| The swim bladder can serve as an auditory organ for most otophysine fishes.The vibration of the swim bladder caused by sound pressure is transmitted to the inner ear through the weberian organ,which can enhance the reception of auditory signals.In the process,the swim bladder acts as a pressure-to-displacement transducer and the vibration of the swim bladder wall acts as a secondary sound source that reradiates sound energy in the form of local particle motion,which then can effectively stimulate the particle motion-sensitive inner ear end organs.Therefore,the morphology of the swim bladder may be related to auditory function of fish.Triplophysa rosa is a species of cavefish and its swim bladder is composed of two connected spherical lateral chambers,which are dumbbell-shaped and enlarged to the body surface.The swim bladders of surface fish T.bleekeri and Homatula variegata also have transverse chambers,but without bulge on the body surface.Carassius auratus has the common two-chamber swim bladder in anterior-to-posterior form.Several questions are worth exploring whether the sizes of those different swim bladders be related to hearing function and mechanosensory function and whether the morphological specialization in the swim bladder of the T.rosa contributes to its auditory function.In this study,different degrees of gas removal were performed on the swim bladder to explore the role of the swim bladder in the auditory and mechanosensory function of fish.We aimed to examine whether the swim bladders of fish involve in auditory and mechanosensory function and whether the roles of swim bladder on auditory and mechanicalsensory function differe between cavefish and surface fish.The individuals of T.rosa,T.bleekeri,H.variegata,and C.auratus were randomly divided into four groups for different treatment,respectively,including the control,the sham control,one side(gas was removed from only one side of swim bladder except for C.auratus)and both sides(gas was removed from both sides of swim bladders).The acoustic startle response experiment was carried out with increasing intensity of sound stimuli at different frequencies(50-6400 Hz)and the behavioral response changes of the fish were observed before and after the sound stimulation.Then,the auditory sensitivity was evaluated to analyze the effect of removing gas from the swim bladder on hearing in each species.Different amplitudes of vibration stimulation were created by changing voltage(0-190 mv)and the speed,percentage of active time,preference,center times,turn times,and twist times were observed to examine and control the effect of gas removal from swim bladder on basic activity behavior of each species.The changes of these parameters before and after vibration were used to evaluate the effect of gas removal from swim bladder on the mechanosensory function of fish and the differences among species were analyzed.By dissecting experimental fish,the swim bladder chamber volume was observed and calculated and the correlations between the volume of swim bladder and hearing threshold of fish was analyzed.The main results are as follows.(1)The hearing thresholds were compared between the control group and the sham control group and were 1.8-6.7 and 2.0-7.0 dB in T.rosa,1.0-15.0 and 1.0-15.0 dB in T.bleekeri,34.0-67.0 and 35.5-67.0 dB in H.variegata,14.5-20.0 and 16.9-20.5 dB in C.auratus,respectively.The parameters of vibration behavior response were compared between the control group and the sham control group,and the speed of T.rosa,T.bleekeri,H.variegata and C.auratus were 68.5 and 61.3,51.0 and 32.8,1.0 and 4.0,35.2 and 30.6cm/min,respectively;the percentage of active time were 42.8%and 33.5%,14.8%and15.7%,0.5%and 9.4%,36.5%and 37.6%,respectively;the preference were 1.6%and4.5%,0%and 0.4%,0%and 0%,0.1%and 0.3%,respectively;the center times were 0.2and 0.3,0 and 0.2,0 and 0,0.04 and 0.1 times,respectively;the turn times were 0.7 and0.8,0.5 and 0.6,0 and 0,0 and 0 times and twist times were 0.2 and 0.5,0.9 and 1.1,1.0and 0.2,0 and 0 times,respectively.The interaction between sham control and frequency had significant effect on hearing threshold of T.rosa(t=-4.187,p<0.001)and H.variegata(t=2.178,p=0.029)at high frequencies.The interaction between sham control and vibration amplitude had significant effect on turn times(z=2.352,p=0.019)and twist times(z=1815.27,p<0.001)of T.rosa.The sham control resulted in an increased speed(t=3.295,p<0.001)and percentage of active time(t=4.359,p<0.001)of H.variegate,while decreased twist times of H.variegate(z=-2.076,p=0.038).The interaction between sham control and vibration amplitude had significant effect on preference(t=-3.232,p=0.001)of C.auratus.These results suggest that sham gas removal can affect the hearing thresholds and parameters of vibration behavior response of those species.(2)The swim bladder volumes of the sham control group,the one side group and the both sides group were corrected with the average body length(6.85 cm)of the four species,and were 98.9,32.2 and 7.8 mm~3 in T.rosa,11.8,5.6 and 1.6 mm~3 in T.bleekeri,9.3,5.2 and 0.4 mm~3 in H.variegate,and 282.4 and 198.6 mm~3 in C.auratus.The volumes of the swim bladders were negatively correlated with the degrees of bladder gas removal of T.rosa(t=-8.143,p<0.001),T.bleekeri(t=-13.714,p<0.001),H.variegata(t=-6.811,p<0.001),and C.auratus(t=-2.211,p=0.040).