313
4 Conclusions and Future Directions
Fish represent the largest extant group of vertebrates and display the greatest diversity
of structures of the vertebrate auditory system. Thus, this taxon has the potential to
provide valuable insights into the ecology and evolution of the vertebrate auditory
system. However, despite the greater than 30,000 known fi sh species only a relatively small number has been examined in terms of ontogenetic development of
structure and sensitivity of the auditory system. Thus, more studies should be performed on representatives of diverse species and families with different anatomical
hearing specializations. The remarkable diversity of inner ear morphologies and
accessory hearing structures in fi shes should provide a rich source for future comparisons to gain insights on the selective pressures that have shaped the evolution of
fi sh auditory systems.
There remain many important questions and areas of research that should be
addressed in future work, several of which are briefl y detailed below:
1. Although the saccule is the main auditory end organ in most teleost species,
more physiology studies are needed for the other putative auditory end organs,
the lagena and utricle, and their characterization in terms of ontogenetic changes
in morphology, sensitivity, and contribution to the development of hearing in
fi shes. Compared to the numerous studies for the fi sh saccule, there are only a
limited number of studies for the lagena and utricle regarding their potential
contribution to hearing during ontogeny (Higgs et al. 2004 ; Webb et al. 2012 ;
Inoue et al. 2013 ).
2. Another topic that needs further study concerns the ontogenetic morphological
changes in hair cell addition and bundle density in the three putative auditory end
organs (saccule, lagena, and utricle). Questions that should be addressed include:
(1) Are there differences in hair cell addition and bundle density in the saccule,
lagena, and utricle during different stages of development? (2) How do changes
in hair cell bundle density relate to the auditory sensitivity of the end organ? (3)
In addition to ontogenetic changes, are there seasonal differences in the proliferation and density of hair cells in these auditory end organs across different
stages of the reproductive cycle? Recently a study by Coffi n et al. ( 2012 ) reported
seasonal changes in hair cell density in the saccules of female plainfi n midshipman fi sh ( Porichthys notatus ) that did not occur in the other two end organs
(lagena or utricle). The saccular- specifi c changes in hair cell density were correlated with reproductive state-dependent changes in auditory saccular sensitivity of female midshipman. Additional studies should be performed in other vocal
and non-vocal species to determine how widespread this phenomenon is among
fi shes.
3. Very few studies have examined the concurrent development of the auditory system and sound production in fi shes in the context of social acoustic communication. In order to better understand how the vocal motor system develops together
Development of Structure and Sensitivity of the Fish Inner Ear
4 Conclusions and Future Directions
Fish represent the largest extant group of vertebrates and display the greatest diversity
of structures of the vertebrate auditory system. Thus, this taxon has the potential to
provide valuable insights into the ecology and evolution of the vertebrate auditory
system. However, despite the greater than 30,000 known fi sh species only a relatively small number has been examined in terms of ontogenetic development of
structure and sensitivity of the auditory system. Thus, more studies should be performed on representatives of diverse species and families with different anatomical
hearing specializations. The remarkable diversity of inner ear morphologies and
accessory hearing structures in fi shes should provide a rich source for future comparisons to gain insights on the selective pressures that have shaped the evolution of
fi sh auditory systems.
There remain many important questions and areas of research that should be
addressed in future work, several of which are briefl y detailed below:
1. Although the saccule is the main auditory end organ in most teleost species,
more physiology studies are needed for the other putative auditory end organs,
the lagena and utricle, and their characterization in terms of ontogenetic changes
in morphology, sensitivity, and contribution to the development of hearing in
fi shes. Compared to the numerous studies for the fi sh saccule, there are only a
limited number of studies for the lagena and utricle regarding their potential
contribution to hearing during ontogeny (Higgs et al. 2004 ; Webb et al. 2012 ;
Inoue et al. 2013 ).
2. Another topic that needs further study concerns the ontogenetic morphological
changes in hair cell addition and bundle density in the three putative auditory end
organs (saccule, lagena, and utricle). Questions that should be addressed include:
(1) Are there differences in hair cell addition and bundle density in the saccule,
lagena, and utricle during different stages of development? (2) How do changes
in hair cell bundle density relate to the auditory sensitivity of the end organ? (3)
In addition to ontogenetic changes, are there seasonal differences in the proliferation and density of hair cells in these auditory end organs across different
stages of the reproductive cycle? Recently a study by Coffi n et al. ( 2012 ) reported
seasonal changes in hair cell density in the saccules of female plainfi n midshipman fi sh ( Porichthys notatus ) that did not occur in the other two end organs
(lagena or utricle). The saccular- specifi c changes in hair cell density were correlated with reproductive state-dependent changes in auditory saccular sensitivity of female midshipman. Additional studies should be performed in other vocal
and non-vocal species to determine how widespread this phenomenon is among
fi shes.
3. Very few studies have examined the concurrent development of the auditory system and sound production in fi shes in the context of social acoustic communication. In order to better understand how the vocal motor system develops together
Development of Structure and Sensitivity of the Fish Inner Ear