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18.10: Ocean Ecosystems

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    What would a field trip into the ocean reveal?

    There are all sorts of incredible life forms in the seas! Most live near the surface or come up to the surface. Some live very far below the surface and never come up. All are well adapted to their watery environment.

    The Intertidal

    Have you been to a rocky shore and seen the intertidal? It's the zone at the beach that is covered when the tide is up and uncovered when the tide is out. An abundance of life is found there, but it's a difficult place to live. Conditions change rapidly. Waves pound and the tide goes in and out (Figure below).

    Organisms in a tide pool include a sea star and a sea urchin

    Organisms in a tide pool include a sea star and a sea urchin.

    Many animals in the intertidal live in tide pools. What specific adaptations do they have to that zone? The mussels on the top left have hard shells. These protect them from waves and from drying out. The sea anemones in the lower right are more often submerged. They can close up during low tides. Both types of organisms have strong attachments so they don't wash out to sea.

    Many young organisms get their start in estuaries and so they must be adapted to rapid shifts in salinity.


    Corals and other animals create limestone rock reefs near the shore. Coral reefs are the “rainforests of the oceans.” They have a tremendous amount of species diversity (Figure below).

    Coral reefs are very beautiful and very diverse

    Coral reefs are very beautiful and very diverse.

    Reefs can form interesting shapes in the oceans. Remember that there are many volcanoes in the ocean. Coral reefs can form on volcanoes in tropical water. Since the volcanoes are cones, the reef forms in a circle around the volcano. Eventually the volcano becomes inactive. The mountain subsides and erodes so that it is below sea level. This leaves a circular coral reef (Figure below).

    The remnants of a volcano are surrounded by the circular reef

    In this image of Maupiti Island in the South Pacific, the remnants of the volcano are surrounded by the circular reef.

    Eventually, all that is left is a reef island called an atoll. A lagoon is found inside the reef.

    Oceanic Zone

    The open ocean is a vast area. Food washes down from the land. Food is also created by photosynthesizing plankton. Zooplankton and larger animals feed on the phytoplankton and on each other. Larger animals such as whales and giant groupers may live their entire lives in the open water.

    In the deepest ocean, survival is almost impossible (Figure below). It’s very rare to find a meal, so the fish there use very little energy. They are very small. The fish move very little and breathe slowly. Missing a meal could be deadly, so the fish have interesting ways of getting and keeping food. Some species have jaws that unhinge to accept a larger fish. Some have backward-folding teeth to keep prey from escaping.

    A drawing of a deep sea angler fish

    An 1896 drawing of a deep sea angler fish with a bioluminescent “lure” to attract prey.

    Hydrothermal Vents

    Hydrothermal vents are among the most unusual ecosystems on Earth. They are located at mid-ocean ridges where water is extremely hot. The food source for these ecosystems is chemosynthesis (Figure below). This ecosystem is entirely separate from the photosynthesis at the surface. Shrimp, clams, fish, and giant tube worms have been found in these extreme places.

    Tube worms, which live at hydrothermal vents

    Giant tube worms are found at hydrothermal vents. They get food from the chemosynthetic bacteria that live within them. The bacteria provide food; the worms provide shelter.


    • Life in the intertidal is difficult, but there are many types of organisms there.
    • There are few organisms that live in the deepest ocean. The ones that do have amazing adaptations to the exceptionally harsh conditions.
    • A hydrothermal vent ecosystem has chemosynthesis as its food source. The ecosystem is independent of photosynthesis at the surface.


    1. Why is there so much biodiversity in the intertidal zone?
    2. Why is survival in the deep ocean difficult? To do this, what adaptations do organisms have?
    3. What is different about the hydrothermal vent ecosystem from a surface ocean ecosystem?

    Explore More

    Use the resources below to answer the questions that follow.

    1. What is the intertidal zone?
    2. What do organisms in an intertidal zone have to deal with each day?
    3. What is zonation?
    4. What adaptation to the algae of the mid-tidal zone have to being exposed to air about half the day?
    5. How have some of the organisms in the intertidal zone adapted to life there?
    1. Why is the deep sea not well explored?
    2. What is the epipelagic zone? Which organisms live there?
    3. What is the mesopelagic zone? Which organisms live there?
    4. What color are most of the animals in this zone and why?
    5. What is bioluminescence?
    6. What is the deep scattering layer?

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