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The Intertidal Zone

Channel Islands - The Intertidal Zone
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Anacapa Island Anacapa Island, the closest to the mainland of the five islands within the sanctuary and park, is composed of three islets that are usually inaccessible from each other except by boat. When the tide recedes to its lowest mark, however, it is actually possible to walk through the drained sea channel that separates two of the islets, Middle and West Anacapa, to explore the back side of the island.

The open-ocean side of West Anacapa is lined with rocky beaches thick with marine organisms. This transition zone, which leads a double life as both channel and beach, is a fascinating part of the marine environment.

The capricious habitat that lies between high and low tide lines is called the intertidal zone. Marked by dramatic fluctuations in moisture, temperature and sunlight, life in the intertidal zone is fraught with challenges that its residents combat in a variety of ingenious ways. At low tides, the flora and fauna that live here must cope with exposure to air, rising temperature, desiccation by the sun, and the shock of a freshwater bath from rainfall. At high tide, near shore fish roam, the submerged rocks in search of food and the pounding of waves threatens to dislodge animals from their hard-fought domain.

Tides and waves are the principal artists of the constantly changing intertidal world. Tides are caused by the gravitational attraction of the moon and the sun combined with the centrifugal force generated by the spinning of the Earth in concert with the moon. These competing tugs cause the Earth' s oceans to bulge at opposite sides, creating variations in the level of seawater.

At times of new and full moon, the Earth, moon and sun fall into line with each other, intensifying the gravitational pull; as a consequence, tides rise and ebb to their most extreme levels, creating what is known as spring tides. Their name comes not from the season of the year but from an Anglo-Saxon word meaning "a rising or welling of the water."

During the first quarter and third quarter phases of the moon, the moon and sun are at right angles with respect to the Earth, and their gravitational pull is minimized. At these times, moderate tides occur. They are called neap tides from an Anglo-Saxon word meaning scanty.

Tides vary considerably from one position on the planet to another. At our latitude, we experience diurnal or twice-daily tides: Two unequal high tides alternate with two unequal low tides each 25 hours. The highest tides in the Santa Barbara area normally occur in January, when the Earth is closest to the sun. The tidal range on the five islands within the sanctuary is approximately seven feet.

The portion of beach extending from low tide to high is known as the sandy intertidal. Of the different parts of the intertidal zone, this area is the harshest and least habitable because of its vulnerable-ity to waves. Because shifting sands provide little stability, not many species have been able to carve out a life here. That does not mean, though, that the sandy intertidal is uninhabited. Animals such as sand crabs, sand fleas, blood worms and sometimes clams can be found here in abundance. When the tide is low, they find protection by burying themselves in the cooler, moister depths, hiding from predators. At night, many of these creatures surface warily, always ready to dive back down for cover.

Rocky intertidal areas present their own challenges to survival, yet they provide homes for a diverse array of marine creatures. Here tides and waves, reinforced by sunlight, air and competition for space, sketch invisible borders up and down the rocks, segregating the organisms into distinct bands or zones. Crossing the border into a higher or lower neighborhood is usually fatal.

The upper portion of the rocks is called the splash zone. This area is rarely covered by the tide; spray from slapping waves provides the moisture. The splash zone is wet only one quarter of the time. The high-tide zone is next; this band is uncovered at nearly every low tide, which means that about half of the time it is exposed to the elements. The mid-tide zone is completely uncovered during spring tides and at least partially uncovered at other low tides; thus, one quarter of the time it is dry. The low-tide zone is the largest; only spring tides expose it to sunlight and air. It lies below sea level.

Though this system of zoned neighborhoods is quite complex, it is easily visible, even to untrained eyes. The presence of one or more indicator species, those whose range is so limited that they can be