Though marshes may look like still seas of grass, they are among the most active and biologically productive habitats in the world. They are in constant motion, with tides continuously rising or falling. Birds and small mammals nest and feed here. Alligators are lurk, crabs scurry about, fish swim and turtles bask in the sun.. They are filled with oysters and shrimp, along with a range of invertebrates and amphibians. And when the wind blows, the cordgrass, rushes and sedges move gently in unison as if swaying to a song.