77
production of sounds such as the tail slap, head bob or body pulse in Chaetodon , and
the head bob or anal fi n pulse in Forcipiger (see Sect. 3 ), a local hydrodynamic fl ow
is generated by the displacement of the adjacent water mass. Directional motions of
the body produce a polar hydrodynamic fl ow fi eld that could generate a whole body
acceleration of a nearby fi sh and thus stimulate its ear depending on the amplitude,
direction, and distance of the acoustic source. At very short distances of a few body
lengths, the hydrodynamic fl ow also produces a pressure gradient across the surface
of the receiver fi sh that may stimulate the lateral line system to provide additional
information about stimulus direction and intensity (Schellart and Popper 1992 ;
Hawkins 1993 ; Webb 1998 ; Braun and Coombs 2010 ). At greater distances, sound
pressure waves penetrate the body of a receiving fi sh and may set into motion the
walls of the gas-fi lled swim bladder and swim bladder horns of Chaeotodon , which
secondarily induce local particle motions in the ear (Fletcher and Crawford 2001 ;
Schellart and Popper 1992 ; Tricas and Boyle 2015b ) and presumably in the lateral
line canals in the vicinity of the LC (Webb et al. 2006 ).
4.2 Stimulation of the Ear in Chaetodon and the Effect
of the Swim Bladder Horns
Several lines of evidence from data obtained in the laboratory by the auditory
evoked potential (AEP) technique (which estimates hearing sensitivity thresholds to
short tone stimuli) indicate that all butterfl yfi shes species tested to date are sensitive
to the hydrodynamic fl ow component (particle acceleration) of an acoustic fi eld,
and that the auditory sensitivity of Chaetodon is enhanced by coincident sound
pressure stimuli that are mediated by the gas-fi lled swim bladder horns (Tricas and
Boyle 2015b ). Sensitivity to hydrodynamic particle acceleration is indicated for
Forcipiger (which lacks swim bladder horns) by higher stimulus thresholds at all
frequencies compared to that for all tested Chaetodon species, and these curves
converge at the lowest stimulus frequency of 100 Hz (Fig. 9 ). The best frequency
sensitivity of Forcipiger to particle acceleration was at the lowest test frequency of
100 Hz (Fig. 9b ), which is predicted for particle acceleration sensitive species (see
Ladich and Fay 2013 for discussion). None of the hearing thresholds for Forcipiger
increased (or changed) following defl ation of its gas-fi lled swim bladder, which
would be required for the transduction of sound pressure stimuli to stimulate the ear
(Fig. 10a ). A relatively low absolute sensitivity to sound pressure for all butterfl yfi shes is indicated by their apparently much higher AEP thresholds compared to
species with anatomical specializations for reception of sound pressure stimuli such
as the Weberian apparatus (Kenyon et al. 1998 ; Ladich 1999 ; Amoser and Ladich
2005 ; Lechner and Ladich 2008 ), anterior swim bladder horns and otic bullae, or a
suprabranchial organ (Ladich and Yan 1998 ). Further, the lowest particle acceleration threshold levels in this low frequency band for butterfl yfi shes ranged from
Acoustic Communication in Butterfl yfi shes…
production of sounds such as the tail slap, head bob or body pulse in Chaetodon , and
the head bob or anal fi n pulse in Forcipiger (see Sect. 3 ), a local hydrodynamic fl ow
is generated by the displacement of the adjacent water mass. Directional motions of
the body produce a polar hydrodynamic fl ow fi eld that could generate a whole body
acceleration of a nearby fi sh and thus stimulate its ear depending on the amplitude,
direction, and distance of the acoustic source. At very short distances of a few body
lengths, the hydrodynamic fl ow also produces a pressure gradient across the surface
of the receiver fi sh that may stimulate the lateral line system to provide additional
information about stimulus direction and intensity (Schellart and Popper 1992 ;
Hawkins 1993 ; Webb 1998 ; Braun and Coombs 2010 ). At greater distances, sound
pressure waves penetrate the body of a receiving fi sh and may set into motion the
walls of the gas-fi lled swim bladder and swim bladder horns of Chaeotodon , which
secondarily induce local particle motions in the ear (Fletcher and Crawford 2001 ;
Schellart and Popper 1992 ; Tricas and Boyle 2015b ) and presumably in the lateral
line canals in the vicinity of the LC (Webb et al. 2006 ).
4.2 Stimulation of the Ear in Chaetodon and the Effect
of the Swim Bladder Horns
Several lines of evidence from data obtained in the laboratory by the auditory
evoked potential (AEP) technique (which estimates hearing sensitivity thresholds to
short tone stimuli) indicate that all butterfl yfi shes species tested to date are sensitive
to the hydrodynamic fl ow component (particle acceleration) of an acoustic fi eld,
and that the auditory sensitivity of Chaetodon is enhanced by coincident sound
pressure stimuli that are mediated by the gas-fi lled swim bladder horns (Tricas and
Boyle 2015b ). Sensitivity to hydrodynamic particle acceleration is indicated for
Forcipiger (which lacks swim bladder horns) by higher stimulus thresholds at all
frequencies compared to that for all tested Chaetodon species, and these curves
converge at the lowest stimulus frequency of 100 Hz (Fig. 9 ). The best frequency
sensitivity of Forcipiger to particle acceleration was at the lowest test frequency of
100 Hz (Fig. 9b ), which is predicted for particle acceleration sensitive species (see
Ladich and Fay 2013 for discussion). None of the hearing thresholds for Forcipiger
increased (or changed) following defl ation of its gas-fi lled swim bladder, which
would be required for the transduction of sound pressure stimuli to stimulate the ear
(Fig. 10a ). A relatively low absolute sensitivity to sound pressure for all butterfl yfi shes is indicated by their apparently much higher AEP thresholds compared to
species with anatomical specializations for reception of sound pressure stimuli such
as the Weberian apparatus (Kenyon et al. 1998 ; Ladich 1999 ; Amoser and Ladich
2005 ; Lechner and Ladich 2008 ), anterior swim bladder horns and otic bullae, or a
suprabranchial organ (Ladich and Yan 1998 ). Further, the lowest particle acceleration threshold levels in this low frequency band for butterfl yfi shes ranged from
Acoustic Communication in Butterfl yfi shes…