The Living Sea    Life Near the Surface

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The sea is home to billions of plants and animals. Many live only near the sunlit surface. Here you'll find everything from microscopic plankton to the giant blue whale. Oceanographers classify marine organisms by separating them into two primary groups: plants and animals.

Ocean Plants

Distribution of phytoplankton in the Atlantic OceanA spring bloom of phytoplankton
Images from data collected from satellites show densities of phytoplankton in the Atlantic Ocean. Red shows where phytoplankton is densest, followed by yellow, green, blue and violet where it is least dense. Phytoplankton's spring bloom (shown right) occurs when days are longer and more nutrients come up from the bottom.

There are two general types of plants found in the ocean, those having roots that are attached to the ocean bottom and those not having roots which simply drift about with the water. The rooted plants in the ocean are only found in shallow water because there is not enough sunlight to sustain photosynthesis in deeper waters. Since sunlight does not penetrate more than a few hundred feet into the ocean, most of the ocean is not capable of supporting rooted plants. Nevertheless, plants are found throughout most of the oceanic surface waters.

The most abundant plants in the ocean are known as phytoplankton. These are usually single-celled, minute floating plants that drift throughout the surface waters of the ocean. A bucket of sea water might hold a million microscopic diatoms which are relatives of seaweed encased in glassy boxes. To grow, phytoplankton need nutrients from the sea water and lots of sunlight. The most light occurs in the tropics but nutrients there, especially nitrogen and phosphorus, are often in short supply. When large quantities of diatoms and other phytoplankton are present they give a color to the sea. Spectacular phytoplankton blooms are found in cooler waters where nutrients are brought up from the sea floor during storms.

Animals of the Sea

Animals of all shapes and sizes live throughout the layers of the ocean
Marine animals are divided into three groups: zooplankton (those that float), nekton (those that actively swim) and benthos (those that live on or in the ocean bottom).

Marine animals are divided into three groups: zooplankton, nekton, and benthos. Zooplankton are drifting animals and are usually small, however, they can grow to fairly large size. For example, the jellyfish and the Portuguese man-of-war are examples of larger types of zooplankton which are unable to propel themselves effectively and are therefore at the mercy of either wind or current. The zooplankton population also includes some temporary members such as fish eggs or larval forms of organisms which may grow up and leave the planktonic community to join the nekton or benthos.

Nekton are the free swimmers and probably the largest portion of familiar animals found in the ocean belong to this class. Common fishes, the octopus, whales, eels and squid are all examples of nekton. The nekton category includes a number of very diverse creatures. The whale, dolphin and porpoise are certainly very different from codfish or trout because whales represent sea mammals whereas cod are true fishes.

The third type of sea animal spends its entire life on or in the ocean bottom. This group of marine animals is called the benthos. It includes lobsters, starfish, various worms, snails, oysters and many more. Some of these creatures, such as lobsters and snails, may be able to move about on the bottom but their lifestyle is so bound up with the ocean floor that they are unable to survive away from this environment.

Sunlight is the source for all food consumed in the ocean

Who eats what

All living organisms need food. The basic difference between plants and animals is that plants make their own food, while animals obtain food from their environment. Through photosynthesis, plants manufacture organic materials (food) from inorganic materials (water, carbon dioxide, and nutrients) using sunlight as their source of energy. Because plants make their own food, they are called producers. Animals are known as consumers. They gather and consume organic material rather than making it themselves. Herbivores are animals that eat plants; carnivores are animals that eat meat. Omnivores are animals that eat both plants and other animals. Scavengers eat all the leftovers and other dead organisms. No matter what an animal eats, all of its food can be traced back to the ability of plants to produce organic materials from the sun.

Discover more about life in the sea

Aquatic Internet Resources
Fish FAQ


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