Remarkably loud snaps during mouth-fighting by a sponge-dwelling worm.

Many aquatic animals, including mammals, fishes, crustaceans and insects, produce loud sounds underwater [1-6]. Soft-bodied worms would seem unlikely to produce a loud snap or pop because such brief, intense sounds normally require extreme movements and sophisticated energy storage and release mechanisms [5]. Surprisingly, we discovered a segmented marine worm that makes loud popping sounds during a highly stereotyped intraspecific agonistic behavior we call 'mouth fighting'. These sounds - sound pressures up to 157 dB re 1 μPa at 1 m, with frequencies in the 1-100 kHz range and a strong signal at ∼6.9 kHz - are comparable to those made by snapping shrimps, which are among the most intense biological sounds that have been measured in the sea [6]. We suggest a novel mechanism for generating ultrafast movements and loud sounds in a soft-bodied animal: thick, muscular pharyngeal walls appear to allow energy storage and cocking; this permits extremely rapid expansion of the pharynx within the worm's body during the strike, which yields an intense popping sound (likely via cavitation) and a rapid influx of water. Clearly, even soft-bodied marine invertebrates can produce remarkably loud sounds underwater. How they do so remains an intriguing biomechanical puzzle that hints at a new type of extreme biology.