SOUND PRODUCTION IN THE SNAPPING SHRIMPS ALPHEUS (CRANGON) AND SYNALPHEUS ,

Abstract

The sound-producing mechanism of snapping or pistol shrimps is confined to the larger member of the first pair of pereiopods (which in the Family Alpheidae are dissimilarly enlarged). A loud snapping noise is produced by contact of the calcified tips upon closure of the dactylopodite on the propodite. The main function of this movement is not production of sound but of a "water jet" that Is a weapon of offense and defense. Closure of the large chela is analogous to the uncocking of a pistol hammer. Evidence supporting the earlier idea that "suckers" in the articulation of the propodo-dactylar joint in Alpheus sp. serve as a cocking mechanism is wanting. It is suggested that suction develops by depression of the membrane between the segments as the dactylus is raised. Overlapping exoskeletal knobs in the joint probably serve to hold the dactylus in place. The structure and physiology of the claw muscles are as in the Crustacea generally. The adductor muscle is innervated by fast, slow, and inhibitory fibers. Analysis of the "crackle" produced by populations of snapping shrimp at various locations around Bermuda shows that they occur at depths of less than 120 feet where sheltering materials are present. Shrimp noise extends to frequencies of over 15 kc under natural conditions, in contrast to water noise, which is limited to the frequency range 0-5 kc. Shrimp spectra from different areas are variable, resulting from differences in the predominating species inhabiting each area and, more importantly, to habitat differences. There is a slight increase in shrimp noise at night but probably no significant seasonal variation at Bermuda, because water temperature remains relatively constant there throughout the year. Under laboratory conditions, the frequency range and amplitude of peak intensity of an Alpheus single snap are greater than in Synalpheus, as a result of the more powerful chela of Alpheus. Variation in recording conditions or size of specimen does not alter the spectrum substantially. The components of a single snaps of specimens in aquaria extend to higher frequencies and cover a greater time interval than individual snaps under natural conditions because of greater echoing.