Why do some fish have more sensitive ears than others? The better to find food with, suggests a study published in a recent issue of the journal Animal Behaviour.
Members of the second-largest superorder of fishes, Ostariophysi (including loaches, minnows, tetras, catfishes and South American electric fishes), possess a hearing specialisation involving a connection between the swimbladder and the inner ear called the Weberian apparatus.
This connection, consisting of a series of modified bones, allows for greater hearing sensitivity and an enhanced ability to detect higher-frequency sounds. Interestingly, there has been little explanation as to why this widespread hearing specialisation evolved in ostariophysians.
In their study, Daniel Holt and Carol Johnston investigate the potential use of sensitive hearing in fishes for acoustic feeding cues. Focusing on species of the Cyprinidae (carps and minnows), they designed experiments to investigate whether these fishes are attracted to sounds associated with potential food sources.
Carrying out their experiments in the Tellico River in Tennessee, the authors placed two underwater speakers 2m apart, and the fish activity in front of the silent speakers recorded on video camera for five minutes.
With the video cameras running, the noise of shuffling rocks was broadcast through one speaker and white noise through the other for the next five minutes, followed by a ten-minute period of silence.
The sounds were broadcast again, this time through opposite speakers, for another five minutes before the experiment was concluded.
The authors found that a significantly greater number of cyprinid fishes congregated in front of the speaker when it was broadcasting rock-shuffling sounds, with fewer fishes appearing the second time the sound was played.
The fishes observed were primarily River chub, Nocomis micropogon, Warpaint shiner, Luxilus coccogenis, Saffron shiner, Notropis rubricroceus, Largescale stoneroller, Campostoma oligolepsis, and Northern hogsucker, Hypentelium nigricans (except for the hogsucker, which is a catostomid, all the others are cyprinids; catostomids are closely related to cyprinids and also possess a Weberian apparatus, which is why the authors also included this species in their data collection).
Other fishes that occurred in the study area and that lacked a Weberian apparatus were seldom encountered and did not appear to respond to the sounds at all.
The fish were attracted to the rock-shuffling sounds because benthic invertebrates are often dislodged or brought to the surface and become readily available food for the cyprinids when a rocky substrate is disturbed.
The authors hypothesise that the use of indirect environmental cues such as shuffling noise to find food sources may be especially advantageous to freshwater fishes such as cyprinids, given that visibility underwater is low relative to air in many freshwater systems.
For more information, see the paper: Holt, DE and CE Johnston (2011) Can you hear the dinner bell? Response of cyprinid fishes to environmental acoustic cues. Animal Behaviour 82, pp. 529–534.
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