Feeding responses of reef-building corals provide species- and concentration-dependent risk assessment of microplastic

The negative impacts of microplastic on reef-building corals are often attributed to the feeding responses to these particles. Although reactions to and ingestion of microplastic are frequently reported, a quantitative comparison to natural particles and of the factors influencing these responses is largely missing. Thus, this study aims to compare the feeding rates of corals to microplastic and natural particles, considering factors influencing these responses. Specifically, we I) studied the feeding responses of corals to microplastic, natural food, and non-food particles, II) examined the influence of biotic factors (i.e., biofilm on the particles and presence of natural food), III) evaluated species-specific differences in feeding responses to microplastic particles, and IV) applied a toxicodynamic model for species- and concentration-dependent risk assessments. We assessed the feeding responses of 11 coral species, spanning different life-history strategies and growth forms in experimental feeding trials. The results showed that the feeding responses of corals to microplastic differ from those to naturally occurring particles. Reactions to microplastic and natural food occurred equally often, while sand was more frequently rejected. Yet, the ingestion process was much more selective, and microplastic was ingested less frequently than natural food. The presence of a biofilm and natural food had activating effects on the feeding behavior of the corals on microplastic. Generally, coral species that exhibit a higher degree of heterotrophic feeding also reacted more often to microplastic. The species- and concentration-dependent toxicodynamic risk model built on these data reveals that most tested coral species are unlikely to be at risk under present environmental concentration levels. However, highly heterotrophic feeders, such as Blastomussa merleti, or generally vulnerable species, such as Pocillopora verrucosa, need special consideration. These findings help to better evaluate the responses of corals to microplastic and their risk in an increasingly polluted ocean.