tidal range, or intertidal zone. Organisms that live in the intertidal zone need to adapt to changing physical and chemical conditions. There are five intertidal zones, each with differing levels of biodiversity. As you view each of these zones in the interactive lesson on the next page, compare the biodiversity of each zone. Also, think about the physical stresses organisms deal with in each zone, and how they might learn to adapt to these conditions. The five zones are as follows:

1. Spray zone - usually exposed, but can be underwater during very high tides, spring tides, or severe storms. The spray zone is constantly splashed by waves and ocean spray.
   
2. Upper intertidal zone - flooded during peak tides, but exposed at other times. This zone is bounded by high tide level and spring high tide level.
   
3. Mid intertidal zone - generally submerged except during low tide. This zone is located between high and low tide.
   
4. Low intertidal zone - exposed only during the lowest tides. This zone is located between the low tide and low spring tide lines.
   
5. Sub-tidal zone - located below the low spring tide line.
   

Then on the following page, explore the organisms that live in tidal pools. Tidal pools form along rocky coastlines, and contain pockets of seawater. Organisms in tidal pools need to adapt to the alternating high and low water levels and the associated changes in water chemistry (e.g. salinity), biological processes (e.g. predation by birds and land-based organisms ,when trapped and exposed as the water recedes), and physical properties such as wave action, exposure to air, and changes in temperature. As part of this interactive lesson, see how mussels, snails, urchins, anemones, rockweed, and sea stars cope with these chemical, physical, and biological challenges.