Oyster larvae swim along gradients of sound

Abstract

1. Marine soundscapes provide navigational information for dispersing organ-isms, but with wide-scale habitat loss, these soundscapes are becoming muted. Consequently, dispersing larvae that use soundscapes for navigation may be lost at sea, limiting the success of restoration efforts that rely upon the recruitment of dispersing organisms to restore habitat. Where limited larval supply constrains restoration efforts, using speakers to create gradients in healthy soundscapes could provide the navigational cue that attract larvae and enhances recruitment.2. Combining laboratory and field studies, we test whether broadcasting soundscapes might act as a directional cue for oysters targeted for national-scale reef restoration; the Australian flat oyster Ostrea angasi. In the laboratory, we tested whether settlement of larvae increases along a gradient of increasing sound in-tensity (8 m laboratory tank) versus a no sound control, and whether settlement increases with soundscapes that approximate healthy reefs.3. In the field, we tested the context dependency and magnitude of using boosted soundscapes for restoration practice in areas of low, medium and high back-ground noise, by observing the settlement rates of naturally recruiting oysters at three restoration sites when exposed to boosted reef sound relative to ambient conditions.4. In the laboratory, we showed that 83% of larvae swim horizontally towards reef sound to settle in greater densities closer to its source, a near doubling of the larvae (44%) that dispersed in the no sound controls. Larval settlement increased by 300% in the presence of reef sound relative to controls in the laboratory. In the field, speakers increased larval settlement in localities of lower background noise. To our knowledge, this is the first evidence that oyster larvae can swim horizontally and choose to move towards a sound source.5. Synthesis and applications. We discovered that oyster larvae can swim horizon-tally towards reef sound and then settle in higher densities, relative to controls. Importantly, this effect of sound on recruitment is enhanced in localities of lower background noise. We propose that where recruitment is limited, restoration practitioners best use acoustic technology in localities of lower background noise to guide larvae to boost recovery.