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organisms remain the most poorly understood fauna on the planet and little is known
about their sense of hearing. Animals that thrive in a cave not only have evolved
cave-specifi c morphological specializations but also have selective behaviors that
allow for their survival in complete darkness. Cavefi shes (obligate cave-dwelling
fi shes) are replicate ecological and evolutionary experiments in adaptation to this
extreme environment. Because the direction of evolution is known (i.e., surface to
subterranean), the colonization of a subterranean environment affords us the opportunity to examine species that have survived similar evolutionary pressures in parallel
and converging ways.
Of the approximately 32,000 fi sh species globally, over 300 species have been
reported to live in subterranean habitats with more than 170 described species living
obligate underground with some degree of troglomorphy (features related to cave
adaptation; Proudlove 2006 , 2010 ). The phylogenetic diversity of cavefi shes representing 10 orders and 21 families distributed on every continent but Europe and
Antarctica (Fig. 1 ) and the assortment of aquatic subterranean habitats from fast
fl owing streams and waterfalls to quiet phreatic waters provide excellent examples
for the study of independent responses to subterranean environments. Many species
are monotypic and their relationships to surface forms are unknown; in other cases
cave forms are grouped together based on convergent morphologies (Niemiller and
Poulson 2010 ; Niemiller et al. 2013). However, the surface ancestors of all cavefi sh
species had to adapt to the strict constraints imposed by caves, particularly perpetual
darkness and limited energy resources. As a result, a suite of unique phenotypes
associated with subterranean adaptation has emerged, with loss of pigmentation and
eyes being the most conspicuous. Cavefi shes, however, are outcomes of not just
regressive evolution but also constructive adaptation. For example, several cavefi shes exhibit enhancement of the mechanosensory lateral line system relative to
their surface relatives (Culver and Pipan 2009 ; Soares and Niemiller 2013), and a
cave catfi sh, Astroblepus pholeter , has adapted skin-teeth to sense water fl ow
(Haspel et al. 2012 ). Despite the obvious power of using cavefi sh as a natural experiment, to date there have been few comparative studies in cavefi sh ecology and
sensory biology (Trajano 1991 , 1997 , 2001 ; Niemiller et al. 2013; Soares and
Niemiller 2013).
There is strong selection to develop and enhance non-visual sensory modalities
in subterranean habitats, with enhanced hearing ability being one possible modifi cation (see below for a review on possible lateral line enhancement). Lower auditory
thresholds and greater frequency ranges in subterranean habitats should be adaptive
for several reasons, including integrating with other non-visual senses to detect
prey, predators, or conspecifi cs. However, hearing sensitivity and range, and acoustic communication for that matter, have received little attention in cavefi shes—with
studies limited to just three groups. Popper ( 1970 ) found no differences in hearing
sensitivities between cave and surface forms of the characid Astyanax mexicanus .
Both forms had sensitivities comparable to other otophysan (Actinopterygii:
Teleostei: Ostariophysi) fi shes with a threshold at 1000 Hz. The best-studied cavefi sh with respect to hearing are cave and surface ecotypes of the Atlantic Molly
( Poecilia mexicana ). Schulz-Mirbach et al. ( 2008 ) documented pronounced
D. Soares et al.
organisms remain the most poorly understood fauna on the planet and little is known
about their sense of hearing. Animals that thrive in a cave not only have evolved
cave-specifi c morphological specializations but also have selective behaviors that
allow for their survival in complete darkness. Cavefi shes (obligate cave-dwelling
fi shes) are replicate ecological and evolutionary experiments in adaptation to this
extreme environment. Because the direction of evolution is known (i.e., surface to
subterranean), the colonization of a subterranean environment affords us the opportunity to examine species that have survived similar evolutionary pressures in parallel
and converging ways.
Of the approximately 32,000 fi sh species globally, over 300 species have been
reported to live in subterranean habitats with more than 170 described species living
obligate underground with some degree of troglomorphy (features related to cave
adaptation; Proudlove 2006 , 2010 ). The phylogenetic diversity of cavefi shes representing 10 orders and 21 families distributed on every continent but Europe and
Antarctica (Fig. 1 ) and the assortment of aquatic subterranean habitats from fast
fl owing streams and waterfalls to quiet phreatic waters provide excellent examples
for the study of independent responses to subterranean environments. Many species
are monotypic and their relationships to surface forms are unknown; in other cases
cave forms are grouped together based on convergent morphologies (Niemiller and
Poulson 2010 ; Niemiller et al. 2013). However, the surface ancestors of all cavefi sh
species had to adapt to the strict constraints imposed by caves, particularly perpetual
darkness and limited energy resources. As a result, a suite of unique phenotypes
associated with subterranean adaptation has emerged, with loss of pigmentation and
eyes being the most conspicuous. Cavefi shes, however, are outcomes of not just
regressive evolution but also constructive adaptation. For example, several cavefi shes exhibit enhancement of the mechanosensory lateral line system relative to
their surface relatives (Culver and Pipan 2009 ; Soares and Niemiller 2013), and a
cave catfi sh, Astroblepus pholeter , has adapted skin-teeth to sense water fl ow
(Haspel et al. 2012 ). Despite the obvious power of using cavefi sh as a natural experiment, to date there have been few comparative studies in cavefi sh ecology and
sensory biology (Trajano 1991 , 1997 , 2001 ; Niemiller et al. 2013; Soares and
Niemiller 2013).
There is strong selection to develop and enhance non-visual sensory modalities
in subterranean habitats, with enhanced hearing ability being one possible modifi cation (see below for a review on possible lateral line enhancement). Lower auditory
thresholds and greater frequency ranges in subterranean habitats should be adaptive
for several reasons, including integrating with other non-visual senses to detect
prey, predators, or conspecifi cs. However, hearing sensitivity and range, and acoustic communication for that matter, have received little attention in cavefi shes—with
studies limited to just three groups. Popper ( 1970 ) found no differences in hearing
sensitivities between cave and surface forms of the characid Astyanax mexicanus .
Both forms had sensitivities comparable to other otophysan (Actinopterygii:
Teleostei: Ostariophysi) fi shes with a threshold at 1000 Hz. The best-studied cavefi sh with respect to hearing are cave and surface ecotypes of the Atlantic Molly
( Poecilia mexicana ). Schulz-Mirbach et al. ( 2008 ) documented pronounced
D. Soares et al.