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Figure 3.84: Goldfish are otophysan fishes and therefore possess Weberian ossicles that allow sound pressure waves impinging upon the swim bladder to be carried directly to the ear, leading to sensitive hearing (wide-frequency range and relatively low thresholds of around 60 dB). Goldfish hear above 3 kHz, with their best hearing between 500–1,000 Hz.


sound is converted from mechanical to electrical signals by the sensory hair cells that are common in them all. Extremely high-intensity sounds are able to fatigue or damage these cells, resulting in temporary or permanent hearing loss. However, fish continue to add sensory hair cells throughout their lives. In addition, there is evidence that fishes can repair sensory cells that have been fatigued due to sound exposures that cause a shift in auditory thresholds. Fish will move along with the sound field from any source.


While this might result in the fish not detecting the sound, the inner ear also contains dense calcium carbonate structures – the otoliths – which move at a different amplitude and phase from the rest of the body while stimulating sensory hair cells. Tis system of relative motion between the otoliths and the sensory hair cells acts as a biological accelerometer and provides the mechanism by which all fish hear. However, in fish with a swim bladder the acoustic sound


In otophysan fish (e.g., carps, minnows, channel catfishes,


and characins; the majority of freshwater fish worldwide), a bony coupling is formed by the Weberian ossicles. Tese bones allow them to use their swim bladder as a sort of drum to detect a greater range of sounds, and create a super-league of hearing-sensitive fish.


3.11.1 Fish Sensory Systems


Fish have evolved two sensory systems to detect acoustic signals: the ear and the lateral line. Fish do not need an outer or middle ear, since the role of


these structures in terrestrial vertebrates is to funnel sound to the ear and overcome the impedance difference between air and the fluids of the inner ear. Since fish live in, and have the same density as water, there is no impedance difference to overcome. Fish do have an inner ear which is similar in structure and function to the inner ear of terrestrial vertebrates. Te most important similarity between the ears of all vertebrates is that


Figure 3.85:


(a) The fish inner ear with three semi-circular canals and three otolith organs.


(b) Schematic cut through an otolith organ.


a Inner ear Membrane Hair cells Otolith organs Nerve Otolith


pressure can indirectly stimulate the fish’s inner ear via the bladder. For the stimulation to be efficient, the swim bladder must either be close to or have a specific connection to the inner ear. In one form, a gas bubble makes the mechanical coupling; in another the inner ear is directly connected to it by a set of small bones called the Weberian ossicles. Since the air in the swim bladder is of a very different density to that of the rest of the fish, in the presence of sound the air starts to vibrate. Tis vibration stimulates the inner ear by moving the otolith relative to the sensory epithelium. In these cases the fishes are sensitive to both particle motion and pressure modes of sound, leading to enhanced pressure detection and a broadened frequency response range. Te lateral line consists of a series of receptors along the body


of the fish enabling detection of hydrodynamic signals (water motion) relative to the fish. It is involved in schooling behaviour, where fish swim in a cohesive formation with many other fish and for detection of nearby moving objects, such as food.


3.11.2 Audiograms


For fish to hear a sound source, the generated sound pressure level should be higher than its auditory threshold and background noise levels from natural sources and anthropogenic sound.


b Endolymph


139


Alexstar/Dreamstime.com


Lasse Amundsen (modified from Karlsen (2010))


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