Ambient underwater sound: measuring the importance of spatial variability and its effect on late stage larval crabs

Abstract

Recent studies have shown that underwater sound emanating from coastal reefs may be used for guiding the movements of a wide range of reef organisms to suitable settlement habitats. However, it is not known whether this underwater sound is also capable of mediating the settlement and metamorphosis processes in these organisms. The present study used laboratory- and field-based methods to determine whether ambient underwater sound is used as a settlement and metamorphosis cue in 10 species of larval crabs. The settlement stage larvae of five common crab species showed marked changes in swimming behaviour consistent with settlement and showed a significant decrease in time to metamorphosis (TTM) when exposed to replayed ambient reef sound compared with a silent control. Ambient underwater sound has the potential to convey valuable information about the type and suitability of the habitat at its source to settlement stage pelagic larvae provided different habitats produce distinctive underwater sound. Analyses of recordings from several different habitat types along the coast of north-eastern New Zealand showed that the sound emanating from different habitat types had marked differences in terms of gross character, i.e., spectral composition and sound level. When habitat specific sounds were used in laboratory- and field-based experiments a significant decrease in TTM was observed for settlement stage crab larvae exposed to favourable settlement habitat sound when compared to unfavourable habitats. Behavioural thresholds for habitat sound were determined experimentally by exposing settlement stage larvae to a range of sound levels from both favourable and unfavourable habitat types for settlement. Larvae did not respond to sound from unfavourable habitat types. However, for sound from favourable habitat types for settlement most crab species showed increasing reductions in TTM as sound levels were increased, suggesting that proximity to the sound source or settlement habitat is important in inducing faster settlement. The results presented in this thesis demonstrate that ambient underwater sound originating from coastal habitats mediates the settlement processes of the megalopae of many common coastal crab species in both temperate and tropical waters. It provides evidence that differences in the spatial and biological characteristics of underwater sound play a significant role in this process. Overall, the results of this research greatly extend our knowledge of the importance of underwater sound to recruitment processes of coastal larvae.