The sea is filled with many different kinds of plants and animals. Some live in warm waters and others live in cold. Some live close to land and some live far away. Some live close to the surface and some live deeper down.
The plants of the ocean, like those of the land, depend upon sunlight for their life, because sunlight has energy in it, and all living things need this energy to stay alive. Sea plants, like most land plants, take the energy in sunlight and turn it into food. They keep this food and use it a little at a time.
Some sea animals get their energy by eating plants. Other animals get their energy by eating the plant-eaters. And animals that live in the deepest, darkest part of the ocean get their energy from plants and animals that live above them, die, and drift down. This way of passing energy along is called the food chain.
The food chain of the ocean begins with very small plants such as those called diatoms. Diatoms drift on or near the surface. They are so small that you need a microscope to see them. But when billions of them drift together, the color of the ocean looks green.
Diatoms look like no plant you have ever seen. A diatom is just green-brown jelly inside a glassy shell.
There are many different kinds of these small plants. The shells of some are square; others are round. Some diatoms drift alone; others join together in long chains.
Drifting and swimming within the huge clouds of diatoms are billions of other little living things. Some are very small animals, no bigger than the plants. Some are the eggs of fish and other creatures. Some are new baby clams, lobsters, and other animals. Some are very small animals called copepods.
These huge masses of drifting plants and animals are called plankton, a word that means ”drifting”. The plankton near the surface is like a great ”grassland” and all kinds of animals live and eat in it. In fact, it is often called ”the pasture of the sea.”
The copepods and some of the other small animals in the plankton eat the diatoms. Then these animals are eaten by small fish and other animals that stay in or near the plankton to be near their food. Large animals, such as tuna and dolphin, eat the small fish. Then even bigger animals, such as sharks, eat both big and little fish.
So, around, above, and below the plankton, there are many kinds of swimming animals.
Most are fish, but there are also whales, turtles, and other creatures.
Though these animals swim about, they cannot live everywhere. Some can live only in warm water; others only in cold water. Some, because of the way their bodies are made, can live only at certain depths. If they swim too far up or down, the change in water pressure may kill them.
There is an important difference between plants and animals in the sea. Animals are found both near the land and in the depths of the ocean. However, plants cannot live any deeper than sunlight can reach. In clear water, sunlight reaches down only about six hundred feet (l80 meters). Then the mater grows darker and darker. Finally, below about seventeen hundred feet (510 meters), you cannot see anything in the water.
Most kinds of the leafy plants that are usually called seaweed grow in water near the land. Masses of seaweed form underwater forests and many kinds of animals live in them. For some of these animals, the seaweed is food. For others, it is a good hiding place. For still others, it is a rich hunting ground to find many animals to eat.
Some animals that live on the bottom of the sea walk or move about. Some can swim and some cannot. Others, such as oysters, just stay on the bottom. They spend their whole life in one place. Some food drifts down to them, and some is carried to them by currents and running water.
When a sea animal dies, it slowly sinks. Its body is usually eaten by animals living at lower depths, and when these animals die, they sink down and become food for creatures that live even deeper, or that live on the bottom.
Dead animals that are not eaten sink to the bottom. Their bodies slowly break down into chemicals. These chemicals spread through the water and give food to plants. So the food chain starts all over again.
In the sea, just like on the land, every creature depends upon other creatures for life -- and all life depends upon the sun.