Click on the page icon for Lesson 3 in Adobe Acrobat format (200K). Includes Activity Page 3.
1. Begin the lesson by reminding students that waves deliver food and circulate oxygen to organisms on the coral reef. Emphasize that tides play a similar role in Maine's rocky coast ecosystem. Explain that tides result mainly from the gravitational pull of the Moon on the rotating Earth (see diagram, above right). On the side of Earth facing the Moon, gravitational pull is greatest. Here, and on the opposite side of Earth, the sea bulges, causing high tide conditions. These bulges take water away from the remaining areas of the oceans, resulting in low tide conditions elsewhere. During a full moon, when the Sun, Earth, and Moon are aligned, the tides are highest. Such high tides, and their corresponding low tides, occur twice each day. The many bays and inlets along the northeast coast of the United States may experience considerable variation in water level between high and low tides.
2. Tell your students that conditions along the Maine coast change from hour to hour, day to day, and season to season. Short summers give way to long, stormy winters. Cold winter temperatures drive intertidal animals close to the low-tide elevations or out of the intertidal region entirely. Many vertebrate animals fly or swim out of the cold shallows into deep water or to the south, where temperatures tend to be warmer. The invertebrates that cannot migrate have breeding cycles that closely align with seasonal and even tidal cycles.
3. Give each student a copy of Activity Page 3. Ask your students to examine the diagram carefully as you describe some of the living things found along the rocky shore. Emphasize that the changing tides expose many organisms to varying amounts of water and direct sunlight. In the splash zone, algae, periwinkles, and other shelled animals fix themselves to rocks to withstand the awesome power of crashing waves. By some estimates, the pressure exerted by a pounding wave may be one and a half tons per square foot of rock.
In the intertidal zone, barnacles build limestone forts around themselves. Then, as some scientists explain, they spend their lives lying on their backs kicking food into their mouths with their feet. Below the barnacles are the rockweeds, which need to be submerged at least an hour during each tide. Mussels live among the rockweeds, attaching themselves to rock by silken threads. These organisms close their shells when the tide is low and then open them to filter-feed when the tide is high.
Sea stars make their homes in the subtidal zone and prey upon mussels and sea urchins. Their five-sided body structure consists of an exoskeleton of tiny spines and a mouth in the center of the ventral, or bottom, side. They use the suction of their tube feet to open the shells of their prey. Lobsters also live in this zone, eating almost anything, alive or dead. The tides, currents, and waves stir up nutrients, delivering food to many organisms that attach themselves to the ocean floor. Inside the mud and among the waving grasses, worms, clams, and bacteria digest dead organisms and recycle wastes.
4. Ask your students to complete Activity Page 3 by writing their answers on a blank piece of paper. When they finish, discuss the correct answers with the class. To conclude the lesson, ask students to describe what the diagram might look like at high tide. (Water would be up to the high-tide line, and only the periwinkles would be out of the water.)