Smaller swimbladder means poorer hearing

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Austrian scientists have found that catfishes with smaller swim bladders hear more poorly.

Publishing their results in the latest issue of the Journal of Experimental Biology, Walter Lechner and Friedrich Ladich studied the correlation between the size of a catfish's swim bladder and its hearing sensitivity.

Catfishes belong to a group of predominantly freshwater fishes known as the Otophysi. Otophysan fishes have a heightened sense of hearing compared to other fishes due to the fact that they have tiny ear bones (Weberian ossicles) that transmit sound from the swim bladder to their inner ear.

But the swim bladder also functions in maintaining buoyancy and some catfishes have reduced the swim bladder because buoyancy control is no longer important for a fish that spends most or all of its time on the bottom.

The authors studied the relationship between swim bladder size and auditory sensitivity by subjecting individuals of 11 catfish species (representing eight catfish families) placed in a tank in a darkened room to sound of varying pitches and intensities.

Sounds of 50"5000 Hz were played and electrodes placed on the skin that measured brainwaves from the medulla oblongata measured the hearing sensitivity of fishes as each sound was played in decreasing intensity.

The authors examined the swim bladders of the catfishes and found them to be of primarily two kinds: a larger, single-chambered, free swim bladder that generally had four Weberian ossicles and a smaller, twin-chambered swim bladder that was encapsulated in bone; the encapsulated swim bladder usually had only one or two Weberian ossicles.

The authors found that the hearing sensitivities of catfishes with either free or encapsulated swim bladders was not significantly different for sounds below 1000 Hz in frequency, but that the fish with larger swim bladders having three or more Weberian ossicles were able to hear sounds above 1000 Hz in frequency while those with encapsulated swim bladders and one or to Weberian ossicles were not.

The authors conclude that etaining the auditory sensitivity for various purposes such as analysing the auditory scene for predators, prey or communicating conspecifics is the most likely explanation for why bottom-dwelling otophysans such as catfishes, which evolved and live in low ambient noise environments, never reduced their swimbladders, contrary to other bottom-dwelling teleosts belonging to hearing generalists.

For more information, see the paper: Lechner, W and F Ladich (2008) Size matters: diversity in swimbladders and Weberian ossicles affects hearing in catfishes. Journal of Experimental Biology 211, pp. 1681"1689.