Sound production in a crab: stridulating mechanism and sound acoustic features in a mating context

One important biophonic component of underwater soundscapes is benthic crustaceans. Passive acoustic monitoring of species allows to reveal ecological patterns of aquatic habitats. In the present study, the sound emission, production mechanism and acoustic parameters in a male-male competing mating context were characterized in the crab Cyrtograpsus angulatus through a laboratory experiment. Two acoustic signals were recognized: (1) single isolated, impulsive signals produced by an undisclosed internal mechanism in presence and absence of receptive females and a competing male, thus not related to mating, and (2) multi-impulsive signals conforming an irregular train pattern, potentially produced by males, only in the presence of a receptive female and a competing male, and thus associated with a competing mating context. The acoustic characterization of the impulsive signals demonstrated differences in two acoustic parameters, sound pressure level peak and 3 dB bandwidth, showing lower values in the multi-impulsive signals conforming the irregular train pattern compared to the single isolated ones. The irregular train pattern signals were made by a stridulation mechanism that involved the cheliped rubbing its merus tubercles against the tubercles of the carapace´s spines. Sound emission by males is proposed to be involved in the dominance assessment of opponents. The stridulatory structures were well developed in adults, both males and females, but not juveniles, indicating a potential function as secondary sexual structures. Under the competing mating context tested herein, only adult males were recognized to produce sounds, although it cannot be excluded that females also emit acoustic cues. The particular acoustic parameters described for the multi-impulsive signals conforming to the irregular train pattern indicate that sound could be related to species-specific behaviors, specifically in C. angulatus, signaling intrasexual communication in a male-male competing mating context.