“Sea grapes” may sound like something Poseidon would snack on, and not a killer algae. Yet Caulerpa racemosa var. cylindracea poses a serious threat to marine life. Spread by the bilge water of boats, this fast-growing alga is quick to take root, squeezing out native species. But there is one spot in the Mediterranean where cylindracea hasn’t yet taken over, and biologists like Juan Manuel Ruiz Fernández are trying to discover why.