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important role in shaping hearing abilities and the corresponding morphological
structures (ear, ancillary auditory structures) (Fay 2011 ; Ladich 2014a , b ).
Ecoacoustical hypotheses have been extensively presented and discussed elsewhere
(Rogers and Cox 1988 ; Schellart and Popper 1992 ; Ladich 2014a , b ) and, as this is
not the focus of our review, we refer the reader to those articles. Notwithstanding
the fundamental infl uence of the acoustical environment on inner ear and hearing
evolution, other factors must also be taken into account to get a more complete picture of how the auditory periphery evolved. In the following, we briefl y discuss the
role of constraints other than ecological/ecoacoustical ones that potentially acted on
the evolution of the inner ear.
Spatial constraints within the ear might, for example, account for modifi ed macula shapes or 3D curvature of maculae, as seen, for example, in melamphaids and
cichlids. Melamphaids are characterized by a bilobate striola region (Table 2 ) that
was assumed to be a consequence of hampered macula growth of the striola in the
region where the ampulla of the anterior semicircular canal opens into the utricle
(Deng et al. 2013 ). A similar reason can be envisioned for the strong 3D curvatures
in the macula lagenae and macula utriculi in the cichlid Etroplus maculatus .
Maculae are signifi cantly enlarged in this species when compared to other cichlid
members without anterior swim bladder extensions such as Hemichromis guttatus
and Steatocranus tinanti (Schulz-Mirbach et al. 2014 ). Enlargement of the anterior
arm of the macula lagenae in 2D is limited in anterodorsal direction by the opening
of the lagena into the saccule. Accordingly, the macula expansion may thus give
way in anterior direction following the curvature of the anterior lagenar wall, which
in turn results in a 3D curved anterior macula portion (Schulz-Mirbach et al. 2014 ).
An interesting topic to tackle in future research would be the question of how
much phenotypic integration and modularity (inner ears and swim bladder are then
seen as separate modules) (see, e.g., Wainwright 2007 ; Klingenberg 2008 ) affect the
evolution of inner ear diversity and ancillary auditory structures. A study on the
interactions between the modules brain, skull, jaw region, and feeding strategy in
cichlids revealed a strong correlation between brain size and morphology with head
morphology; the conclusion was that either spatial constraints of the skull act on
brain size and morphology or that evolution of larger brains demands coevolution of
brain and head (Tsuboi et al. 2014 ). Accordingly, trait coupling in terms of earswim bladder/gas bladder connection of initially separated modules such as the ear
and the swim bladder should lead to coevolution of these structures. Coevolution,
however, may restrict the diversity of possible ear and/or swim bladder modifi cations (see, e.g., Frédérich et al. 2014 ). This would potentially explain why only a
limited number of different “modifi ed” orientation patterns on the macula sacculi
evolved in taxonomically unrelated species that possess ancillary auditory structures (Popper and Coombs 1982 ).
Developmental and genetic constraints may also infl uence inner ear evolution.
Some of the genes responsible of inner ear development are also involved in fundamental processes during ontogenetic development. Several mutants in the zebrafi sh
display mutations in the inner ear that are associated with the absence of the swim
bladder, brain and heart defects or skull deformations (Malicki et al. 1996 ; Schibler
Diversity of Inner Ears in Fishes: Possible Contribution Towards Hearing…
important role in shaping hearing abilities and the corresponding morphological
structures (ear, ancillary auditory structures) (Fay 2011 ; Ladich 2014a , b ).
Ecoacoustical hypotheses have been extensively presented and discussed elsewhere
(Rogers and Cox 1988 ; Schellart and Popper 1992 ; Ladich 2014a , b ) and, as this is
not the focus of our review, we refer the reader to those articles. Notwithstanding
the fundamental infl uence of the acoustical environment on inner ear and hearing
evolution, other factors must also be taken into account to get a more complete picture of how the auditory periphery evolved. In the following, we briefl y discuss the
role of constraints other than ecological/ecoacoustical ones that potentially acted on
the evolution of the inner ear.
Spatial constraints within the ear might, for example, account for modifi ed macula shapes or 3D curvature of maculae, as seen, for example, in melamphaids and
cichlids. Melamphaids are characterized by a bilobate striola region (Table 2 ) that
was assumed to be a consequence of hampered macula growth of the striola in the
region where the ampulla of the anterior semicircular canal opens into the utricle
(Deng et al. 2013 ). A similar reason can be envisioned for the strong 3D curvatures
in the macula lagenae and macula utriculi in the cichlid Etroplus maculatus .
Maculae are signifi cantly enlarged in this species when compared to other cichlid
members without anterior swim bladder extensions such as Hemichromis guttatus
and Steatocranus tinanti (Schulz-Mirbach et al. 2014 ). Enlargement of the anterior
arm of the macula lagenae in 2D is limited in anterodorsal direction by the opening
of the lagena into the saccule. Accordingly, the macula expansion may thus give
way in anterior direction following the curvature of the anterior lagenar wall, which
in turn results in a 3D curved anterior macula portion (Schulz-Mirbach et al. 2014 ).
An interesting topic to tackle in future research would be the question of how
much phenotypic integration and modularity (inner ears and swim bladder are then
seen as separate modules) (see, e.g., Wainwright 2007 ; Klingenberg 2008 ) affect the
evolution of inner ear diversity and ancillary auditory structures. A study on the
interactions between the modules brain, skull, jaw region, and feeding strategy in
cichlids revealed a strong correlation between brain size and morphology with head
morphology; the conclusion was that either spatial constraints of the skull act on
brain size and morphology or that evolution of larger brains demands coevolution of
brain and head (Tsuboi et al. 2014 ). Accordingly, trait coupling in terms of earswim bladder/gas bladder connection of initially separated modules such as the ear
and the swim bladder should lead to coevolution of these structures. Coevolution,
however, may restrict the diversity of possible ear and/or swim bladder modifi cations (see, e.g., Frédérich et al. 2014 ). This would potentially explain why only a
limited number of different “modifi ed” orientation patterns on the macula sacculi
evolved in taxonomically unrelated species that possess ancillary auditory structures (Popper and Coombs 1982 ).
Developmental and genetic constraints may also infl uence inner ear evolution.
Some of the genes responsible of inner ear development are also involved in fundamental processes during ontogenetic development. Several mutants in the zebrafi sh
display mutations in the inner ear that are associated with the absence of the swim
bladder, brain and heart defects or skull deformations (Malicki et al. 1996 ; Schibler
Diversity of Inner Ears in Fishes: Possible Contribution Towards Hearing…