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The Rocky Intertidal Zone |
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Seastar Biology The Study Bibliography and Weblinks
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The intertidal zone is defined as the area between the high tidal mark and
the low tidal mark. Organisms living within the intertidal must be hardy
and adaptable to tolerate conditions underwater as well as exposure to
air. During high tide, when the intertidal is covered with water,
organisms must be able to bear the brunt of oncoming waves that can easily
dislodge organisms not secured to their substrate. During low tide, when
the intertidal is exposed to air, desiccation and overheating provide the
greatest obstacles for intertidal organisms. This is perhaps the most
important factor in determining where an organism may reside in the
intertidal. A central paradigm of the intertidal is that the upper
distribution of a species is limited by its ability to tolerate physical
factors such as desiccation. The lower distribution is limited by
biological factors, primarily competition and predation. This creates a
distinct pattern of vertical zonation across the intertidal.
In the high intertidal, organisms must be well adapted to withstand long periods of exposure to air. Primary producers in this zone include lichens and cyanobacteria. Herbivorous periwinkle species (Littorina sp.) and limpets graze on the various forms of algae that grow there. In the middle intertidal, barnacles, mussels and seaweeds all compete for space – a precious limited resource. Mussels are generally the dominant competitor on rocky shores and without the presence of predatory seastars would outcompete other taxa and occupy all available space. The low intertidal remains submerged the longest of all the intertidal zones, making it easy for predators to feed. It is dominated by red, green and brown seaweeds that cannot tolerate even the slightest desiccation. |
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