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Heart cockle shell. The shell of the heart cockle (Corculum cardissa), a species of marine bivalve mollusc in the family Cardiidae. This species is found in the Indo-Pacific region. It has a symbiotic relationship with dinoflagellates (zooxanthellae), which live in its mantle, gills and liver. The heart cockle has a translucent shell and often lies near the water surface, allowing photosynthesis in the dinoflagellates. The mollusc benefits from the metabolites produced and the dinoflagellates benefit from a protected environment. (KEYSTONE/SCIENCE PHOTO LIBRARY/SMITHSONIAN INSTITUTION/SCIENCE PHOTO LIBRARY)