Habitat-Associated Soundscapes and their Potential Role as a Settlement Cue for Estuarine Larvae

Following a planktonic dispersal period, the larvae of bottom-dwelling marine organisms must locate suitable seafloor habitat in which to settle and metamorphose. Settlement is a critical phase in the life history of these benthic animals and recruitment patterns are important to marine population dynamics and community structure. For animals that are sessile or sedentary as adults, settlement onto substrates that are adequate for survival and reproduction represents a challenge to microscopic larvae since patchily distributed settlement sites may be difficult to find along a coast or within an estuary. Larval encounter with settlement habitats is likely facilitated through the use of habitat-specific physical and chemical cues, but the scales over which particular habitat-related environmental cues may operate are rarely measured. Recent studies have demonstrated that the underwater soundscape, the distinct sounds that emanate from habitats and contain information about their biological and physical characteristics, may serve as a broad-scale environmental cue for marine larvae to find settlement sites. Benthic habitats likely produce distinct soundscapes due to differences in the physical and biological contributors to ambient sound. Despite their potential importance to ecological processes, such as larval settlement, the soundscapes of most coastal and estuarine habitats have not been characterized, and larval responses to sound remain untested for all but a handful of marine invertebrate species. This dissertation investigated estuarine soundscapes as a larval settlement cue using subtidal oyster reef and off-reef soft bottom habitats in Pamlico Sound, North Carolina, USA as a model system and the Eastern oyster (Crassostrea virginica ), an economically and ecologically important reef-building bivalve, as a study organism.;The first part of this study investigated whether variation in an estuarine soundscape is a reliable indicator of habitat-type by comparing the sounds of oyster reefs and nearby off-reef soft bottom areas. Acoustic sampling in three locations found distinct acoustic patterns in oyster reef habitats compared to surrounding off-reef areas, with reefs producing high levels of sound within frequency bands dominated by snapping shrimp sounds and the vocalizations of reef-dwelling fish species. Compared to soft bottom habitat, oyster reefs had significantly higher sound pressure levels in the 2-23 kHz frequency band and higher acoustic diversity index values. The spectral dissimilarities between the two habitats and the reef sound propagation patterns were consistent over the summer and fall sampling season, and across two sampling years. The strength of the acoustic signal differed between reef sites, possibly reflecting differences in physical and biological characteristics that contribute to sound production. Manipulative laboratory playback experiments found increased settlement in larval oyster cultures exposed to oyster reef sound compared to unstructured soft bottom sound or no sound treatments. In field experiments, ambient reef sound produced higher levels of oyster settlement in larval cultures than did off-reef sound treatments. The results suggest that oyster larvae have the ability to respond to sounds indicative of preferred settlement sites, and this is the first evidence that habitat-related differences in estuarine sounds influence the settlement of a mollusk.