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11.19: Types of Marine Organisms

Difficulty Level: Basic Created by: CK-12
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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 ft), 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 shows the three groups.

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. Some examples are shown in 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.

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. 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 oxygen from the water. Figure below shows examples of nekton.

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. Figure below shows two other examples.

These animals live on the ocean floor.

Some benthos live near vents on the deep ocean floor. Tubeworms are an example (see 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.

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


  • benthos: Organisms that live on the ocean floor.
  • nekton: Organisms that swim through the water. Fish are a good example.
  • phytoplankton: Tiny plants that photosynthesize and create food energy and oxygen.
  • plankton: A diverse group of tiny animals and plants that freely drift in the water.
  • zooplankton: Tiny animals that float at the surface their whole lives or only part of their lives.


  • 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.


Use this resource to answer the questions that follow.


  1. When did the census begin?
  2. How many expeditions occurred during this census?
  3. What was the purpose of this census?
  4. What did the researchers find in Australia?
  5. What did they discover about the tuna off of Northern Europe?
  6. What did this census create?


  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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Organisms that live on the ocean floor.


Organisms that swim through the water; fish are a good example.


Tiny plants that photosynthesize and create oxygen and food energy.


Diverse group of tiny animals and plants that freely drift in the water.


Tiny animals that float at the surface their whole lives or only part of their lives.

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Jan 04, 2013
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Aug 29, 2016
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