Bone system connecting the wall of the anterior part of the swim bladder to the inner ear cavity (labyrinth) in some bony fish (carp, catfish). Serves as a resonator.

Weber's apparatus is an organ in some bony fish that connects the swim bladder to the inner ear. It allows the ear to detect changes in the volume of the swim bladder and its vibrations. It is named after Ernst Weber, who described it in 1820. The presence of Weber's apparatus is one of the common features of fishes of the suborder Ostariophysi, which make up the majority of modern freshwater fishes (gonorhynchiformes, carpiformes, characiformes, catfishes, gymnotiformes). Elements of this apparatus are also found in other fishes, such as herring fish. In its simplest form (in perch and herring), Weber's connection between the swim bladder and the inner ear (labyrinth) is made by the contact of the appendages of both at that part of the skull where its wall remains membranous. In more complex forms (catfish, loaches and carps) this connection is achieved by inserting a series of 4 bones between the inner ear and the swim bladder appendages on each side, which are modified upper vertebral arches and a transformed third rib. The degree of hearing in fish is determined by the presence or absence of the vomeronasal apparatus. Fish with it can hear sounds up to 13 kHz, while those without it can only hear up to 2.5 kHz.