66
Chaetodon species are known to sometimes make quick excursions along reef walls
from 2 to 8 m in only 5–15 s during which they experience rapid pressure changes
close to 1 atm (Tricas, unpublished observations). The ability to precisely control
gas pressure in the swim bladder would also enable fi shes to consistently and accurately monitor sounds produced by mates, conspecifi cs, and competitors (see Sect.
3 ). The fi ne control of buoyancy may also facilitate precise maneuvers made with
paired fi ns during paired swimming and interactions, which is when sounds are
produced (Sect. 3 ). Finally, swim bladder shape and the presence of anterior swim
bladder horns may alter the center of gravity or center of buoyancy in Chaetodon .
This may help to control posture, especially when feeding on benthic invertebrate
prey (Woods 2006 ), an ability also suggested in other fi shes (Parmentier et al. 2011 ).
Thus, the evolution of swim bladder morphology is likely the result of responses to
a range of selective pressures involved with sound production, hearing, locomotion,
and/or feeding behavior.
2.3 The Ear and Its Proximity to the Swim Bladder
Prior to the discovery of the LC, the structure and function of swim bladder horns
defi ning the otophysic connection was explored to some extent in holocentrid fi shes,
which provide a useful context for interpreting the LC in Chaetodon . The holocentrid, Myripristis kuntee , has robust anterior swim bladder horns that make contact
with a thinned otic capsule wall, and a saccular macula (sensory epithelium) that is
modifi ed in shape relative to that in Sargocentron (another holocentrid) and in other
percormorph fi shes that lack an otophysic connection (Nelson 1955 ; Popper 1977 ;
Fig. 5b ). Myripristis also has higher sensitivity to sound stimuli over a broader frequency range, when compared to Sargocentron (Coombs and Popper 1979 ; Fig. 5a ).
In his analysis of the ears of Hawaiian fi shes, Popper ( 1977 ) described the sensory
epithelium of the sacculus and lagena in one chaetodontid, C. miliaris , and found
that it was similar to that in other teleosts that lack an otophysic connection.
However, we now know that C. miliaris has an indirect LC (see Sect. 2.1 ) and is not
representative of all chaetodotid species. Thus, Webb et al. ( 2010 ) examined ear
morphology in Chaetodon species with different LC types and in Forcipiger fl avissimus in light of the hypothesized acoustic functions of the LC and the known correlation of swim bladder and LC morphology among Chaetodon species (Sect. 2.2 ).
Ear morphology was found to be similar in all chaetodontid species examined. The
otic capsule sits ventral to the hindbrain, the left and right ears share a common wall
in the midline of the otic capsule (Figs. 1a and 2c ), and the shape of the sensory
maculae of the lagenar, saccular, and utricular otolithic organs was similar in
Chaetodon and Forcipiger , and in other perciform fi shes that lack swim bladder
horns or an otophysic connection (discussed in Webb et al. 2010 ). Thus, swim bladder morphology in butterfl yfi shes is in direct contrast to that in holocentrids, which
demonstrates a correlation between the presence of swim bladder horns, modifi ed
T.C. Tricas and J.F. Webb
Chaetodon species are known to sometimes make quick excursions along reef walls
from 2 to 8 m in only 5–15 s during which they experience rapid pressure changes
close to 1 atm (Tricas, unpublished observations). The ability to precisely control
gas pressure in the swim bladder would also enable fi shes to consistently and accurately monitor sounds produced by mates, conspecifi cs, and competitors (see Sect.
3 ). The fi ne control of buoyancy may also facilitate precise maneuvers made with
paired fi ns during paired swimming and interactions, which is when sounds are
produced (Sect. 3 ). Finally, swim bladder shape and the presence of anterior swim
bladder horns may alter the center of gravity or center of buoyancy in Chaetodon .
This may help to control posture, especially when feeding on benthic invertebrate
prey (Woods 2006 ), an ability also suggested in other fi shes (Parmentier et al. 2011 ).
Thus, the evolution of swim bladder morphology is likely the result of responses to
a range of selective pressures involved with sound production, hearing, locomotion,
and/or feeding behavior.
2.3 The Ear and Its Proximity to the Swim Bladder
Prior to the discovery of the LC, the structure and function of swim bladder horns
defi ning the otophysic connection was explored to some extent in holocentrid fi shes,
which provide a useful context for interpreting the LC in Chaetodon . The holocentrid, Myripristis kuntee , has robust anterior swim bladder horns that make contact
with a thinned otic capsule wall, and a saccular macula (sensory epithelium) that is
modifi ed in shape relative to that in Sargocentron (another holocentrid) and in other
percormorph fi shes that lack an otophysic connection (Nelson 1955 ; Popper 1977 ;
Fig. 5b ). Myripristis also has higher sensitivity to sound stimuli over a broader frequency range, when compared to Sargocentron (Coombs and Popper 1979 ; Fig. 5a ).
In his analysis of the ears of Hawaiian fi shes, Popper ( 1977 ) described the sensory
epithelium of the sacculus and lagena in one chaetodontid, C. miliaris , and found
that it was similar to that in other teleosts that lack an otophysic connection.
However, we now know that C. miliaris has an indirect LC (see Sect. 2.1 ) and is not
representative of all chaetodotid species. Thus, Webb et al. ( 2010 ) examined ear
morphology in Chaetodon species with different LC types and in Forcipiger fl avissimus in light of the hypothesized acoustic functions of the LC and the known correlation of swim bladder and LC morphology among Chaetodon species (Sect. 2.2 ).
Ear morphology was found to be similar in all chaetodontid species examined. The
otic capsule sits ventral to the hindbrain, the left and right ears share a common wall
in the midline of the otic capsule (Figs. 1a and 2c ), and the shape of the sensory
maculae of the lagenar, saccular, and utricular otolithic organs was similar in
Chaetodon and Forcipiger , and in other perciform fi shes that lack swim bladder
horns or an otophysic connection (discussed in Webb et al. 2010 ). Thus, swim bladder morphology in butterfl yfi shes is in direct contrast to that in holocentrids, which
demonstrates a correlation between the presence of swim bladder horns, modifi ed
T.C. Tricas and J.F. Webb