in the cavefishes (*F2,23 ¼ 15.3, p ¼ 0.0007). Inserts show photomicrograms
of the ears stained with phalloidin. Scale bar, 100 mm. (Online version in colour.)
Here, we show the first report of differences in hearing
characteristics in a cavefish compared with its surface relative. We compared the auditory evoked potentials (AEPs)
of three species in the family Amblyopsidae, as well as the
acoustic profiles of their subterranean habitats in order to
investigate whether a relationship exists between noise in
cave habitats and cavefish hearing. Amblyopsid caveshes
are a model system for studying the ecological and evolutionary processes of cave adaptation because the cave-restricted
species in the family represent a range of troglomorphy
that reflects variable durations of isolation in caves [6].
Cave amblyopsids are one of the most comprehensively
studied caveshes, with six genera and eight species [7]. In
this study, we examine the hearing characteristics of three
related amblyopsids: the surface dwelling, Forbesichthys
agassizii and two cave species, Typhlichthys subterraneus and
Amblyopsis spelaea (figure 1a).

pools with some current (4– 12 m2, 0.1– 0.8 m depth,
0 – 0.6 ms21 (low flow), cobble/bedrock substrate) in L&N
Railroad Cave, Barren Co., KY, USA.

(a) Auditory evoked potentials
This method measures the compound electrical potential created
by the eighth cranial nerve and auditory brainstem nuclei in
response to sound [9,10]. We restrained submerged fish and
played 10 msec tones, ranging from 0.1 to 2 kHz at 0.1 Hz intervals. We increased the sound level in 5 dB intervals until a
stereotypical evoked potential waveform was detected (figure 2,
insert). We determined auditory threshold to be the lowest intensity for which AEP traces were detected [11]. Sound output
was measured with a hydrophone (model LC-10, Reson Inc;
Calibration sensitivity of 2208.9 dB re 1V uPa21, 0 – 100 kHz)
and an accelerometer (model 4524 cubic triaxial deltatron,
Bruel & Kjær). We calibrated sound level and particle acceleration at the beginning of each trial. Thresholds were compared
between species and frequencies with a two-way ANOVA.

2. Material and methods
All procedures followed IACUC guidelines dictated by the University of Windsor. All data are available in http://datadryad.
org under doi:10.5061/dryad.9sj49 [8]. Fishes were collected
under scientific permits issued by the states of TN (no. 1605)
and KY (no. SC1211135), USA. We collected nine individuals
of Forbesichthys agassizii from a quiet pool (10 m2, mean depth
0.6 m, mud/silt substrate with abundant vegetation) of a
spring run fed by Jarrell’s Spring, Coffee Co., TN, USA; seven
individuals for each of the two cave-dwelling species: Amblyopsis
spelaea from several quiet pools (20– 150 m2, 0.2– 2þ m depth,
silt/sand/cobble substrate) in Under the Road Cave, Breckinridge Co., KY, USA and Typhlichthys subterraneus from several

(b) Hair cell histology
Fish were euthanized with an overdose of 2-phenoxy-ethanol
and fixed in 4 per cent paraformaldehyde. Epithelia were dissected and stained with Oregon Green phalloidin (Invitrogen)
followed by fluorescent imaging. Hair cells were manually
counted across eight different regions of saccular epithelia and
quantified as density (hair cells/2500 mm2) to correct for differences in epithelium size. There were no apparent differences in
fluorescent intensity sufficient to affect manual counts. Within
species, there were no significant density differences between
epithelial areas (ANOVA F7,40 ¼ 0.437, p ¼ 0.873), so the density
estimates were averaged across epithelial areas. ANOVA was

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Biol Lett 9: 20130104

threshold (dB)


5 mS
10 mV










frequency (Hz)




Figure 2. Auditory thresholds of amblyopsid fishes. Values are means+standard errors. The suface fish Forbesichthys reaches up to 2 kHz while the cavefish
Typhlichthys (1) and Amblyopsis (2) are limited to 1 kHz. Fast Fourier Transformation (FFT) of sound recorded in a Drowned Rat Cave pool. The pool was
carved in bedrock by a small stream. The recording was made 0.5 m deep and approximately 1 m from the waterfall. The ceiling of the cave was also dripping
onto the pool. Insert: auditory evoked potential traces of all species to a 400 Hz tone burst at 60 dB.
used to assess differences in hair cell density, followed by a
Tukey post-hoc test.

(c) Environmental sound profiles
We characterized aquatic environmental sound profiles in cave
and surface habitats, using a hydrophone (type 10CT hydrophone, calibration sensitivity of 2195 dB re. 1 V mPa21;+3 dB,
0.02– 10 kHz, omnidirectional, G.R.A.S., Denmark) connected
through a preamplifier (Spikerbox, Backyard Brains) to an iPad
(Apple). Three recordings of 5 min were taken per site. Within
caves, we obtained sound profiles from two habitat types: shallow stream riffles at depths of 0.05 –0.1 m and pools with no
current at depths of 0.1– 2 m. We also recorded at the same
depths in surface streams and pools inhabited by Forbesichthys.
Characterization of sound spectra and corresponding SPLs was
performed using AUDIOTOOLS software (Studio Six Digital). We
matched cave and surface habitats profiles as much as possible (e.g. area, substrate and water flow), with the exception of
vegetation in surface habitats.

