Sound production occurs in well over 50 families of cartilaginous and bony fishes (Myrberg 1981, 2002; Hawkins & Myrberg 1983; Hawkins 1993; Ladich & Fine 2006). Sound production most commonly involves: (i) prey responses to being startled or handled by predators (“stay away” and “release” signals); (ii) mate attraction, arousal, approach, or coordination sounds; (iii) agonistic interactions with competitors for mates and resources (“stay away” signals); and (iv) attraction of shoal mates.
Startle and release calls occur in families as different as eagle rays, herring, characins, catfishes (of many families), cods, squirrelfishes, sea robins, grunts, and porcupinefishes. They are elicited when a fish is grabbed, poked, or even surprised. A sudden grunt, croak, or drumbeat might distract a predator, perhaps causing it to release its grip on the prey or hesitate in its attack long enough for the prey to escape. A release call could also attract additional predators, including predators on the individual holding the signaler. A small predator with prey in its mouth could be handicapped in its own efforts to evade a larger predator and might abandon its meal rather than risk becoming one (Mathis et al. 1995). Release sounds could also function as alarm calls (see Discouraging capture and handling) that notify conspecifics of a predator’s presence and activity. The caller would have to have close relatives nearby that could benefit from the sound to offset fitness losses to the signaler from being eaten.
Sound is an integral part of the courtship and spawning behavior of many fishes. Some sounds produced by male damselfishes (Pomacentridae) and European croakers (Sciaenidae) drive off intruding males. Territorial males also produce vocalizations to bring females closer during courtship (e.g., toadfishes, centrarchid sunfishes, gobies). Signaling rate frequently increases as a female draws nearer, or during the spawning act itself (cods, serranids), suggesting that acoustic communication synchronizes activities between members of a pair. In at least one species of an African mouth-breeding cichlid, male vocalizations stimulate gonadal activity in females, paralleling a widely observed phenomenon in seasonally breeding birds (Myrberg 1981; Lobel 1992).
During agonistic encounters associated with territorial behavior, sounds are usually produced by an aggressive or dominant animal; the response of the submissive animal is usually to retreat from the signal sender. Sound production during agonistic interactions occurs in many teleosts, including sea catfishes (Ariidae), loaches (Cobitidae), squirrelfishes (Holocentridae), butterfl yfishes (Chaetodontidae), damselfishes (Pomacentridae), gouramis (Osphronemidae), and triggerfishes (Balistidae). Unique structures and behaviors associated with sound production and reception have been found in butterfl yfishes, a family previously thought not to produce sounds. Improvements in sound recording devices have shown that sounds are produced during territorial and pair maintenance interactions, including low-frequency pulses <100 Hz, sounds with peak energy between 100 and 500 Hz, and a high-frequency click at 3.6 kHz (Tricas et al. 2006). A novel chaetodontid swimbladder–lateral line connection, termed the laterophysic connection, is formed from extensions of the anterior swim bladder that connect with the lateral line and even project towards the inner ear (Webb 1998; Webb et al. 2006). The laterophysic connections probably aid in detection of agonistic vocalizations. A unique sound-producing structure found in anemonefishes and other pomacentrids is the “sonic ligament”, a connection between the hyoid bar (ceratohyal) and the inner part of the mandible that helps the fish close its mouth rapidly, bringing its teeth together and producing popping sounds (Parmentier et al. 2007). The catalog of sound-producing fishes and interesting acoustic adaptations will undoubtedly grow as more studies are conducted.
Submissive animals also produce sounds that may reduce aggression in an opponent, as recorded from anemonefishes (Amphiprion, Pomacentridae) (Myrberg 1981; Hawkins 1993). The importance of sound production during territoriality is evident in the loach, Botia horae (Cobitidae), which vocalizes and displays visually to repel intruders of shelter sites. When experimentally muted, residents are unable to repel intruders, whereas sham-operated and intact animals defend their territories successfully (Valinsky & Rigley 1981).
Sound production also functions during shoal formation and maintenance. Most group maintenance sounds are produced by vibrating the swim bladder or stridulating of teeth, bones, and fin spines (see Rice & Lobel 2004; Amorim 2006). However, other mechanisms exist. Pacific and Atlantic Herring, Clupea pallasii and C. harengus, emit trains of pulsed sounds, termed fast repetitive ticks (FRTs) that last up to 7 s. These sounds are accompanied by the expulsion of small bubbles from the anal duct and are probably produced in the gut or swim bladder. FRTs are emitted more often at night and FRT frequency increases as school size increases, suggesting that they serve to maintain contact between schoolmates (Wilson et al. 2003b).
Other group maintenance sounds result from water displacement by fins and bodies during swimming and are detected via the lateral line of neighboring fish. Such water displacement informs aggregating fishes of their location relative to schoolmates, serving as a minor repulsive force that combines with visual input to maintain distance between individuals. When pollack (Gadidae) are experimentally blinded, they swim slightly further from schoolmates than when intact. In unblinded fish in which the lateral line nerve is severed and acoustic information therefore eliminated, they swim closer than normal to schoolmates (Pitcher et al. 1976). Interestingly, more actively schooling species within a family (e.g., among cods and damselfishes) are relatively quiet, and sound production in group-spawning fishes is not as common as it is in solitary, territorial species. Whether this silence helps prevent detection by predators or results from other factors is unknown (Hawkins & Myrberg 1983; Hawkins 1993).
Eavesdropping by predators may be a significant cost of sound production. Many predatory fishes (sharks, groupers, snappers, black basses, jacks, barracuda, tunas) are attracted to the incidental, low-frequency sounds produced by feeding or injured fishes. Bottlenose dolphins (Tursiops truncatus) include a disproportionate number of soundproducing fishes (e.g., croakers, grunts, toadfishes) in their diet (Barros & Myrberg 1987). Interception of signals, whether by predators, competitors, or potential prey, is always a potential cost affecting the evolution and use of communication signals by a species.