Fishes produce different types of sounds using different mechanisms and for different reasons. Sounds (vocalizations) may be intentionally produced as signals to predators or competitors, to attract mates, or as a fright response. Sounds are also produced unintentionally including those made as a by-product of feeding or swimming. The three main ways fishes produce sounds are by using sonic muscles that are located on or near their swim bladder (drumming); striking or rubbing together skeletal components (stridulation); and by quickly changing speed and direction while swimming (hydrodynamics). The majority of sounds produced by fishes are of low frequency, typically less than 1000 Hz.
Fishes such as drums and croakers (Family Sciaenidae) have sonic muscles attached to or near to their swim bladder. These muscles, the fastest contracting muscles known in vertebrates, cause the swim bladder to contract and expand at a rapid rate, thus creating drumming sounds. The majority of sounds produced in this way are short pulses with fundamental frequencies ranging from about 45 - 60 Hz (i.e., goliath grouper and black drum) to about 250 - 300 Hz (i.e., toadfish spp. and silver perch). Higher frequency harmonics produced by drumming are sometimes present above 1000 Hz (e.g., silver perch).
The sonic muscles of the toadfish are located along the lateral surfaces of the heart shaped swim bladder. Contraction of the sonic muscles produces a sound similar to a foghorn. In other species the muscles may be configured differently (such as anchored to the base of the skull) or may be attached to another anatomical feature, which is then triggered to vibrate the wall of the swim bladder. For example, some marine catfishes possess a modified swim bladder mechanism, called the "Springfederapparat" or "elastic spring apparatus." Thin elastic bones function in sound production. Specialized sonic muscles on the upper surface of this elastic spring cause the vibration of the swim bladder.
Drumming sounds have been described as thumps, purrs, knocks, and pulses all of which occur in different variations depending on the fish producing the sound. In this way fishes are able to produce species-specific sounds which can be used to identify them in recordings.
Stridulatory sounds are produced when hard skeletal parts or teeth are rubbed together, like the method used by crickets to make sounds. In fishes, stridulation often occurs during feeding when jaw teeth or pharyngeal teeth are gnashed together. Stridulation may be used intentionally to produce sound as a fright response or territorial display. Stridulatory sounds may be modified or amplified by the swim bladder. The component frequencies of stridulatory sounds range from < 100 to >8000 Hz, while predominant frequencies are generally between 1000 and 4000 Hz. Stridulatory sounds influenced by the swim bladder have predominant frequencies well below 1000 Hz.
Examples of fish species that produce sound by stridulation include marine catfishes and sea horses. In some species such as the grunts (Family Haemulidae), the swim bladder is hypothesized to function as a resonator to amplify stridulatory sounds.
Marine catfishes (Arius felis and Bagre marinus) have specialized pectoral fin spines that make a stridulatory squeaking sound. The base of the pectoral fin spine is modified in these catfish. A part of the base, known as the dorsal process, looks like a ridged potato chip. Sound is created when the dorsal process of the pectoral fin is rubbed against the pectoral girdle. This is commonly heard by anglers who catch a sea catfish.
The northern seahorse is also known for producing stridulatory sounds. In these fish, clicking and/or snapping sounds are produced when bony edges of the skull and coronet, a crown-shaped plate on the fish’s head, rub together. These sounds are possibly amplified by the swim bladder.
Hydrodynamic sound production occurs when a fish quickly changes direction and/or velocity. These sounds are extremely low frequency. These sounds are simply a by-product of swimming. It is possible that hydrodynamic sounds may be important to predator and prey interactions and communication. For example, it has been postulated that sharks can detect the low frequency hydrodynamic sounds emitted by smaller fishes. Therefore, schooling fishes may inadvertently attract a shark simply by the sounds produced during swimming.
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