Fish Ears 2 – Fishes hear where they are…

Schooling Goatfish – Photo from Cardonna

In our last newsletter we dug into the hearing thresholds of fishes. We presented our work at an Acoustical Society conference in 2008 that included a review thresholds curves from ~150 papers, representing maybe 30 species (there are some estimated 50,000 species of fishes in the sea).

Most of the testing in the literature used “Auditory Brainstem Response” and “Audio Evoked Potential” testing – measuring electro-neurological responses to stimulus, or “Operant Conditioning” testing – punishing or rewarding an animal for responses to frequency and amplitude-scaled stimulus signals as the inputs change. The subject’s “positive” responses could be “pushing a lever” for a reward, or freaking out in anticipation of a shock. (Unlike warm-blooded animals where “fight or flight” response to fear increases the heart rate, fear response in fishes is evidenced by a slowing of the heart rate.)

Of course the testing environment – being in a lab, were not very representative of the habitats that the subject animals inhabit in the wild. In one reward-operant-conditioning test setting, a particular fish was impressing the researchers with its frequency and amplitude sensitivities until they realized that the fish was not responding to the input signal, rather it was watching the researcher pressing the stimulus trigger through the aquarium glass and harvesting the rewards for “positive responses”…

We had to nix a lot of the papers due to faulty assumptions – such as the testing aquariums being much smaller than the wavelengths of the test signals, or the metrics being inconsistent with common practice. And then there was the dual acoustical sensing channels mentioned before – pressure gradient, for which we have well-established metrics, and particle motion – for which we still don’t.

Varied morphological adaptations to habitats are also hard to reproduce in a lab: Flounders and halibut are benthic residents and don’t need depth-stabilizing swim bladders – which often also serve as resonators to confer sound into fishes hearing organs, (but they do have cilia on their top side). There are schooling fishes that need to sense the low-frequency swimming motion of adjacent schooling partners in order to keep their tight, defensive school formations in sync, and gamete-dispersal breeders that chorus to aggregate with conspecifics for their collective romantic encounters.

And perhaps most befuddling; intertidal, wave break, and brook-residing fishes dwelling in turbulent habitats where frisky brooks, wave-tormented tide pools, and crashing waves that all generate noise with hundreds-to-thousands of times more noise energy than the acoustical cues these critters need to feed, communicate, and avoid predation.

So how do fishes hear in loud thrashing, splashing, and bubbly environments? In our visual environment, light radiates from various sources – the sun, the moon, light bulbs, reflections off the clouds and bodies of water, etc. As light travels through our environment, it’s just stochastic noise. We can’t see light until it strikes an object, at which point the “light noise” lines up to reflect a set of coherent wave fronts that allow us to reconcile the visual world around us.

In water, where sound serves as “light,” the same thing is probably happening. Intertidal fishes don’t hear the noise in their environment until it strikes an object and returns a coherent wave front – which their hearing organs coagulate to sense their surroundings.

So all of this testing of frequency-amplitude hearing thresholds of fishes in a lab is, as my father would say, “measuring a manhole with a micrometer.”

Once we noticed the deviation of fishes hearing thresholds away from our “Wenz Curves” hypothesis, we probably should have nixed all of the fish papers, except perhaps that they help us prove this point.