396
caused by damage to the cochlea, particularly its hair cells, or the auditory nerve
(Pickles 1996 ).
Fishes do not have an external or middle ear, but some species possess anatomical specializations that are functionally similar to middle ear bones. For example,
fi shes of the Superorder Ostariophysi have a Weberian apparatus, which is composed of modifi ed vertebral bones (i.e., ossicles) and associated ligaments that connect the swim bladder to the inner ear. These structures allow the transmission of
sound-induced vibrations from the swim bladder to the inner ear (Weber 1820 ).
Although the Weberian apparatus is associated with more sensitive hearing and
broader frequency detection (Higgs et al. 2003 ; Ladich and Wysocki 2003 ; Lechner
et al. 2011 ), conductive hearing loss in fi shes has only been shown in a few studies.
Bang et al. ( 2002 ) found that approximately 1 % of the zebrafi sh ( Danio rerio ) that
were exposed to a 400 Hz tone did not exhibit an escape refl ex. Those that did not
respond had abnormalities in conductive elements of the peripheral auditory system, e.g., either the swim bladder or Weberian ossicles. This shows that hearing loss
can occur in fi shes without inner ear hair cell damage, but it is unknown how common such conductive system defi cits are in nature.
In an attempt to understand the roles of the swim bladder and Weberian apparatus on hearing in ostariophysan fi shes, researchers have removed the tripus (one of
the Weberian ossicles; Ladich and Wysocki 2003 ) or defl ated the swim bladder (Yan
et al. 2000 ). Both manipulations signifi cantly reduced hearing sensitivity. Similarly,
the removal of gas from gas-fi lled chambers near the ears of non-ostariophysan species, such as the suprabranchial chambers of gouramis or the otic gasbladder of
mormyrids, also signifi cantly decreased hearing sensitivity (Yan 1998 ; Yan and
Curtsinger 2000 ). Thus, underwater sound impulses that could potentially rupture
these gas-fi lled chambers may be able to cause conductive hearing loss in fi shes.
Unlike conductive hearing loss, there are many examples of sensorineural hearing loss in fi shes. To date, sensorineural research has focused primarily on damage
to the sensory hair cells. This review will fi rst discuss what is known about congenital and acquired causes of hearing and vestibular sensorineural loss in fi shes before
covering the consequences of and recovery from hair cell damage.
3 Congenital Hearing Loss
In humans, congenital hearing loss can result from non-genetic factors such as maternal physiological status, infections/diseases, and trauma during pregnancy such as
toxemia or anoxia, and from genetic factors, e.g., Waardenburg, Usher, and Down
syndromes (ASHA 2014 ). Congenital hearing loss from genetic factors in fi shes has
also been reported, although no research has yet been performed examining nongenetic, i.e., maternal factors. In large-scale genetic screens in zebrafi sh, many mutations affecting the development of the inner ear have been identifi ed (Malicki et al.
1996 ; Nicolson et al. 1998 ; Whitfi eld et al. 2002 , 2005 ; Nicolson 2005 ). For example, the mariner phenotype of the circler zebrafi sh mutant is defective in myosin
M.E. Smith and J.D. Monroe
caused by damage to the cochlea, particularly its hair cells, or the auditory nerve
(Pickles 1996 ).
Fishes do not have an external or middle ear, but some species possess anatomical specializations that are functionally similar to middle ear bones. For example,
fi shes of the Superorder Ostariophysi have a Weberian apparatus, which is composed of modifi ed vertebral bones (i.e., ossicles) and associated ligaments that connect the swim bladder to the inner ear. These structures allow the transmission of
sound-induced vibrations from the swim bladder to the inner ear (Weber 1820 ).
Although the Weberian apparatus is associated with more sensitive hearing and
broader frequency detection (Higgs et al. 2003 ; Ladich and Wysocki 2003 ; Lechner
et al. 2011 ), conductive hearing loss in fi shes has only been shown in a few studies.
Bang et al. ( 2002 ) found that approximately 1 % of the zebrafi sh ( Danio rerio ) that
were exposed to a 400 Hz tone did not exhibit an escape refl ex. Those that did not
respond had abnormalities in conductive elements of the peripheral auditory system, e.g., either the swim bladder or Weberian ossicles. This shows that hearing loss
can occur in fi shes without inner ear hair cell damage, but it is unknown how common such conductive system defi cits are in nature.
In an attempt to understand the roles of the swim bladder and Weberian apparatus on hearing in ostariophysan fi shes, researchers have removed the tripus (one of
the Weberian ossicles; Ladich and Wysocki 2003 ) or defl ated the swim bladder (Yan
et al. 2000 ). Both manipulations signifi cantly reduced hearing sensitivity. Similarly,
the removal of gas from gas-fi lled chambers near the ears of non-ostariophysan species, such as the suprabranchial chambers of gouramis or the otic gasbladder of
mormyrids, also signifi cantly decreased hearing sensitivity (Yan 1998 ; Yan and
Curtsinger 2000 ). Thus, underwater sound impulses that could potentially rupture
these gas-fi lled chambers may be able to cause conductive hearing loss in fi shes.
Unlike conductive hearing loss, there are many examples of sensorineural hearing loss in fi shes. To date, sensorineural research has focused primarily on damage
to the sensory hair cells. This review will fi rst discuss what is known about congenital and acquired causes of hearing and vestibular sensorineural loss in fi shes before
covering the consequences of and recovery from hair cell damage.
3 Congenital Hearing Loss
In humans, congenital hearing loss can result from non-genetic factors such as maternal physiological status, infections/diseases, and trauma during pregnancy such as
toxemia or anoxia, and from genetic factors, e.g., Waardenburg, Usher, and Down
syndromes (ASHA 2014 ). Congenital hearing loss from genetic factors in fi shes has
also been reported, although no research has yet been performed examining nongenetic, i.e., maternal factors. In large-scale genetic screens in zebrafi sh, many mutations affecting the development of the inner ear have been identifi ed (Malicki et al.
1996 ; Nicolson et al. 1998 ; Whitfi eld et al. 2002 , 2005 ; Nicolson 2005 ). For example, the mariner phenotype of the circler zebrafi sh mutant is defective in myosin
M.E. Smith and J.D. Monroe