This is a cartoon of a typical rocky shore. The length of time each part of the rock, from lower to upper shore, is exposed determines the types of organisms that live there. Some are better adapted to survive longer periods of exposure to the air, whilst others are more competitive nearer the low water mark. This causes ‘zonation’- patterns of distribution of organisms. The intertidal is very challenging! The markers on the left show some of the key stressors on a tidal shore; temperature, salinity, aerial exposure, competition and sunlight. In adapting to these challenges that cycle with the tides and day and night, organisms have evolved tidal ‘clocks’ so that they can move or prepare themselves for predictable changes in their environment resulting from tidal cycles.