300
number, total axon area, and hair cell number of the macula neglecta increased
linearly with size/age of the skate, R. ocellata , and that there were signifi cant differences in hair cell numbers of the macula neglecta in females and males for any given
size of skate with females having a greater number of total hair cells. In the teleost
A. ocellatus , the number of ganglion cells innervating the saccule increase 4.8-fold
(Popper and Hoxter 1984 ). The rate of hair cell addition drastically outpaces nerve
growth in Astronotus ocellatus and in both studies the disproportionate addition of
hair cells results in an increase in neural convergence ratio of hair cells to auditory
afferents (Corwin 1983 ; Popper and Hoxter 1984 ). By retrograde fi lling of the nerve
axons using cobalt, Corwin ( 1983 ) found that each nerve innervates several hair
cells with terminals that branch over a small area, with the greatest arborization in
the center of the macula and lesser arborization at the periphery. These hair cells
appeared to innervate by only one auditory afferent neuron (Corwin 1983 ). Corwin
( 1983 ) also found that as the ray grows, the axons increase in diameter and terminal
fi eld size.
2.4 Ontogenetic Structure-Function Relationships
in the Fish Auditory System
The functional signifi cance of many of the observed morphological changes in the
fi sh auditory system during development is not known because relatively few studies have related quantifi ed morphological changes to some measure of auditory sensitivity. It is likely, as shown in other vertebrate groups, that changes in some aspect
of sensory morphology will be correlated with functional and/or sensitivity changes
of the auditory system (Weiss et al. 1976 ; Lewis et al. 1985 ). In the elasmobranch
Raja clavata , Corwin ( 1983 ) found a 500-fold increase in auditory nerve sensitivity
that is likely due to the addition of sensory hair cells. This increase in sensory hair
cells was not accompanied by a corresponding increase in auditory nerve innervation thus leading Corwin ( 1983 ) to postulate that the observed increase in auditory
sensitivity resulted from the increased convergence ratio of sensory hair cells to
auditory afferent neurons. In teleost fi shes, Lu and DeSmidt ( 2013 ) found an
increase in the microphonic response and sensitivity of the saccule in the zebrafi sh,
Danio rerio , which correlated with increases in the number and density of saccular
hair cells. In contrast, Higgs et al. ( 2001 ) found no changes in hearing sensitivity or
bandwidth in D. rerio that correlated with hair cell addition. It is important to note
that Lu and DeSmidt ( 2013 ) measured hearing sensitivity of the hair cells in the
saccule, the end organ where the morphological changes were observed, whereas
Higgs et al. ( 2001 ) measured hearing sensitivity using the auditory evoked potential
(AEP) recording technique, which measures overall neural responses potentially
including higher-order brain regions of the central auditory system (see next section).
Lu and DeSmidt ( 2013 ) also used fi sh during an earlier stage of zebrafi sh development than Higgs et al. ( 2001 ), which may have allowed them to capture a period of
R.O. Vasconcelos et al.
number, total axon area, and hair cell number of the macula neglecta increased
linearly with size/age of the skate, R. ocellata , and that there were signifi cant differences in hair cell numbers of the macula neglecta in females and males for any given
size of skate with females having a greater number of total hair cells. In the teleost
A. ocellatus , the number of ganglion cells innervating the saccule increase 4.8-fold
(Popper and Hoxter 1984 ). The rate of hair cell addition drastically outpaces nerve
growth in Astronotus ocellatus and in both studies the disproportionate addition of
hair cells results in an increase in neural convergence ratio of hair cells to auditory
afferents (Corwin 1983 ; Popper and Hoxter 1984 ). By retrograde fi lling of the nerve
axons using cobalt, Corwin ( 1983 ) found that each nerve innervates several hair
cells with terminals that branch over a small area, with the greatest arborization in
the center of the macula and lesser arborization at the periphery. These hair cells
appeared to innervate by only one auditory afferent neuron (Corwin 1983 ). Corwin
( 1983 ) also found that as the ray grows, the axons increase in diameter and terminal
fi eld size.
2.4 Ontogenetic Structure-Function Relationships
in the Fish Auditory System
The functional signifi cance of many of the observed morphological changes in the
fi sh auditory system during development is not known because relatively few studies have related quantifi ed morphological changes to some measure of auditory sensitivity. It is likely, as shown in other vertebrate groups, that changes in some aspect
of sensory morphology will be correlated with functional and/or sensitivity changes
of the auditory system (Weiss et al. 1976 ; Lewis et al. 1985 ). In the elasmobranch
Raja clavata , Corwin ( 1983 ) found a 500-fold increase in auditory nerve sensitivity
that is likely due to the addition of sensory hair cells. This increase in sensory hair
cells was not accompanied by a corresponding increase in auditory nerve innervation thus leading Corwin ( 1983 ) to postulate that the observed increase in auditory
sensitivity resulted from the increased convergence ratio of sensory hair cells to
auditory afferent neurons. In teleost fi shes, Lu and DeSmidt ( 2013 ) found an
increase in the microphonic response and sensitivity of the saccule in the zebrafi sh,
Danio rerio , which correlated with increases in the number and density of saccular
hair cells. In contrast, Higgs et al. ( 2001 ) found no changes in hearing sensitivity or
bandwidth in D. rerio that correlated with hair cell addition. It is important to note
that Lu and DeSmidt ( 2013 ) measured hearing sensitivity of the hair cells in the
saccule, the end organ where the morphological changes were observed, whereas
Higgs et al. ( 2001 ) measured hearing sensitivity using the auditory evoked potential
(AEP) recording technique, which measures overall neural responses potentially
including higher-order brain regions of the central auditory system (see next section).
Lu and DeSmidt ( 2013 ) also used fi sh during an earlier stage of zebrafi sh development than Higgs et al. ( 2001 ), which may have allowed them to capture a period of
R.O. Vasconcelos et al.