Diel patterns in sea urchin activity and predation on sea urchins on the Great Barrier Reef
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Understanding diel patterns in sea urchin activity is important when assessing sea urchin populations and when interpreting their interactions with predators. Here we employ a combination of surveys and a non-invasive tethering technique to examine these patterns in an intact coral reef system on the Great Barrier Reef (GBR). We also assess local scale variation in relative diurnal predation pressure. Surveys revealed that sea urchins were active and exposed at night. Echinometra mathaei and Echinothrix calamaris were the most abundant species with significantly higher night densities (0.21 and 0.03 ind. m−2, respectively), than daytime densities (0.05 and 0.001, respectively). Bioassays revealed that exposed adult E.mathaei (the most abundant sea urchin species) were 30.8 times more likely to be eaten during the day than at night when controlling for sites. This observation concurs with widely held assumptions that nocturnal activity is a risk-related adaptive response to diurnal predation pressure. Despite relatively intact predator communities on the GBR, potential predation pressure on diurnally exposed E. mathaei assays was variable at a local scale and the biomass of potential fish predators at each site was a poor predictive measure of this variation. Patterns in predation appear to be more complex and variable than we may have assumed.
KeywordsCoral reefs Sea urchin Predation Echinometra mathaei Tethering Ecosystem function
We thank the staff of the Lizard Island Research Station (an Australian Museum facility) for the provision of logistical support and facilities. Thanks also to A. Miskelly for assistance with sea urchin identifications and to C. Lefevre and R. Schroeder for invaluable support in the field. We also thank H. Sweatman, A. Davis, and two anonymous reviewers for their constructive comments. Financial support for the project was made available by the ARC Centre of Excellence for Coral Reef Studies.
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