Microtopographic refuges enhance recruitment and survival but inhibit growth of propagules of the tropical macroalga Sargassum swartzii

For organisms with discrete life-histories, any mechanism that enhances growth and/or survival at critical life-history transitions may significantly influence adult population size. On coral reefs, structurally complex microhabitats enhance the early post-settlement growth and/or survival of fishes and corals; however, the importance of such microhabitats to the early life stages of tropical macroalgae is largely unknown. Here, we investigate the effect of crevices on the recruitment, growth and survival of propagules of a common coral reef macroalga, Sargassum swartzii. We settled S. swartzii propagules onto terracotta settlement tiles that had a series of 3 mm deep crevices evenly spaced on their top (exposed) surface. Recruitment of S. swartzii was 21% greater, but propagules were 18% shorter, 18 d post-settlement within crevices than on adjacent exposed surfaces. Exposing tiles to local fish assemblages for 5 d showed that survival of propagules was 90% greater in crevices than on exposed areas of the tiles on the reef crest, but not on the reef flat. Underwater video footage revealed that few fishes fed from within the crevices (18% of all bites), with the majority of feeding being concentrated on the exposed surface of tiles. Interestingly, small-bodied fishes from the family Blenniidae (predominantly Ecsenius spp.) accounted for the majority of the feeding activity on the tiles and likely contributed to the mortality of propagules. Structurally complex microhabitats, such as crevices, that shelter vulnerable early post-settlement propagules from herbivory may therefore be important for the persistence of macroalgae on coral reefs.