A diurnal tideexposesthe organisms once a day, a
semidiurnaltide
twice. If the tide is
mixed,
the lowerof the two successive high tides may notsubmerge the high part of the zone, andthe higher of the two lows may not ex-pose the bottom part. Thus, even inplaces with a semidiurnal tide, some partsof the intertidal may be submerged or ex-posed only once a day. With so much variation in the time spent out of the water, different heights within the middleintertidal may support very differentkinds of marine life. In other words, themiddle intertidal is often made up of sev-eral separate vertical zones.