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10.8: Ocean Ecosystems

Difficulty Level: At Grade Created by: CK-12
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Which ecosystem doesn't depend on photosynthesis?

When scientists first dove in Alvin and witnessed hydrothermal vents, they were not surprised by the eruptions of hot water. But they never anticipated finding life there. Without sunlight, they knew that photosynthesis could not be the basis of this community. Eventually they discovered a different way of producing food, chemosynthesis. Many more hydrothermal vents were discovered and many more types of vent organisms.

The Intertidal

Conditions in the intertidal zone change rapidly as water covers and uncovers the region and waves pound on the rocks. A great abundance of life is found in the intertidal zone (Figure below). High energy waves hit the organisms that live in this zone, so they must be adapted to pounding waves and exposure to air during low tides. Hard shells protect from waves and also protect against drying out when the animal is above water. Strong attachments keep the animals anchored to the rock.

Organisms in a tide pool include sea stars and sea urchins.

In a tide pool, as in the photo, what organisms are found where and what specific adaptations do they have to that zone? The mussels on the top left have hard shells for protection and to prevent drying because they are often not covered by water. The sea anemones in the lower right are more often submerged and have strong attachments but can close during low tides.

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


Corals and other animals deposit calcium carbonate to create rock reefs near the shore. Coral reefs are the “rainforests of the oceans,” with a tremendous amount of species diversity (Figure below).

Coral reefs are among the most densely inhabited and diverse areas on the globe.

Reefs can form interesting shapes in the oceans. Remember that hot spots create volcanoes on the seafloor. If these volcanoes rise above sea level to become islands, and if they occur in tropical waters, coral reefs will form on them. Since the volcanoes are cones, the reef forms in a circle around the volcano. As the volcano comes off the hot spot, the crust cools. The volcano subsides and then begins to erode away (Figure below).

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 either washes down from the land or is 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.

How do fish survive in the deepest ocean? The few species that live in the greatest depths are very specialized (Figure below). Since it’s rare to find a meal, the fish use very little energy; they move very little, breathe slowly, have minimal bone structure and a slow metabolism. These fish are very small. To maximize the chance of getting a meal, some species may have jaws that unhinge to accept a larger fish or backward-folding teeth to keep prey from escaping.

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

Many ocean-related videos are found in National Geographic Videos, Environment Video, Habitat, Ocean section: http://video.nationalgeographic.com/video/player/environment/. Just a few are listed below.

  • How we can know what lives in the ocean is in “Deep-Sea Robo Help.”
  • Some of the results of the Census of Marine Life have been released and are discussed in “Record-Breaking Sea-Creature Surveys Released.”
  • Bioluminescence is common in the oceans and seen in “Why Deep Sea Creatures Glow.”

Hydrothermal Vents

Hydrothermal vents are among the most unusual ecosystems on Earth since they are dependent on chemosynthetic organisms at the base of the food web. At mid-ocean ridges at hydrothermal vents, bacteria that use chemosynthesis for food energy are the base of a unique ecosystem (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.

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

A video explaining hydrothermal vents with good footage is seen here: http://www.youtube.com/watch?v=rFHtVRKoaUM&feature=related.


  • In the ocean, phytoplankton photosynthesize as the main food source. They are eaten by zooplankton and other larger animals.
  • Organisms that live in the deepest ocean have amazing adaptations to the exceptionally harsh conditions, such as unhinging jaws, backward-folding teeth, or a bioluminescent lure.
  • A hydrothermal vent ecosystem has chemosynthesis as its food source. The ecosystem is independent of photosynthesis at the surface.

Making Connections


Use these resources 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. How have some of the organisms in the intertidal zone adapted to life there?


5. Why is the deep sea not well explored?

6. What organisms live in the epipelagic zone?

7. What organisms live in the mesopelagic zone?

8. What color are most of the animals in this zone?

9. What is bioluminescence?

10. What is found in the deep scattering layer?


1. Why is there so much biodiversity in the intertidal zone?

2. Why is survival in the deep ocean difficult? What adaptations to organisms have to do this?

3. What is bioluminescence and how does it help organisms survive in the ocean?

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hydrothermal vent A stream of heated water that enters into the ocean at a mid-ocean ridge.
reef A large underwater structure created from the calcium carbonate skeletons of coral.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 07, 2016
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