Estuaries are usually filled with shallow waters, and sunlight reaches all levels of the water. Marsh grasses, algae, and other kinds of plants live in estuaries and provide food for a variety of fish, crabs, oysters, and shrimp. Estuaries are especially important since they act as nurseries for many different types of young fish and other animals before they head out toward the open ocean. Many sea birds also nest in estuaries.