137
(Fig. 7). However, post-experiment examination of swim bladder deflation revealed
that the majority (88 %) of the “deflated” females that displayed positive phonotaxis
had partially re-inflated swim bladders. In total, 21 of the 22 tested fish (95 %) that
localized the sound source had at least partially inflated swim bladders which indicated that pressure reception was likely required for sound source localization.
In a separate experiment, Coffin et al. (2014) found that midshipman could solve
the 180° ambiguity of source direction in the shallow water test tank, which was
similar in depth to their natural nesting environment. The authors found that there
were no differences in the positive phonotaxic response rates when fish were
allowed to swim in any direction upon release using an “unbiased” release cage
versus when fish were directed toward the sound source upon release using a
“biased” release cage. The positive phonotaxic response rate was greater than 60 %
in both release cases. If the gravid females were unable to solve the 180° ambiguity,
then biasing their release toward the source would have increased the positive phonotaxic response rate relative to the unbiased release condition because the “unbiased” released fish would have been expected to be unable to determine “front”
from “back” and would therefore swim away from the source in about half of trials.
The researchers did not observe any fish to exit away from the sound source and
then correctly turn and move to the source (Fig. 7). While it is true that a subset of
fish tested failed to localize the sound source, the majority of the non-responding
fish did not swim 180° in the opposite direction, as would be expected if they were
motivated to locate the source but could not solve the 180° ambiguity. Thus, based
on these observations the authors posited that midshipman could effectively resolve
the 180° ambiguity problem during sound source localization.
In a final set of experiments (Coffin et al. 2014), gravid female midshipman
underwent ablation of the lateral line system soon after field collection and then
were tested within 36 h after treatment in a monopolar sound source localization
task. In preliminary lateral line ablation experiments, fish were initially treated with
0.001 % gentamicin sulfate for 24 h (after Van Trump et al. 2010) or 0.05 % streptomycin sulfate for 3 h (after Montgomery et al. 1997) in an attempt to chemically
ablate both the canal and superficial neuromasts of the lateral line system (Brown
et al. 2011). Treated and untreated females were then labeled with the fluorescent
vital dyes DASPEI and FM1-43 to assess the extent of aminoglycoside-induced hair
cell death and hair cell survival by fluorescence microscopy. Extensive neuromast
survival was evident in both cases after the antibiotic treatments. Brown et al.
(2011) then subsequently probed the effect of higher aminoglycoside concentrations on the midshipman lateral line system, treating additional fish in seawater
containing doubled concentrations of gentamicin (0.002 %) for 24 h or streptomycin (0.1 %) for 3 h. As before, extensive neuromast survival was evident but because
comorbid nonsensory effects have been associated with exposure to high concentrations of aminoglycosides (Kaus 1987; see Janssen 2000), the researchers elected to
abandon aminoglycosides altogether as a means of lateral line ablation, eventually
selecting a physical method of ablation—direct application of a liquid nitrogenchilled probe to mechanosensory superficial neuromasts and surgically exposed
Directional Hearing and Sound Source Localization in Fishes
(Fig. 7). However, post-experiment examination of swim bladder deflation revealed
that the majority (88 %) of the “deflated” females that displayed positive phonotaxis
had partially re-inflated swim bladders. In total, 21 of the 22 tested fish (95 %) that
localized the sound source had at least partially inflated swim bladders which indicated that pressure reception was likely required for sound source localization.
In a separate experiment, Coffin et al. (2014) found that midshipman could solve
the 180° ambiguity of source direction in the shallow water test tank, which was
similar in depth to their natural nesting environment. The authors found that there
were no differences in the positive phonotaxic response rates when fish were
allowed to swim in any direction upon release using an “unbiased” release cage
versus when fish were directed toward the sound source upon release using a
“biased” release cage. The positive phonotaxic response rate was greater than 60 %
in both release cases. If the gravid females were unable to solve the 180° ambiguity,
then biasing their release toward the source would have increased the positive phonotaxic response rate relative to the unbiased release condition because the “unbiased” released fish would have been expected to be unable to determine “front”
from “back” and would therefore swim away from the source in about half of trials.
The researchers did not observe any fish to exit away from the sound source and
then correctly turn and move to the source (Fig. 7). While it is true that a subset of
fish tested failed to localize the sound source, the majority of the non-responding
fish did not swim 180° in the opposite direction, as would be expected if they were
motivated to locate the source but could not solve the 180° ambiguity. Thus, based
on these observations the authors posited that midshipman could effectively resolve
the 180° ambiguity problem during sound source localization.
In a final set of experiments (Coffin et al. 2014), gravid female midshipman
underwent ablation of the lateral line system soon after field collection and then
were tested within 36 h after treatment in a monopolar sound source localization
task. In preliminary lateral line ablation experiments, fish were initially treated with
0.001 % gentamicin sulfate for 24 h (after Van Trump et al. 2010) or 0.05 % streptomycin sulfate for 3 h (after Montgomery et al. 1997) in an attempt to chemically
ablate both the canal and superficial neuromasts of the lateral line system (Brown
et al. 2011). Treated and untreated females were then labeled with the fluorescent
vital dyes DASPEI and FM1-43 to assess the extent of aminoglycoside-induced hair
cell death and hair cell survival by fluorescence microscopy. Extensive neuromast
survival was evident in both cases after the antibiotic treatments. Brown et al.
(2011) then subsequently probed the effect of higher aminoglycoside concentrations on the midshipman lateral line system, treating additional fish in seawater
containing doubled concentrations of gentamicin (0.002 %) for 24 h or streptomycin (0.1 %) for 3 h. As before, extensive neuromast survival was evident but because
comorbid nonsensory effects have been associated with exposure to high concentrations of aminoglycosides (Kaus 1987; see Janssen 2000), the researchers elected to
abandon aminoglycosides altogether as a means of lateral line ablation, eventually
selecting a physical method of ablation—direct application of a liquid nitrogenchilled probe to mechanosensory superficial neuromasts and surgically exposed
Directional Hearing and Sound Source Localization in Fishes