Microherbivores are significant grazers on Palau's forereefs

Herbivory plays an important role in controlling benthic dynamics on coral reefs. The previous studies have highlighted the importance of grazing herbivorous fishes in removing algal turf biomass, but fewer studies have investigated the impact of invertebrate microherbivore grazing. This study examined the impact of microherbivore grazing in areas of high- and low-wave exposure on the forereefs of Palau, Micronesia, in June 2015. Experimental tiles were placed on open benthos, and in benthic and suspended herbivore exclusion cages at exposed and sheltered sites to partition the grazing impacts of microherbivores from fish grazers while examining the effect of exposure on algal turf productivity. Microherbivore grazing significantly impacted algal turf biomass, and this impact was greater in exposed sites than sheltered sites. Exposure did not significantly affect algal turf biomass on experimental tiles in the suspended exclusion cages. Surveys of microherbivore density revealed only Paguroidea (hermit crabs, especially of family Diogenidae) were more abundant at exposed sites than sheltered sites. Furthermore, tank trials of grazing rates showed diogenid hermit crabs removed over four times as much algal turf biomass as Columbellidae (marine gastropods), the second most abundant microherbivores. These results show that microherbivores are significant grazers on Palau's forereefs, and may play an important role in maintaining reef resilience as reef health continues to decline worldwide. The significant role of invertebrate microherbivores in removing algal turf biomass should be investigated when considering the ecological role of herbivory on coral reefs.