How do swim-bladder vibrations reach the inner ear in otophysan fishes?

In otophysan fishes, sound waves in the water cause the walls of the swim bladder to vibrate. These vibrations are transmitted to the inner ear through the Weberian apparatus, a specialized chain of four pairs of small bones known as ossicles.123

Derived from the first few vertebrae behind the skull, these ossicles—the claustrum, scaphium, intercalarium, and tripus—connect the swim bladder to the ear. The tripus is in contact with the anterior wall of the swim bladder, while the claustrum connects to a membranous window of the inner ear. As the swim bladder changes volume in response to sound, the ossicles move, increasing the amplitude of the vibrations and conducting these pressure changes to the ear. This mechanism functions similarly to a hydrophone, granting these fishes an exceptionally high sensitivity to sound.213