3. Results
Density of saccular hair cells differed between species
(F2,6 ¼ 15.3, p ¼ 0.0007), with the two cave species having
lower hair cell densities (mean ¼ 34 and 29 hair cells/
2500 mm2) than the surface species (mean ¼ 45 hair cells/
2500 mm2; figure 1). There was no difference in threshold
between species below 800 Hz (F2,15 ¼ 1.087, p ¼ 0.342;
figure 2), and thresholds increased with frequency (F11,15 ¼
25.9, p , 0.001) with no significant frequency–species interaction
(F15,95 ¼ 47.9, p ¼ 0.702). All three amblyopsid species were
most sensitive at 100 Hz (mean threshold range 112–122 dB re
1 mPa), and thresholds increased between 100 and 800 Hz.


In the two cave species, only one Typhlichthys responded to
tones 700–1000 Hz and just two Amblyopsis responded to tone
bursts above 600 Hz, with only one responding at 1000 Hz.
The surface species showed clear evoked responses well above
this limit, with defined responses detected up to 2000 Hz.
Underwater sounds were variable depending on habitat. In
cave streams with rock and sand substrate, there was a peak in
background noise at about 1000 Hz followed by peaks at low
frequencies (below 200 Hz; figure 2). Overall sound intensity
was less prominent between 200 and 5000 Hz in pool habitats
away from the small streams. Nonetheless, the same general
profile was present but with a smaller, less defined 1000 Hz
peak. Surface streams showed low-frequency noise (less than
100 Hz) and high-frequency noise (more than 8000 Hz) with
a small peak at 1200 Hz, but the overall noise level was much
higher at intermediate frequencies (1000–3000 Hz) in the
cave streams than surface streams.

4. Discussion
Adaptation to cave environments is often associated with
hypertrophy of non-visual sensory modalities. Cave amblyopsids exhibited similar hearing sensitivities as their surfacedwelling relative at 800 Hz and below, consistent with
previous findings in other cavefishes [5,6]. Surprisingly however, cave amblyopsids have lost a significant portion of their
hearing range. Both Amblyopsis and Typhlichthys are unable
to hear frequencies above 800 Hz, unlike their surface relative
Forbesichthys, which can hear up to 2 kHz. In addition, both
cave species had lower hair cell densities than Forbesichthys.
To our knowledge, this is the first report of auditory regression
in a subterranean organism.

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All procedures followed IACUC guidelines dictated by the University of Windsor. All data are available in
under doi:10.5061/dryad.9sj49 [8]. Fishes were collected under scientific permits issued by the states of TN (no. 1605) and KY (no.
SC1211135), USA.
We thank Daniel Escobar Camacho for help and Dr Gal Haspel and
Dr Kim Hoke for comments. This work was supported by the Yale
Institute of Biospheric Studies (M.L.N.) and by ADVANCE grant
no. 1008117 to D.S.






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Biol Lett 9: 20130104

adaptation in this group and suggests it may be due to different
equilibrium demands. If the sensory epithelium is growing in
pace with the otolith without concomitant increase in hair
cells, a decrease in hair cell density would result. If, however,
the loss of high-frequency hearing ability in cave species was
due to selective loss of high-frequency hair cells, this could
also lead to a decrease in overall hair cell density. There is no
evidence for tonotopy in fish ears, but there is some evidence
for differential frequency selectivity in hair cells across the
epithelia [15]. More work needs to be done on frequency
responses at the level of individual hair cells before this idea
can be supported.
Our study provides evidence that two cavefish species
have evolved loss of high-frequency hearing and reduced
hair cell densities compared with a surface-dwelling relative.
These traits may have evolved in response to loud highfrequency background noise that mask acoustic signals in
their aquatic subterranean habitats; however, the mechanism
(i.e. neutral loss versus selection) underlying hearing loss
remain to be understood.

Like the loss of eyes, loss of hearing range in cave amblyopsids represents an example of regressive evolution in
subterranean organisms. Audio recordings from native cave
habitats of cave amblyopsids showed that flowing streams
(riffles) and water droplets dripping from the ceiling of
cave passages contribute to loud high-frequency background
noise generally above 800 Hz (figure 2), although the precise
contribution of all noise sources have not been characterized.
Lower frequencies are not likely to propagate far in these
shallow environments [12] but the higher frequency components would propagate further and contribute to the more
to the high background noise levels of the caves. The apparent
match between hearing ability and background noise profiles
has been hypothesized to be an evolutionary driver of hearing
ability across the Teleostei [13], and the hearing of two species
of goby (Padogobius martensii and Gobius nigricans) living in
noisy waterfall environments is most sensitive in a frequency
range corresponding to a quiet window in these environments
[14]. Noisy stream environments mask high-frequency hearing
in ostariophysan fishes [15] but hearing specializations of closely related species in different acoustic environments have
rarely been tested. Our findings raise the intriguing possibility
that cave amblyopsids may have lost hearing at high frequencies in response to the noisy acoustic environments in which
they live.
The reduction in hair cell density indicates peripheral
involvement in high-frequency hearing loss. Fewer hair cells provide fewer sites for signal transduction and also may lead to less
relative stimulation upon relative motion of the otolith. Poulson
[9] reports an increase in otolith size with increasing cave