The kinds of animals that live along a particular shoreline are determined by zone—upper, intertidal, or lower—and the type of surface—rocky, sandy, or muddy. This is also true of different species of the same animal. Hermit crabs, for example, live comfortably in the lower and intertidal zones of rocky shores, whereas mudcrabs prefer muddy estuaries.

Lower zone animals, including sea anemones, shrimp, and small fish, are underwater almost all of the time. Upper zone animals, such as ghost crabs, prefer dry conditions and therefore live on land. The middle, or intertidal, zone along rocky shores supports the most lifeforms, in spite of its being the harshest zone of all. Cockles, barnacles, clams, sea urchins, and fish can all be found in this zone. Some, such as clams, can survive during low tide when they are exposed to air by closing their shells to...