Fiddler crabs with snapping claws. Needlefish with sharp, slashing teeth. Slimy eels who wiggle when they swim. Bull sharks with dangerous bites. These animals and many more make their homes in estuaries-places where freshwater rivers join saltwater oceans. So many animals lay their eggs here that scientists call these locations "nurseries of the seas." Rivers rush out. Waves crash in.

Estuaries and the animals that live there stay in constant motion.