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Inter-Tidal Zones - Part 2

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 found only in one zone, tells you where on the hierarchy you are. For example, periwinkle snails prefer to be out of the water and are thus always found in the splash zone. Barnacles and chitons (see the mollusk with eight-plated shell, top right) may be attached to the rocks just below the periwinkles, straddling the splash and high-tide zones.

Goose barnacles Purple-shelled mussels define the limits of the high-tide zone. A mussel uses slender elastic strands called byssal threads, which are secreted through its foot, to grip the face of the rocks. A stripe of goose barnacles, whose name quite adequately describes their beak-like appearance, can also be found in this zone. These two animals, both of which are sessile, or stationary, close up to hold in moisture.

The aggregating anemone forms the lower border of the mid-tide zone, and below that lives the resourceful hermit crab, who borrows someone else's shell to use as a home.

A conspicuous fringe of green surfkrass, an unusual flowering plant that the Chumash used to make baskets, skirts and fishing line, indicates the beginning of the low-tide zone.

Each of the animals in this system has an upper threshold over which it may not tread. This limit is determined by a physical factor: its tolerance for dryness. Meanwhile, biological forces such as predation and competition for space rein each species in from below. The periwinkle snail could live in a lower zone, for example, but since it lacks the firm grip of a mussel or barnacle, it could get washed away. It also makes good food for its downstairs neighbors. Sea stars may live among the mussels in the lower limits of the high-tide zone, but they don't go any higher because they would dry out. Crossing the band of anemones below them carries another risk: contact with the stinging cells called nematocysts that anemones use to keep interlopers from crossing their zone.