The functional role of tabular structures for large reef fishes: avoiding predators or solar irradiance?
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Large reef fishes may often be seen sheltering under tabular structures on coral reefs. There are two principle explanations for this behaviour: avoidance of predation or avoidance of solar irradiance. This study sought supporting evidence to distinguish between these two explanations by examining the usage of tabular structures on a shallow mid-shelf reef of the Great Barrier Reef at midday and sunset. If predation avoidance is most important, usage should increase towards sunset; conversely, if avoidance of solar radiation is most important, more fishes should use cover at midday. Underwater video observations revealed that tabular structures were extensively used by large reef fishes at midday, being characterised by numerous species, especially Lutjanidae and Haemulidae. In contrast, at sunset, tabular structures were used by significantly fewer large reef fishes, being characterised mostly by species of unicornfish (Naso spp.). Resident times of fishes using tabular structures were also significantly longer at midday (28:06 ± 5:55 min) than at sunset (07:47 ± 2:19 min). The results suggest that the primary function of tabular structures for large reef fishes is the avoidance of solar irradiance. This suggestion is supported by the position of fishes when sheltering. The majority of large reef fishes were found to shelter under the lip of tabular structure, facing outwards. This behaviour is thought to allow protection from harmful downwelling UV-B irradiance while allowing the fish to retain photopic vision and survey more of the surrounding area. These findings help to explain the importance of tabular structures for large reef fishes on coral reefs, potentially providing a valuable energetic refuge from solar irradiance.
KeywordsAcropora Coral reef fish Predation Shelter Twilight UV irradiance
This study was conducted on Jiigurru in the traditional sea country of the Dingaal people. Thanks to S. Bainbridge, L. Grutter, A. Hoey and N. J. Marshall for helpful discussions, Alastair Harborne and two anonymous reviewers for constructive feedback, E. McClure for assistance in the field and helpful discussions, and the staff of Lizard Island Research Station (A facility of the Australian Museum) for invaluable support and facilities. Funding for the project was provided by the Australian Research Council (DRB) and a James Cook University GRS grant (JTK). Research was conducted under GBRMPA permit #G11/33857.1.
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