Unlike the larvae of marine pearl oysters, the specialized larvae of freshwater pearl mussels must attach to a fish for survival. Their tiny hooks cling to the fish's fins or gills until the larvae develop enough to drop off and settle on a lake bottom or streambed. The attached larvae feed on particles from the fish's food supply, and the fish carry them to new habitats. Because the environment must sustain both the larvae and the fish on which they depend, the presence or absence of pearl mussels may indicate the relative health of a river or lake.