The intertidal refers to the zone between tides. Thus, this is the marine environment that is exposed at low tide and submerged during high tide. Due to periods of exposure and submersion the organisms that live in this environment support extreme conditions, such as temperature and salinity oscilations, mechanical wave action, among others.

The distribution of organisms int he intertidal is conditioned by factors (physical and chemical elements of the environment) and biotic (effects caused by living organisms in and ecosystem). Thus, species separate into communities that occupy well-defined levels.

This phenomenon is called by zonation and, in general, in rocky shores it comprises three zones:

- Upper intertidal – zone of lichens and periwinkles;

- Middle intertidal – zone of barnacles, limpets and mussels;

- Lower intertidal – zone occupided by several species of algae and invertebrates.

There are several factors that allow the establishment of species along the coast. In general, the species found at the upper border of the intertidal are better able to tolerate thermal oscilations and desiccation than those at the lower limit. On the other hand, species that settle int he lower part of the coast are limited mainly by biotic factors such as predation and competition.

On sandy beaches the zonation is not visible to the surface of the sediment. The distribution of species in this case occurs horizontally and vertically given the tridimensionality of the environment.