How are Clupeiformes identified by features of the swim bladder and inner ear?

Clupeiformes are distinguished by a unique internal connection between the swim bladder and the inner ear, which typically manifests as two large vesicles (cavities) located within the skull bones.1

This anatomical arrangement serves as a specialized mechanism for sound reception. In clupeids, such as herring, long anterior extensions of the swim bladder form air sacs that lie directly adjacent to each utricular macula within the labyrinth. This proximity allows for the efficient stimulation of hair cells by creating a significant acoustic discontinuity between the gas-filled sac and the surrounding tissue, which enhances the fish's sensitivity to sound. Unlike the Weberian apparatus found in ostariophysan fishes, which utilizes a chain of small bones to connect the swim bladder to the ear, the clupeiform system relies on these direct air-filled extensions to facilitate auditory function.23