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Tropical seascapes are comprised of a range of patch habitat types, yet we have only a partial understanding of how local patch condition and seascape position may influence patterns of marine biodiversity, particularly for invertebrate taxa. We investigated how the epifaunal abundance and biomass of tropical Sargassum varied with canopy size (volume, total length and dry weight), local patch conditions (macroalgal composition, canopy structure and invertivorous fish biomass) and seascape setting (nearshore, lagoon and back reef) within the Ningaloo fringing reef ecosystem, Australia. A total of 49431 epifauna, dominated by crustaceans and molluscs, were extracted from the thalli of 81 tropical Sargassum polycystum individuals. Epifaunal abundance and biomass were most strongly correlated with host Sargassum canopy volume and dry weight, respectively. Epifaunal abundance and biomass also varied significantly among separate Sargassum meadow patches, with a significant interaction between canopy size and seascape position. Considerable site-level variations in epifaunal biomass density (mg per g Sargassum dry weight) were best predicted by either seascape context or local invertivorous fish biomass. Sargassum within meadows furthest from the back reef tended to have the highest epifaunal biomass (dominated by molluscs), while meadows closest to the back reef were dominated by crustacea. Sargassum within meadows with a high local abundance of invertivorous labrids and serranids tended to have the lowest epifaunal biomass. Strong Sargassum canopy size-epifauna relationships indicate that even small differences in canopy extent have major flow-on effects for the trophic function of tropical marine ecosystems by affecting the epifaunal secondary productivity available to higher-order consumers, such as fishes.

The University of Newcastle and NSW Department of Primary Industries provided funding with research conducted under NSW DPI Animal Care and Ethics Committee permit 10/09

L.N.W. was supported by an Honours Scholarship from the Australian National University (ANU). Funding was provided by the Australian Academy of Science (Thomas Davies Research Award to C.J.F.), the Research School of Biology at ANU and a Julian E. Tenison Woods Award to L.N.W. from the Linnean Society of NSW.

Behaviour can offset the limitations of the morphology and physiology of a species to profoundly influence patterns of resource use. In fishes, flow refuging is a behavioural mechanism that may facilitate the occupation of turbulent, high-flow habitats by reducing extreme environmental demands on their swimming capabilities. Using a novel combination of experimental and field techniques, we examined the refuging patterns of coral reef fishes to demonstrate the potential energetic advantages of flow refuging on coral reefs. Field evaluations of 3 common reef flat species (Halichoeres margaritaceus, Pomacentrus chrysurus and Chrysiptera brownriggi) revealed clear species-specific refuge selectively towards substratum holes and coral heads, with all species using these 2 refuge types in a manner that provided significant shelter from ambient flow. Indeed, at experimental flow speeds similar to those in the field (10 to 90 cm s–1), measurements taken in and around replicas of these refuges identified flow reductions of 60 to 100% depending on ambient flow speed, type and size of the refuge. In addition, the degree of refuging displayed by these reef fishes appeared to be strongly linked with swimming ability, with smaller individuals and slower-swimming species refuging more frequently than individuals with stronger swimming abilities (i.e. refuging frequency in P. chrysurus > C. brownriggi > H. margaritaceus). Ultimately, it appears that refuging is not just a predator-avoidance mechanism, but also enhances the ability of coral reef fishes to sustain themselves in high-flow environments, which may underpin their occupation of otherwise inhospitable wave-swept habitats.

Patterns of foraging behaviour are described for a local assemblage of wrasses (Labridae) at the within-habitat scale of 2 fringing reef sites at Lizard Island, northern Great Barrier Reef, to examine the relationship between locomotor abilities and foraging patterns. Focal individual censuses were used to record the distances travelled by individuals over 30 s and 5 min time periods, and their substratum microhabitat preferences during searching and feeding. Size of short-term foraging ranges, estimated by linear start-to-finish distances, appeared to be driven largely by the shape of the foraging path. Two major foraging modes were identified: directed (widely-foraging) and convoluted (restricted). Within each mode, a strong positive relationship was observed between estimated locomotory ability and foraging distances. Four major groups of foraging-microhabitat preferences were apparent: neutral foraging and positive selection towards aggregates, dead coral heads, or live coral. A weak relationship between foraging mode and estimated patch size of preferred microhabitats was observed, with species having a directed foraging mode most frequently selecting more spatially discrete microhabitats.

We examined the extent to which wave exposure and swimming ability shape the ontogenetic distribution patterns of wrasses across the Great Barrier Reef at 2 spatial scales: cross-shelf and within-habitat. Visual censuses were used to record the distribution of juveniles and adults among shelf habitats, and their relative position in the water-column within a single, wave-swept habitat zone. Pectoral fin shape was used as a measure of swimming ability. Examination of fins from a broad size range of individuals indicated an ontogenetic shift in swimming ability in 5 of the 8 species examined. Despite apparent differences in their swimming abilities, juveniles and adults of each species displayed similar distributions across the 11 shelf habitats. Significant differences in habitat distributions were only apparent among species. Microhabitat use within the reef flat, however, displayed a distinct trend of increasing water-column use with increasing size in those 5 species that displayed a marked ontogenetic shift in swimming ability. This shift in microhabitat use appeared to be due largely to flow-refuging behaviour in the small individuals and slow-swimming species.