Habitat soundscapes and sound production by tropical and temperate sea urchins and the swimming behaviour of their larvae

Abstract

The distribution, abundance and behaviour of soniferous organisms influence the spatial variability of underwater noise characteristics or ‘soundscape’. In this way, the soundscape provides useful information on habitat and assemblages that enables navigation in larvae and can be used for environmental assessment and monitoring. Despite the ecological importance of sea urchins and evidence they produce sound of their sound, knowledge gaps remain regarding the role of adult and larval sea urchins in acoustic ecology. In this thesis the sounds produced by 3 temperate and 3 tropical species of sea urchins were characterised. The soundscape of sea urchin habitat from important ecological systems (sea grass, temperate rocky reef, coral reef) in eastern Australia were also characterised. The sea urchins made a ‘crunching’ sound when feeding that was commonly produced around dawn, dusk or midnight. This sound ranged from 2-22 kHz with highest intensity from 2-8 kHz. Centrostephanus rodgersii appeared to contribute to a diurnal chorus between 2-8 kHz in the ‘barrens’ habitat of Jervis Bay, NSW. Analysis of the 5000 Hz 1/3rd octave band revealed a difference between barrens sites of up to 10 dB re 1 µPa2. A similar difference was found between two coral reef habitats recorded at One Tree Island, QLD. Arm angle development reflects swimming ability in sea urchin larvae and so was documented for two-armed and multi-armed larvae forms to characterise their swimming biology. In a behavioural study of early and settlement stage larvae of H. tuberculata and C. rodgersii, early stage larvae exhibited signs of negative phototaxis swimming down during the day, suggesting that they exhibit diurnal migration in the field. However, larvae did not modify this swimming behaviour in response to a reef noise. This research will enable detection of sea urchin sounds in ambient noise recordings and provides insight into the impacts of sea urchin populations on the marine soundscape.