This statement is true for all animals whether they live on land or in the water.
Plants alone can synthesize living matter by the process of photosynthesis.
In the sea are photosynthesizing plants called phytoplankton. This group of plants are the foundation of animal energy in the sea. Secondary in importance are zooplankton. These two groups are know together as plankton and form the basis of the food web of the sea. Become familar with the use of a plankton tow net, learn how to classify the different types of plankton using a microscope.