The intertidal zone -- the area between high and low tides -- is a harsh and unforgiving habitat, subject to the rigors of both the sea and the land. It has four distinct physical subdivisions based on the amount of exposure each gets -- the spray zone, and the high, middle, and lower intertidal zones. Each subzone has a characteristic and distinct biological community.

Click on each of the zones below to see pictures of life within that zone and to read a description of the zone itself, then answer the questions below.


The spray zone is perhaps more a part of the land than the ocean. It is submerged only during rare, very high tides or severe storms, but is repeatedly wetted by splashing waves and wind-blown spray.

Lichen Periwinkle Snails
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The high intertidal is flooded during the peaks of the once or twice daily high tides, and out of water for the long stretches in between.

Purple Shoreline Crab Barnacles Blue-Green Seaweed
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The middle intertidal zone is generally submerged, except for a fairly short period during the turn of the low tide.

Mussel Bed Sea Urchin Snail Sea Stars and Anemones
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The lower intertidal zone is exposed only during the lowest spring tides.

Anemones Red Algae Dungeness Crab Sea Star and Sea Urchins Kelp
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  1. How does the abundance and diversity of life change across the various intertidal zones?
    Check Answer
    Life is very abundant and very diverse in the lower intertidal, and becomes less diverse towards the high intertidal. Both diversity and abundance are lowest in the spray zone.
  2. Describe how the physical stresses on life vary from the top of the intertidal zone to the bottom.
    Check Answer
    At the top of the intertidal zone, organisms spend more time exposed than underwater, so they will have to cope with desiccation and large temperature swings. Those pressures will be less important lower in the intertidal zone because of the protection provided by greater time underwater. But plants and animals there are at, or just below, the surf zone, so they will be pounded by waves.
  3. How are these stresses reflected in the types of animals that inhabit the intertidal subzones?
    Check Answer
    Many of the creatures that live in the highest intertidal zones either have the ability to close themselves up into their shells as a shelter against drying out, or are mobile enough to take cover. In the lower parts of the intertidal zone, many animals and plants have a means of attaching themselves in place and are either very sturdy or very flexible to stand up to wave energy.
  4. Living conditions in the intertidal zone are difficult, yet most of it is abundantly populated. What are some of the benefits of intertidal living?
    Check Answer
    The harsh conditions in the intertidal zone cut down on competition for food and for space by excluding many animals. And the shallow water transmits the sunlight for bottom-dwelling plantsí need for photosynthesis.