14.4 Ocean Life

Lesson Objectives

• Identify three major groups of marine life.
• Describe marine food chains.

Vocabulary

• benthos
• marine organism
• nekton
• phytoplankton
• plankton
• zooplankton

Introduction

Living things in the oceans are called marine organisms. They range 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.

Living Things in the Ocean

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

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

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

Nekton

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

Benthos are living things 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 let the bacteria live 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.

Marine Food Chains

Figure below shows a marine food chain. Phytoplankton form the base of the food chain. Phytoplankton are the most important primary producers in the ocean. They use sunlight and nutrients to make food by photosynthesis. Small zooplankton consume phytoplankton. Larger organisms eat the small zooplankton. Larger predators eat these consumers. In an unusual relationship, some enormous whales depend on plankton for their food. They filter tremendous amounts of these tiny creatures out of the water.

Many marine food chains look like this example.

The bacteria that make food from chemicals are also primary producers. These organisms do not do photosynthesis since there is no light at the vents. They do something called chemosynthesis. They break down chemicals to make food.

When marine organisms die, decomposers break them down. This returns their nutrients to the water. The nutrients can be used again to make food. Decomposers in the oceans include bacteria and worms. Many live on the ocean floor. Do you know why?

Lesson Summary

• Three main groups of ocean life are plankton, nekton, and benthos. Plankton float in the water. Nekton swim through the water. Benthos live on the ocean floor.
• Phytoplankton are the primary producers in the ocean. They form the base of most marine food chains.

Lesson Review Questions

Recall

1. Define marine organism.

2. What are phytoplankton and zooplankton?

3. Why do phytoplankton live in the photic zone?

4. Define nekton.

5. Where do benthos live?

Apply Concepts

6. Create a marine food chain that includes the following living things: jellyfish, worm, shark, algae.

Think Critically

7. Compare and contrast plankton, nekton, and benthos.

8. Explain the importance of phytoplankton to other forms of ocean life.

Points to Consider

This chapter describes how the oceans influence Earth’s atmosphere.

• What else do you know about the atmosphere? For example, what gases does it contain?
• The ocean is divided into zones. Do you think the atmosphere is divided into zones as well?