Where the land dips steeply into the sea, then the gaps between the tides is condensed and the creatures who live here are threatened by two dangers - being dried out at low tide or being preyed on by creatures from the sea at high tide. These two factors interact to produce distinct horizontal bands on rocky shores, with each band dominated by particular species. The bottom band is only exposed at the lowest of tides when the moon and the sun are in alignment so both pulling in the same direction. The animals here cannot tolerate being out of water for long as they have no special devices to prevent them from drying out. Off the coast of North America, for example, sea urchins gnaw away at encrusting algae when the water is low, but out of water all they can do is hang on to the rocks. Here, giant sea anemones can only droop their tentacles when they are out of water, and many will even withdraw their tentacles since there is nothing for them to feed on in the air. Starfish are meat eaters and there is one species here that feeds on mussels. It envelopes them with its adhesive arms, wrenches apart their shells, then feeds on the flesh within. But since they can not feed out of water, conditions favour the mussels if they above the low-water mark. Mussel beds are home to many other creatures including small starfish, worms, crustaceans, winkles and other molluscs. The mussels hold onto the rocks with bundles of threads but they can not withstand the pull of the roughest waves, and in winter storms many of them are ripped away. In more exposed places goose barnacles replace the mussels, clasping the rock with a fleshy foot. They feed by holding out stiff fan-like arms that catch particles from the waves as they subside.