299
1994 ), so there appears to be variation among fi shes in where hair cells are added
within the sensory epithelia.
Additionally, there is a large variation in the rate of hair cells addition during
ontogeny in the auditory maculae of sexually immature juvenile fi shes: 302 cells
per day in the European Hake, M. merluccius (Lombarte and Popper 1994 ), 167
cells per day in the saccule of cichlid Astronotus ocellatus (Popper and Hoxter
1984 ), 13 hair cells per day in the zebrafi sh Danio rerio (Lu and DeSmidt 2013 ),
and the elasmobranch Raja calvata adds 1–3 sensory hair cells per day to the macula neglecta (Corwin 1983 ). Lombarte and Popper ( 1994 ) also found signifi cant
postembryonic proliferation of hair bundles in the lagenar and utricular epithelia in
M. merluccius , although at a much-reduced rate, 47 hair cells per day in the utricle,
and 37 hair cells per day in the lagena. The only study to examine hair bundle orientation found no changes in orientation patterns or percentage of area occupied by
different orientation groups in all three otolithic epithelia during ontogeny in the
European hake (Lombarte and Popper 1994 ).
As fi sh grow, so do the inner ears and the sensory maculae. In the zebrafi sh,
D. rerio , the area of the otic vesicle as well as the area of saccular and utricular
otoliths grow linearly, although the area of the saccular otolith grows at a greater
rate than that of the utricular otolith (Lu and DeSmidt 2013 ). Lombarte and Popper
( 1994 ) found in M. merluccius that the utricular and lagenar epithelial areas grow at
a slower rate than that of the saccular epithelial area, which grows isometrically
with total length (TL). The shape of the sensory macula may or may not change as
fi sh grow. In the ray, R. clavata , the macula neglecta elongates in the direction of the
long axis of the posterior canal duct as the elasmobranch grows (Corwin 1983 ). This
is in contrast to the zebrafi sh, which does not change shape during growth and
development (Lu and DeSmidt 2013 ).
Another area where there seems to be variation in developmental patterns of the
fi sh inner ear is hair bundle density. In the saccule of both A. ocellatus and M. merluccius hair bundle density decreased with age/size even though the total number of
hair cells increased dramatically (Popper and Hoxter 1984 ; Lombarte and Popper
1994 ). In M. merluccius , the hair bundle density in the lagenar and utricular epithelia also decreased with size (Lombarte and Popper 1994 ). In D. rerio , hair bundle
density in the saccule did not change in juvenile fi sh aged 3–18 months posthatch
(Higgs et al. 2001 ), however during the fi rst week of posthatched growth the density
of hair cells increased linearly (Lu and DeSmidt 2013 ). It is possible during early
postembryonic development that hair bundles rapidly increase in numbers relative
to the growth of the sensory maculae causing an increase in density, which slows
and reverses later during development due to a decrease in hair cell density as the
area of the auditory macula grows and expands. More work is needed in other fi shes
over a broader range of developmental time periods to determine if this is the case.
Another area of the peripheral auditory system of fi shes where ontogenetic plasticity has been demonstrated is the eighth cranial nerve. Relatively few data exists
examining ontogenetic changes in the auditory nerve morphology, however Corwin
( 1983 ) found that the number of nerves that innervate the macula neglecta do not
change in the skate, Raja clavata . In contrast, Barber et al. ( 1985 ) found that axon
Development of Structure and Sensitivity of the Fish Inner Ear
1994 ), so there appears to be variation among fi shes in where hair cells are added
within the sensory epithelia.
Additionally, there is a large variation in the rate of hair cells addition during
ontogeny in the auditory maculae of sexually immature juvenile fi shes: 302 cells
per day in the European Hake, M. merluccius (Lombarte and Popper 1994 ), 167
cells per day in the saccule of cichlid Astronotus ocellatus (Popper and Hoxter
1984 ), 13 hair cells per day in the zebrafi sh Danio rerio (Lu and DeSmidt 2013 ),
and the elasmobranch Raja calvata adds 1–3 sensory hair cells per day to the macula neglecta (Corwin 1983 ). Lombarte and Popper ( 1994 ) also found signifi cant
postembryonic proliferation of hair bundles in the lagenar and utricular epithelia in
M. merluccius , although at a much-reduced rate, 47 hair cells per day in the utricle,
and 37 hair cells per day in the lagena. The only study to examine hair bundle orientation found no changes in orientation patterns or percentage of area occupied by
different orientation groups in all three otolithic epithelia during ontogeny in the
European hake (Lombarte and Popper 1994 ).
As fi sh grow, so do the inner ears and the sensory maculae. In the zebrafi sh,
D. rerio , the area of the otic vesicle as well as the area of saccular and utricular
otoliths grow linearly, although the area of the saccular otolith grows at a greater
rate than that of the utricular otolith (Lu and DeSmidt 2013 ). Lombarte and Popper
( 1994 ) found in M. merluccius that the utricular and lagenar epithelial areas grow at
a slower rate than that of the saccular epithelial area, which grows isometrically
with total length (TL). The shape of the sensory macula may or may not change as
fi sh grow. In the ray, R. clavata , the macula neglecta elongates in the direction of the
long axis of the posterior canal duct as the elasmobranch grows (Corwin 1983 ). This
is in contrast to the zebrafi sh, which does not change shape during growth and
development (Lu and DeSmidt 2013 ).
Another area where there seems to be variation in developmental patterns of the
fi sh inner ear is hair bundle density. In the saccule of both A. ocellatus and M. merluccius hair bundle density decreased with age/size even though the total number of
hair cells increased dramatically (Popper and Hoxter 1984 ; Lombarte and Popper
1994 ). In M. merluccius , the hair bundle density in the lagenar and utricular epithelia also decreased with size (Lombarte and Popper 1994 ). In D. rerio , hair bundle
density in the saccule did not change in juvenile fi sh aged 3–18 months posthatch
(Higgs et al. 2001 ), however during the fi rst week of posthatched growth the density
of hair cells increased linearly (Lu and DeSmidt 2013 ). It is possible during early
postembryonic development that hair bundles rapidly increase in numbers relative
to the growth of the sensory maculae causing an increase in density, which slows
and reverses later during development due to a decrease in hair cell density as the
area of the auditory macula grows and expands. More work is needed in other fi shes
over a broader range of developmental time periods to determine if this is the case.
Another area of the peripheral auditory system of fi shes where ontogenetic plasticity has been demonstrated is the eighth cranial nerve. Relatively few data exists
examining ontogenetic changes in the auditory nerve morphology, however Corwin
( 1983 ) found that the number of nerves that innervate the macula neglecta do not
change in the skate, Raja clavata . In contrast, Barber et al. ( 1985 ) found that axon
Development of Structure and Sensitivity of the Fish Inner Ear