English: A group of giant clams (Tridacna spp.) with their shells open in shallow water at the Steinhart Aquarium of the California Academy of Sciences in San Francisco, California. The exposed mantles of the clams, brightly colored due to their pigment cells, harbor symbiotic zooxanthellae (photosynthetic dinoflagellate algae). The larger, elongated inhalent siphon, surrounded by small tentacles, and smaller, more tube-like exhalent siphon (between which are the filter-feeding adapted gills) are visible in each individual. A goldbelly damsel swims at top center.