The common perception that the underwater world is silent is fundamentally inaccurate. Sound travels through water nearly five times faster than it does in air, making the aquatic environment a complex acoustic landscape. Fish are not silent creatures; they actively produce a variety of sounds for different purposes. Their ability to send and receive acoustic signals mediates a significant portion of their behavior, playing a large role in survival and reproductive success.
Mechanisms of Sound Generation
Fish generate sounds using a diverse array of specialized biological structures, primarily relying on two distinct physical mechanisms. The most common method involves manipulating the swim bladder, a gas-filled organ primarily used for buoyancy control. Certain families, such as the drums and croakers (Sciaenidae) and toadfishes (Batrachoididae), have developed fast-contracting “sonic muscles” attached to the swim bladder wall.
These sonic muscles vibrate the bladder rapidly, much like striking a drumhead, to produce low-frequency sounds described as grunts, croaks, or hums. The oyster toadfish, for example, uses some of the fastest-contracting vertebrate muscles to create its characteristic “boatwhistle” call. These swim bladder sounds generally range below 500 hertz, a frequency that travels well over long distances underwater.
A second major mechanism is stridulation, which involves rubbing or grinding together skeletal parts. This process creates higher-frequency sounds, often described as clicks, rasps, or snaps. For instance, clownfish produce sounds by rapidly gnashing their pharyngeal teeth, which are located in the throat.
Marine catfishes use stridulation by rubbing a fin spine against a socket in their shoulder girdle to create a grating sound. Additionally, some sounds are produced hydrodynamically as unintended by-products of rapid swimming movements, such as a quick change in direction or a fin snap. These sounds are species-specific, allowing fish to identify and interpret signals from members of their own kind.
Functions of Fish Vocalizations
The sounds produced by fish are intentional signals that serve various behavioral functions, particularly reproduction. Male oyster toadfish emit their tonal, low-frequency “boatwhistle” call from a nest site to advertise their presence and attract females. The intensity and duration of these calls can influence a female’s choice, suggesting sound quality is a factor in reproductive fitness.
Other sounds, such as short pulses, grunts, or pops, are commonly used in agonistic interactions related to territorial defense or competition. These sounds act as warnings or threats, allowing fish to settle disputes without engaging in physical combat. For example, clownfish use clicks and pops to maintain social hierarchy within their sea anemone home.
Acoustic signaling also plays a role in group cohesion, particularly in species that form large aggregations or schools. Some species use sounds to maintain contact, especially in low-visibility environments like turbid water or at night. This complex acoustic repertoire ensures successful reproduction and social organization.
How Fish Hear Underwater Sounds
All fish detect sound through two interconnected sensory systems: the inner ear and the lateral line. The inner ear contains dense, calcium carbonate structures called otoliths, which are suspended near sensory hair cells. Since otoliths are much denser than the surrounding fish body, they move at a different rate when a sound wave passes.
This differential movement, known as particle motion, causes the otoliths to lag behind the fish’s tissue, bending the sensory hair cells. This bending transduces the sound energy into a neural signal interpreted by the brain. All fish are capable of detecting this particle motion component of sound, which is the direct vibration of water molecules.
In many species, the swim bladder enhances hearing by acting as a pressure-to-particle-motion transducer. Sound pressure waves cause the gas-filled bladder to vibrate, transmitting this vibration to the inner ear. This significantly increases their sensitivity to the sound pressure component of a signal. Separately, the lateral line system, a row of sensory organs along the sides of the fish, detects near-field particle displacement and local water movement, primarily used for close-range sensing and schooling.
The Impact of Anthropogenic Noise
The aquatic acoustic environment is increasingly affected by human-generated (anthropogenic) noise from sources like commercial shipping, offshore construction, and seismic surveys. This noise often falls within the same low-frequency range (below 1,000 hertz) that fish use for communication and hearing. This overlap leads to auditory masking, where human-generated sound effectively drowns out biologically relevant signals.
Masking can reduce the effective distance over which a female fish detects a male’s mating call, potentially lowering reproductive success in noisy areas. Chronic noise exposure also induces physiological stress responses, including elevated levels of stress hormones like cortisol. This stress compromises overall health and diverts energy away from foraging or reproductive activities.
Behavioral alterations are common, with fish sometimes abandoning preferred habitats or altering migration routes to avoid excessively noisy areas. For example, studies show that ferryboat noise masks the hearing of the Lusitanian toadfish, interfering with their communication signals. The disruption of this natural acoustic environment poses a significant challenge to the survival of many soniferous species.