Sound amplification by the sand goby nests in the field and comparison with laboratory results

Abstract Male sand gobies call and spawn inside cavities beneath stones, shells, and other submerged objects (including artificial shelters) which they cover by piling sand on them. The male emits low-frequency courtship sounds only during the mating interactions taking place in the nest hollow. These vocalizations are crucial for female spawning decisions. A previous laboratory study showed shelters used as a nest amplify the sound pressure in the frequency range of 50–150 Hz. Another laboratory study showed the sand pile further increases the low-frequency amplitude gain of the hollow by up to 12 dB. In the present study, the role of nest hollow and the sand pile in sound amplification were examined using two types of bivalve shells in the field. Results were similar to those obtained in laboratory for the shelters with and without the nest pile, both in terms of frequencies enhanced and amplitude gain. The relationship between shell weight and sound amplification was examined by pooling field and laboratory data. Results showed the amplitude gain increased with the weight of the shell, whereas peak frequency was not affected. These results provide further support to the conclusions from early laboratory studies that (1) the amplification properties of the nest hollow may contribute to explain the evolution of the low-frequency acoustic communication among sand gobies and that (2) sand digging and piling is an activity of environmental manipulation performed by the male sand goby to increase sound amplitude and efficiency of the acoustic signaling in the nest.