A giant clam's dazzling colors come from algae living inside its own flesh
Giant clams host millions of photosynthetic algae called zooxanthellae directly in their mantle tissue, the same partnership corals rely on, and the algae's pigments produce the clam's vivid blues, greens, and golds. In return for shelter and sunlight exposure, the algae hand over a steady supply of sugars, letting the clam grow to over a meter across on a diet it largely grows itself. Losing this partnership to warming water can bleach and starve the clam, just as it does coral.