Shore Life The essentially marine organisms that inhabit the region bounded on one side by the height of the extreme high tide and on the other by the height of the extreme low tide. Within these boundaries, organisms face a severe environment imposed by the rise and fall of tides. For up to half of a 24-hour period, the environment is marine, the rest of the time it is exposed, with terrestrial extremes in temperature and the drying effects of wind and sun. Life on rocky shores, best developed on northern coasts, is separated into distinct zones that reflect the length of time each zone is exposed. At the highest position on the rocks is the so-called black zone, marked by encrusted bluegreen algae. This transition area between land and the marine environment is flooded only during the high spring or fortnightly 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 a so-called 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 are attached to rocks by holdfasts. Brown algae (Phaeophyta) are rockweeds that grow more than 8 feet (2.5 meters) long, the most common are the bladder wracks, with branching thalli up to 6 inches (15 centimeters) wide. Looking like fronds, the thalli have conspicuous, bubble-like swellings that buoy the plants at high tide. Blue mussels also inhabit this tide zone, especially where sediment accumulates. They attach themselves to the substrate by threadlike byssuses secreted by a gland in the foot. In the lowest zone, uncovered only during the spring tides, is the large brown alga Laminaria, one of the kelps. Beneath its frond-like thalli live starfish, sea cucumbers, limpets, mussels, and crabs. Also see Marine Life.