Experimental observation of microplastics invading the endoderm of anthozoan polyps

Coral reefs are being degraded worldwide by land reclamation and environmental factors, such as high seawater temperature, resulting in mass bleaching events. In addition, microplastics disturb the formation of coral-algae symbiotic relationships in primary polyps. In our experiments, we observed this effect in the bleached primary polyp Seriatopora caliendrum that lost its symbiont Symbiodiniaceae as a result of high water temperature. There was a higher incorporation of microspheres into bleached corals than in healthy ones. To understand the interference in symbiosis, we used the sea anemone Exaiptasia (as an anthozoan model organism) and fed it with microspheres. TEM results suggested the incorporation of microspheres and symbionts from the same phagocytosis zones in the mesenterial filament and endocytosis by the cells. In the tentacles, microspheres were in the same cell layer as the symbionts. These results suggest that microplastics occupy the spaces inhabited by Symbiodiniaceae, thereby hindering their symbiotic association.