There is a need to identify and better understand the impacts of underwater anthropogenic sound on the marine environment. For example, very few investigations concerning anthropogenic sound impacts on crustaceans have been published. Therefore, the research described in this thesis characterises potentially significant anthropogenic sound sources in New Zealand’s coastal waters and begins to determine their influence on the settlement-stage larvae of estuarine crabs. Several field-based experiments established that common sources of underwater anthropogenic sound in New Zealand were of appropriate intensities and frequencies to mask, over large areas, the natural underwater sounds found from several habitats. Laboratory-based experiments showed some anthropogenic sounds influenced the natural settling behaviour of settlement-stage larvae in several species of estuarine crabs. Notably, turbine sound was found to delay metamorphosis beyond a silent control. The research also characterised the ambient underwater acoustics from one of the largest estuaries in the world. Subsequent comparisons found that the sound from an operating tidal turbine was of greater intensity across the frequency range 0.1 – 20 kHz compared to the natural estuarine sound. The results also demonstrated that geometric spreading models are not accurate in shallow coastal waters and a simple model for the preliminary assessment of sound spreading was proposed based on field data. The findings presented in this thesis extend our knowledge of the role of natural underwater sound in mediating larval settlement in important coastal organisms. Specifically, this work demonstrated that anthropogenic sound may mask natural underwater sounds in estuaries and is likely to influence the patterns of settlement of estuarine crab larvae. Overall, the results raise concerns about potential long-term ecological impacts of anthropogenic sound in coastal marine habitats.