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18.8: Types of Marine Organisms

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    How can you visit marine life?

    It's easiest to get to the relatively shallow seafloor just offshore. Experienced divers can descend to 130 feet (43 meters), which gives them access to most marine life. Manned submersibles can descend to the bottom of the deep ocean. Where would you like to go? A coral reef or a hydrothermal vent?

    Living Things in the Ocean

    Marine organisms range in size from tiny bacteria to the largest known animal, the blue whale. All are adapted for life in salt water. Most are adapted for extreme pressures.

    When you think of life in the ocean, do you think of fish? Actually, fish are not the most common life forms in the ocean. Plankton are the most common. Plankton make up one of three major groups of marine life. The other two groups are nekton and benthos (Figure below).

    Plankton, nekton, and benthos are the three groups of living things in the oceans

    Living things in the oceans are placed in these three groups.


    Plankton are living things that float in the water. Most plankton are too small to see with the unaided eye (Figure below). Plankton are unable to move on their own. Ocean motions carry them along.

    There are two main types of plankton:

    1. Phytoplankton are “plant-like” plankton. Since they make food by photosynthesis, they live in the photic zone. Most are algae.
    2. Zooplankton are “animal-like” plankton that include tiny animals and fish larvae. They feed on phytoplankton.

    Pictures of phytoplankton and zooplankton

    The phytoplankton (left) and zooplankton (right) shown here have been magnified. Otherwise, they would be too small for you to see.


    Nekton are living things that swim through the water (Figure below). They may live at any depth, in the photic or aphotic zone. Most nekton are fish, although some are mammals. Fish have fins and streamlined bodies to help them swim. Fish also have gills to take in oxygen from the water.

    Nekton, like a fish, swim through ocean water

    Nekton swim through ocean water.


    Benthos are living organisms on the ocean floor. Many benthic organisms attach themselves to rocks and stay in one place. This protects them from crashing waves and other water movements. Some benthic organisms burrow into sediments for food or protection. Benthic animals may crawl over the ocean floor. Examples of benthos include clams and worms. Pictured below are two other examples (Figure below).

    Picture of an anemone and sea cucumber, which live on the ocean floor

    These animals live on the ocean floor.

    Some benthos live near vents on the deep ocean floor. Tubeworms are an example (Figure below). Scalding hot water pours out of the vents. The hot water contains chemicals that some specialized bacteria can use to make food. Tubeworms have the bacteria living inside them. The bacteria get protection and the tubeworms get some of the food.

    Picture of tubeworms living near hot water vents

    Tubeworms live near hot water vents on the deep ocean floor.


    • Plankton are tiny organisms that are swept along on currents. Phytoplankton are tiny plant-like organisms. Zooplankton are tiny animals.
    • Nekton are organisms that go through the water under their own power. Fish and marine mammals are nekton.
    • Benthos bury in the sediment, attach to rocks, or crawl over the seafloor.


    1. Compare and contrast phytoplankton and zooplankton.
    2. How are nekton adapted to life in the sea?
    3. How are benthos adapted to life in the sea?

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