The ocean floor holds its own secrets, and some are deadly. Bonamia ostreae is a microscopic parasite that targets the flat oyster (Ostrea chilensis), also known as the Bluff oyster. It invades the oyster's blood cells, causing systemic infection and eventual death. The disease is called bonamiosis. It was first detected in New Zealand waters in the late 1960s and has since become endemic in many shellfish beds. The impact is severe. Infected oysters weaken, stop growing, and die. For commercial fisheries, this is an economic disaster. For the ecosystem, it is a shift in the balance. Flat oysters are filter feeders. They clean the water. When their numbers drop, water quality can suffer. Other species may fill the gap, but the community structure changes. The parasite spreads through water currents and by the movement of infected oysters. There is no treatment. Management relies on monitoring and restricting the movement of stock from infected areas. The Bluff oyster fishery has adapted, but the threat remains. Every season is a gamble. The parasite does not care about tradition or taste. It only cares about the host. The oyster carries on, filtering the tide, hoping its immune system holds. Sometimes it does. Often it does not.