The coastal zone that is sometimes wet and sometimes exposed to air because of the action of the tides. The intertidal zone is also called the littoral zone. Because of the extreme variability of conditions on a daily basis, creatures of the intertidal zone are well adapted to these stresses, and there is a vertical differentiation of the zone into sub-zones based on how much water they are usually exposed to.
The high intertidal is inhabited by creatures such as crabs, limpets, and barnacles, which are happy to be completely dry for a large part of the day. In the middle region are the creatures typically found inside rock pools, such as sea anemones, mussels, and starfish. They need regular immersion but have enough resources to survive the periods without. At the lowest level are essentially marine creatures like sea urchins which have adapted to survive some degree of exposure.
Above the high tide mark is the supralittoral or spray zone. This normally is exposed only to the spray from breaking waves, so typically contains salt-adapted land plants, but may expect to be submerged in extreme conditions. There are not enough nutrients or oxygen for marine species to survive well here.
A rocky coast produces the most diverse intertidal communities, but a sandy substrate, which favours burrowers, also shows vertical differentiation.
Apart from the amount of water covering them, other stresses affecting intertidal dwellers are the force of waves, acidity, temperature, the amount of dissolved oxygen and nutrients, and the salinity. Many of these factors vary as salt water evaporates during the lower phases of the tides. The strong seashells of barnacles and the tough holdfasts of brown seaweeds (Phaeophyta) are adaptations to withstand severe wave action.
Since in general the more stable and nutrient-rich lower zones are friendlier to life than the upper zones, a species' range tends to be limited at the low end by predators, but at the upper end by their physical adaptations to survive extremity of condition.
http://www.vanaqua.org/shock/intro.htm - This contains a Shockwave educational game of placing different creatures in their correct zone
http://oceanlink.island.net/oinfo/intertidal/intertidal.html