Exploring the Intertidal

Check out the intertidal zone and its biodiversity!

Biodiversity? | Marine Diversity | Intertidal Zone
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Click on the phylum listed below for pictures and information about representative members!

Porifera Cnidaria Annelida Arthropoda

Mollusca Echinodermata Urochordata Chordata

Rhodophyta Phaeophyta Chlorophyta

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What is the intertidal zone?

At the border between land and ocean there exists a wondrous diversity of life that can only be viewed by land lubbers at certain times of the day, and at other times surrenders itself to the fish and the crabs. This favourite location of beach combers is, of course, the intertidal zone!
It's nothing at all when the tide is high
It's just a bunch of waves
They whip all around the rocks
And chase all the fish into caves
But if you get there when the tide is low
And the pool is clear and clean
You can see to the bottom
The damnedest collection of creeps
you ever seen
Hungry flowers that feed on fish
Scooping in whatever comes
Crabs that grab another crab
And chew his legs, the dirty bum!
Starfish having himself a lunch
Eats a mussel off a shell
Shrimps and limpets and snails and eels
What a smelly tale they tell
Biting each other and eating each other
and lousing up the sea
Stupid sons of fishes, if you're asking me! 
by Richard Rogers and
Oscar Hammerstein III, Pipe Dream


The purpose of the following pages is to provide an introduction to intertidal (littoral) ecology and to provide a basic field guide for the rocky beaches of south west British Columbia (north west Washington State). If you don't enjoy the pleasure of living by the ocean, this guide can give you an online experience. Due to seasonal changes in the abundance's of some species, this guide is by no means complete, but hopefully will be added to as the occasion arises. Feel free to print off, or download, photos and information for educational use. 

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Intertidal organisms have become specially adapted to survive the rapid and significant changes in temperature, salinity, moisture, pH, dissolved oxygen, and food supply that occur on a daily basis due to the movement of the tides. Different plants and animals have excelled at dealing with different problems, and, as a result, the rocky intertidal is composed of a series of bands - representing dominant species at different tidal heights. This distribution pattern of different species along the shore is referred to as vertical, or intertidal, zonation. Typically, physical stresses set upper boundaries and biological stresses (competition, predation etc.) set lower boundaries.

The size and species composition of the intertidal will vary with degree of exposure. Large waves increase the size of the supralittoral ("splash") zone, and allow species to live higher in the intertidal. High levels of wave action also select for certain species and morphologies that can withstand the large forces without being ripped off of the rocks. The following is a brief overview of physical and biological characteristics of different regions in the intertidal.

The Supra-littoral Zone

The size of this zone depends on slope, splash, climate, and amount of shade. This zone is only covered by water during the highest high tides and marine organisms found here are adept at surviving in these arid conditions. Dissolved nutrients and oxygen are limited in the supralittoral and it is sparsley populated. Organisms found here are: black lichen, coralline algae, some small green algae and small, herbivorous snails and limpets. Some littorine snails have become so adapted to a dry environment that they will drown if submersed for any length of time! A barnacle dominated zone is found just below the lichen, in the upper mid-littoral.

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The Mid-littoral Zone

Increased submergence time makes this zone more favourable for many species and the major biological factor at work is competition for space. This zone is dominated by mussels, which would threaten to take over the entire zone if it wasn't for the voraciousness of their main predator - the seastar, Pisaster ochraceous.

Pisaster is called a "keystone species" because it affects the diversity of the area more so than one would expect based on its numbers. Some examples of other species found here are: snails, limpets, barnacles, chitons, Fucus (rock weed), polychaetes, small anemones, and shore crabs. Obviously, this is a very busy place!



The Low-littoral Zone

The highest level of dissolved nutrients (nitrogen and phosphorus) and plankton are found in this region, as it is normally covered by water. The high concentration of nutrients makes the low intertidal a favourite place for macro algae. As a result, many grazers are also found here. Large anemones, seastars, sponges, flat worms, crabs and nudibranchs all make their home in the low littoral.

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Tide Pools

Each tide pool is a unique environment formed in rocky depressions by the receding tide. Tide pool organisms face large and sudden changes in salinity, temperature, pH and other factors due to tidal movements. As a result, residents have many special adaptations. Tide pools differ from each other depending on depth and height in the intertidal. Anemones, sea urchins, barnacles, dog whelks, and sculpins create intricate interactions in these tiny, isolated micro-habitats.

check out this online field guide from the Monterey Bay Aquarium!


Kozloff, E. N. Seashore Life of the Northern Pacific Coast: An illustrated guide to Northern Calfornia, Oregon, Washington, and British Columbia. University of Washington Press, Seattle. 1983.

Lee, R.E. Phycology 3rd Edition.Cambridge University Press, Cambridge.1999.Sept, J. D. The Beachcombers Guide to Seashore Life in the Pacific Northwest. Harbour Publishing, Madeira Park, B.C. 1999.

Levington, J.S. Marine Biology: Function, Biodiversity, Ecology. Oxford University Press, Toronto. 1995.

Meinkoth, N. A. The Audobon Society Field Guide to North American Seashore Creatures.Chanticleer Press, Inc., New York. 1981.

Pechenik, J.A. Biology of the Invertebrates 4th Edition. The McGraw-Hill Companies, Inc. Toronto. 2000.

Sept, J.D. The Beachcomber's Guide to Seashore Life in the Pacific Northwest. Harbour Publishing, Madeira Oark, B.C. 1999.

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