At the junction of the oceans and the continents there is a narrow ribbon of ecosystems: the estuaries, which are among the most productive on earth. An estuary is an area near the mouth of a river or a bay where salinity is intermediate between fresh water and the salinity of the oceans. Estuaries, beaches, and the rocky shores of seacoasts are continuously subjected to the action of the tides that affect the plants and animals in this intertidal zone. Tides are the key to the high productivity of these ecosystems as they keep nutrients in constant circulation, removing the waste products of metabolism quickly. Although physical factors, such as salinity and temperature, are far more faviable in these environments, there is such an abundance of food that they are literally packed with life.Barnacles and mussels have special means of attaching themselves firmly to rocks so that they can withstand the pounding of waves and not be dislodged. They open at high tide to receive the food they filter from the water. When the tide recedes, they close their valves and are protected from drying while they are exposed to the air. Fiddler crabs retire to burrows in mud during the high tide and emerge to forage for food when the tide is out. Thus, the lives of these animals are geared to the tides through "internal clocks" which regulate their daily activities.The value of salt marsh or a mud flat is not immediately apparent and we are often tempted to "improve" our environment by dredging a bay to make room for boats and marinas, or filling a salt march to provide sites for homes and factories. But these environments are extremely important sources of food and nutrients for deeper offshore waters that supply most of the marine life we harvest. They act as breeding grounds and nurseries for our most desirable commercial and sport fish. The coastal marshes also act as a buffer against storms and hurricanes, dissipating the energy of waves before they reach higher ground. When we tamper with these ecosystems we violate important ecological principles that might have, in the long run, yeilded the greatest benefits to man.