The Formation Of Rocky Shores

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the length of time each is exposed. At the highest point on the rocks is the black zone, marked by blue-green algae. This transition area between land and the marine environment is flooded only during the high spring tides. The algae, enclosed in gelatinous sheaths to resist drying, are grazed on by periwinkles, which are protected under tightly sealed, conical shells. Below the black zone lies the white zone, where barnacles are tightly glued to rocks. Living among the barnacles are rock-clinging mollusks called limpets. At low tide, barnacles keep their four movable plates closed to avoid drying. At high tide they open the plates and extend six pairs of wand-like tentacles to sweep the water for microscopic life. Preying on the barnacles are hole-drilling snails called dog whelks. Below the white zone and in some places overlying the barnacles are rockweeds, which have no roots but attach themselves to rocks by holdfasts. Brown algae are rockweeds that grow more ...

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