World Builders™
World Builders™
Session Five  --  Seaweeds
Session Five  --  Seaweeds

The Littoral Zone
The Littoral Zone

The littoral zone is the area between the high tide mark and the low tide mark.

     The littoral zone, also called the intertidal zone,  is a challenging one for life forms.  It lies between the high tide mark and the low tide mark, and so every day it is both wet and dry.

     Every day the tide comes in twice and goes out twice.  The seaweeds and animals that live in this zone are exposed to the temperatures of the land twice a day.  On some days the air and the dry sand are hot, and heat and wind dry out the life forms as the water pulls back and leaves them high and dry.  Sometimes the nights are very cold.  These large, rapid temperature changes do not happen in the sea,  

     Desiccation (drying out) is another problem that littoral zone inhabitants have to solve.  Seaweeds sometimes have very thin blades, and the blades can dry out in only a few hours.  Some seaweeds can be dried out until they are  stiff and hard and still recover when they are returned to the seas.  Others may grow in thick clumps that hold water and so survive.

     This picture shows some brown seaweeds clinging to the rocks after the tide has gone out.  They are still moist, so perhaps it is cool today.  Notice the small brown seaweeds that are getting started on the rock on the right.  They are still pretty small, and probably at the greatest risk of drying out.

     Another possible problem is sunburn.  When the earth was young, ultraviolet radiation from the sun poured down on the planet, making life on the surface impossible.  Until the cyanobacteria had made enough oxygen for the ozone layer to form, life could not survive on the planet's exposed surface.  However, water absorbs some of the ultraviolet light, making it possible for seaweeds to live under water.  

      Some seaweeds actually have pigments that protect them from over-exposure to the sun.

     Today, when there are many organisms on earth, the creatures of the littoral zone must deal with predators who might eat or harvest the seaweed, or even just trample on it.  They have a double set of predators -- sea creatures eat the seaweeds when the water covers them, land creatures eat the seaweeds when the water has withdrawn from the high tide line.

     The littoral zone animals must also keep moist somehow.  Look at the periwinkles, the barnacles, and the crabs.  What do you see?  All of them have exoskeletons (skeletons on the outside that protect them from predators, and from drying out.

     The littoral zone is a zone of transition.  It was in this difficult environment that seaweeds and small animals began to learn how to live in air.  The animals and plants that you create for your planet will need to cross this strip of sand and rocks if your continents are to be populated by living beings.

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Header and pictures by Viau at Olympic National Park 
© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .