Anatomy of finfish and shellfish

Unit 11 - Sensory System in fishes

11.5. Inner ear
The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see the fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. It is thought that many fishes communicate with each other in a crude way by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.
In fishes the middle ear apparatus is absent and only inner ear is present which is concerned with two senses. i.e., hearing and balancing. It is contained partly in the auditory capsule.
In sharks the anterior vertical canal joins with horizontal canal to form a “crus”. The pars inferior which receive sound is composed of two vesicles. The anterior sacculus and posterior legena. In bony fishes, the utriculus, succulus and legena comprise lapilles sagitta and astericus respectively. These are calcified structure and are secreted by ectoderm.

Besides the sound detection, the inner ear functions to “orient “or “balance” the animal providing it feeling of direction in which gravity is acting when suspended in lightless, pelagic habitats.
Minnows, carp, catfishes and other teleosts connects the auditory system to the swim bladder with a chain of small bones called “Weberian ossicles”., The ossicles connect the pulsating swim bladder wall with Y – shaped lymph sinus.