Effects of microplastics on the growth and behaviors of larval sea urchins heliocidaris crassispina

The increasing abundance and prevalence of microplastics have drawn attention to their potential impacts on marine ecosystems. Field surveys have revealed that microplastics in nature are far from a homogenous mixture of beads, instead, there is a diverse range of physical (size, shape) and chemical characteristics. While there is a growing number of laboratory experiments on the biological impacts of microplastics, these studies often used microplastics of a single polymer type, with a narrow size range, and/or of homogenous shape, i.e., lacking environmental relevance. To fill in this knowledge gap, we compared the effects of commercial polystyrene microbeads with those of microbeads extracted from toothpaste on the growth and development of larval sea urchins, a tractable study system of ecological and commercial importance. Larval urchins were robust and survived under microplastics exposure, even at high concentrations. Larvae fed only with commercial polystyrene microbeads grew relatively longer arms, a known plasticity response to starvation, earlier than those fed only with toothpaste beads. Such observation cautions against overgeneralizing observations based on a single bead type. We further investigated the ability of larval urchins to differentiate between algae and microplastics. Preferential ingestion of algae over polystyrene microbeads was compromised after prolonged exposure to microplastics, even at a low concentration. Investigating the feeding processes in details, the roles of exposure to microplastics and short-term starvation on the food handling of larval sea urchins were also examined. Increased fasting duration promoted food capture and ingestion while exposure to microplastics retarded digestion, assimilation and egestion. Such observations highlight the dynamic nature of larval feeding with behaviors changing within hours of exposure to a new environment. Overall, microplastics negatively impact marine invertebrate larvae through behavioral changes, and such adverse impact is likely to worsen with rising concentrations in the environment.