Animals and their epibiota as net autotrophs: size scaling of epibiotic metabolism on snail shells

Animals are heterotrophic by definition, but species from many taxonomic groups are hosts to epibiota that may alter their net metabolism. We tested the degree to which snail-shell epibiota can generate net ecosystem productivity for snails and their epibiota (snail-epibiota ecosystems; SEEs) after accounting for snail respiration. We focused on 3 species from the Lavigeria snail assemblage in Africa's Lake Tanganyika and quantified the scaling of SEE metabolism with shell size under light and dark conditions. The metabolism of snails and their epibiota shifted significantly across the size gradient. SEEs of large snails (>20 mm) were consistently autotrophic during the daytime, reflecting increases in shell algae as snails move into well-lit microhabitats after reaching a size refuge from predation by crabs. We extrapolated daytime individual SEE metabolism patterns to snail assemblages at 11 sites and found that SEEs range from heterotrophic to autotrophic in aggregate, reflecting spatial differences in size distributions. Our integration of organismal traits, species interactions, and assemblage structure reveals the important role of epibiota in organismal metabolism. Large epibiotic contributions to organismal metabolism could be widespread among animals that live in well-lit environments and fertilize their epibiota, but depend on the scaling of epibiotic and host metabolism with body size.