Many teleosts emit sounds from crevices, holes or cavities beneath stones and other types of submerged objects. Yet, the acoustical properties of fish shelters are virtually unexplored. This study investigates the resonance properties of shelters commonly used by Mediterranean gobies as hiding places and/or nest sites in the field, i.e., flat stones, shells belonging to five bivalve species (Tapes philippinarum, Crassostrea gigas, Mytilus galloprovincialis, Scapharca inaequivalvis, Cerastoderma edule) or within aquarium tanks (PVC tubes, concrete blocks, clay flower-pots cut in halves). Both natural and artificial shelters were stimulated using a small underwater buzzer as sound source, placed inside or around the shelter to mimic a fish calling from the nest site (source levels comparable to the those of the goby sounds emitted under similar circumstances), and different types of driving stimuli (white noise, pure tones and artificial pulse-trains). Results showed the presence of remarkable amplitude gains (3-15 dB) at lower frequencies (100-150 Hz) in all types of natural shelters but one (Mytilus), and in terracotta flowerpots and concrete blocks. Gain was higher for stones and artificial shelters than for shells. The experimental piling of sand on the shelter (mimicking a nest-related activity common in sand gobies) increased gain in all types of shelter. Conclusions were verified by performing analogous acoustical tests on natural shelters in the field. The enhancement of low frequencies by stones and shells may explain the widespread use of low-frequencies for acoustic communication by many teleosts calling from enclosures. Results draw attention to the use of suitable shelters for the recording of sounds by fishes kept within laboratory aquaria.
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