Otophysine fish are fantastically successful by any standards. Boasting over 8000 species, they have colonised the waters of every continent, except Antarctica. However, the only feature that otophysines have in common is an acute sense of hearing due to their tiny ear bones (Weberian ossicles) which transmit vibrations from the swim bladder to the inner ear. While otophysine lifestyles vary enormously, one of the otophysine orders, catfish, spend most of their time grubbing around on waterway bottoms. Which puzzled Ladich. Why would fish that lurk in the watery depths retain their swim bladders when they have no need for a buoyancy aid? `They haven't kept it for nostalgic reasons'says Ladich. Realising that reducing the swim bladder's size to stick close to the floor could compromise the fish's sensitive hearing, Ladich and his student Walter Lechner decided to investigate various cat fishes' hearing to see if there was any correlation between the sensitivity of the fishes'hearing and the size of their acoustic apparatus(p. 1681).
As catfish are popular with aquarium owners, the pair identified eleven species that could be obtained relatively easily for Lechner to investigate the size and location of the fishes' swim bladders and ear ossicles. The team chose six catfish species with large swim bladders located in the fishes'trunk, while the remaining five had a pair of tiny swim bladders surrounded by a bony sheet located in their heads. Ladich admits that locating the Weberian ossicles was particularly tricky, as some were less than a millimetre long,but eventually it was clear that most of the fish with large swim bladders had 3 or 4 ossicles, while the fish with tiny paired swim bladders only had one or two. But what effect did this diverse range of hearing apparatus have on the fishes' hearing?
Isolating individual catfish in a tank in a darkened room, Lechner played the fish sound at pitches ranging from 50 Hz up to 5 kHz. He monitored the fishes' hearing responses by measuring brainwaves through their skin as he decreased the sound's intensity. At frequencies below 1000 Hz, there was little difference between the fishes' hearing sensitivity, but as Lechner raised the frequency above 1000 Hz, a clear difference emerged. The high frequency hearing of the fish with large swim bladders and 3 or more Weberian ossicles was significantly more sensitive than the fish with tiny paired bladders. It was clear that the fish which had reduced their swim bladders to limit their buoyancy had paid a price when it came to picking up high frequencies.
The hearing of bottom dwelling catfish is so important that they have retained their swim bladders despite the buoyancy penalty. And even though the hearing of catfish with tiny paired swim bladders is worse than catfish with large swim bladders, it is still significantly better than that of many other fish.