Microhabitat selectivity shapes the seascape ecology of a carnivorous macroalgae-associated tropical fish

Habitat quantity and quality can be key drivers for the distribution and abundance of animals in heterogeneous landscapes. Macroalgal meadows are often a prominent component of tropical marine ecosystems, yet we have little understanding of how dynamic shifts in this habitat type may affect patterns of fish biodiversity. We examined whether a tropical carnivorous wrasse, Xenojulis margaritaceus, was a macroalgal specialist that responded to changes in canopy habitat structure across space and time. Field surveys revealed that X. margaritaceus exclusively occupied macroalgal meadows, where it foraged for invertebrate epifauna on a range of macroalgal genera such as Sargassum and Lobophora. During summer, X. margaritaceus preferentially occupied the canopy-forming macroalgae Sargassum. Sargassum canopy height, percent cover of understorey algae and the abundance of strong competitors and roving predators provided the best predictors for the abundance of X. margaritaceus abundance across the seascape. Despite seasonal shifts in habitat use to include more understorey algae during winter, X. margaritaceus displayed significant declines in abundance from summer to winter according to the extent of seasonal canopy habitat loss within each meadow. We conclude that carnivorous fishes can be dependent on tropical macroalgal meadows, where they may be vulnerable to a loss of canopy habitat quality arising from local factors and climatic forcing.