(3)The hearing threshold of the sham control group,the one side group,and the both sides group were 2.0-7.0,2.5-5.9 and 16.4-34.1 dB in T.rosa,1.0-15.0,1.1-15.0 and 24.6-39.7 dB in T.bleekeri,35.5-67.0,49.5-67.9 and 54.0-74.0 dB in H.variegata,16.9-20.5and 38.4-48.3 dB in C.auratus,respectively.At low frequencies(50-400 Hz),the degrees of gas removal from the swim bladder were positively correlated with the hearing thresholds of T.rosa(t=7.036,p<0.001),T.bleekeri(t=4.108,p<0.001),H.variegata(t=3.031,p=0.005),and C.auratus(t=3.587,p<0.001).At high frequencies(800-6400 Hz),the degrees of gas removal from the swim bladder were also positively correlated with hearing thresholds of T.rosa(t=4.771,p<0.001),T.bleekeri(t=3.537,p<0.001),H.variegata(t=2.069,p=0.047),and C.auratus(t=3.035,p=0.002).The results suggest that the swim bladder is involved in auditory function of all four species.(4)The hearing threshold of some H.variegata individuals exceeded the upper limits of the device(50-70 dB)even before removing the gas from the swim bladder.Therefore,H.variegata was not suitable for analysing the effect of removing gas from the swim bladder on hearing threshold and the differences hearing thresholds were compared only between the cavefish T.rosa and the surface fish H.variegata and C.auratus.At low frequencies,the effects of the degrees of gas removal from the swim bladder on the hearing thresholds of T.rosa were not different to those of T.bleekeri(t=-0.163,p=0.871),but different to those of C.auratus(t=-3.145,p=0.002).At high frequencies,however,the effects of the degrees of gas removal from the swim bladder on the hearing thresholds of T.rosa were not different to either T.bleekeri(t=-0.405,p=0.685)and C.auratus(t=-1.059,p=0.289).However,the increasd hearing threshold by gas removal from both sides of swim bladders varied across species except at 200 Hz and the increase of T.rosa was smaller than that of T.bleekeri at any other frequencies,with the difference ranged from 2.5 to 16.5 dB.At 50-200 Hz,the increased hearing threshold of C.auratus by gas removal from the both sides of swim bladders was lower than that of T.rosa with difference range of 6.3 to 9.4 dB,while at 6400 Hz,was only 7.4 dB higher than that of T.rosa.The two species showed similar hearing changes at 400,800 and 1600 Hz with the difference of only 1.3-2.1 dB.The results suggest that the swim bladder of the cavefish T.rosa may not play more important roles in hearing than that of the surface fish.(5)With the increasing degree of gas removal from the swim bladder,only the vibration induced speed changes of T.rosa decreased(t=-2.302,p=0.029),while the changes of other parameters did not change significantly(all p>0.05).However,compared with sham control group,the vibration induced preference changes in the C.auratus after removal of the gas from the both sides of swim bladders only decreased at10 mv(t=-2.063 p=0.048)and 100 mv(t=-2.191,p=0.037).The vibration induced speed changes of T.rosa decreased only at 50 mv(t=-2.714,p=0.010).The vibration induced percentage of active time changes only decreased at 10 mv(t=-2.103,p=0.042).After removal of the gas from the both sides of swim bladders,the vibration induced changes of all parameters of H.variegata did not change significantly,indicating that except for the H.variegate,removal the swim bladder gas of the other three species could lead to the decrease of mechanosensory function under partial vibration amplitudes.(6)The effects of gas removal from the swim bladder on the vibration induced changes of preference and percentage of active time were not different among species.Th gas removal from the swim bladder had no significant effect on the difference in the vibration induced changes of speed between T.rosa and T.bleekeri(t=1.156,p=0.250),but was different from that of H.variegata(t=2.015,p=0.046)and C.auratus(t=2.555,p=0.012).Due to gas removal from the both sides of swim bladders,the vibration induced changes of preference increased from-3.5%to 0.2%in T.rosa,decreased from0.5%to 0.4%in T.bleekeri,and decreased from 0.3%to-0.3%in C.auratus,respectively.H.variegata did not enter the middle vibration region.The vibration induced changes of speed and percentage of active time decreased from 34.3 cm/min to 0 cm/min and from7.5%to 3.1%in T.rosa,decreased from 32.8 cm/min to 17.5 cm/min and from 13.8%to5.2%in T.bleekeri,decreased from 3.8 cm/min to 1.6 cm/min and from 4.6%to 1.4%in H.variegata,and increased from 3.9 cm/min to 13.5 cm/min and from 5.5%to 6.4%in C.auratus.Removing gas from the swim bladder had the stronger effect on the vibration induced changes of speed of T.rosa,but not the preference and percentage of active time changes,compared to other fish species.These results suggest that gas removal from the swim bladder affected the hearing thresholds of the all four species,suggesting that the swim bladders are involved in the auditory function of those species.Removal of gas from swim bladder can lead to decreased mechanosensory function of the three species except H.variegata under some vibration amplitudes,suggesting that the swim bladder is involved in the mechanosensory function of those three species.The increases of hearing threshold in most frequencies of T.rosa were smaller than those of T.bleekeri after gas removal from both sides of swim bladders.It suggests that the swim bladder may not play greater roles in the hearing of cavefish than that of surface fish.The results also suggest that the removal of gas from the swim bladder did not more severely affect the mechanosensory function of the cavefish in T.rosa than that of the surface fish. |
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