Intertidal Zones

An intertidal zone, also called the littoral zone, is the zone between mean high water and mean low water levels.

This zone is unique in that organisms are arranged in distinct belts that run parallel to the shore based on how much sunlight, air, rain, temperature, salinity, and pH difference they can handle.

All ranges for species are set by physical factors; usually the upper limit is set by the height of the water, and the lower limit is set by predation and competition.

There are three distinct zones in which different organisms live: –The High Zone –The Mid Zone –The Low Zone

The High Zone This zone is only covered by water during the highest high tides. The size of this zone depends on slope, splash, climate, and amount of shade. Dissolved nutrients and oxygen are limited and it is sparsely populated.

Organisms in the High Zone Many of these organisms have hard, flat shells, and have the ability to stay attached to the rocks as the waves crash down. They either feed only at low tide, or only at high tide.

1) Crabs: Have a hard shell that protects them and allows them to squeeze into tight places. They can survive for extended periods of time out of the water. During low tide they scavenge for bits of plants and animals.

2) Barnacles: They have a protective shell, but they do not move after reaching adulthood. They eat by kicking their feet into the water to capture plankton.

3) Limpets and Snails: They are soft bodied animals that produce a shell. They mostly scrape algae from the rocks, and only during high tide.

The Mid Zone This zone is more favourable for many species because it is submerged for more of the time. It is the most active zone and the main factor among organisms in this zone is competition for space.

Organisms in the Mid Zone 1) Anemones: They have soft, jelly-like bodies that absorb the impact of waves. When they reach adulthood they stay in one place, holding on with suction cups. When the tide goes out they fold their tentacles inward to preserve moisture.

2) Sea Stars: They have a malleable exterior which allows them to wrap tightly around rocks’ algae during tidal changes. They are carnivores and feed by covering their prey in digestive juices and sucking up the liquid

3) Chitons: These are prehistoric looking mollusks. They have 8 segments that attach to rocks with a foot. They are mostly herbivores and feed during high tide.

4) Mussels: They are bivalves and filter plankton from the water when they are covered. They remain in the same place their entire lives.

The Low Zone The highest level of dissolved nutrients and plankton are found here, as it is normally covered by water. It is only exposed during the lowest low tides.

Organisms in the Low Zone 1) Sea Urchins: They protect themselves from drying out by carving a hole in the rocks with their tough spines. To feed, they break off pieces of algae.

2) Nudibranchs: They are small, brightly coloured slug-like creatures that live in the deepest parts of tide pools. They are carnivores.

Tide Pools Tide pools are rocky areas on the edge of an ocean that are filled with sea water. They are created when the tide goes out, and each is unique based on the depth and width of the pool.