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Types of Marine Organisms

Many types of organisms live in the sea, such as tiny plankton, submerged fish, and shorebirds.

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Types of Marine Organisms

How does the ocean seem all the same, yet have so much biodiversity?

Although it may seem like the ocean is all the same, there are many different habitats based on temperature, salinity, pressure, light, currents, and other factors. Organisms have adapted to these conditions in many interesting and effective ways. Covering 70% of Earth's surface, the oceans are home to a large portion of all life on Earth.

Types of Ocean Organisms

The smallest and largest animals on Earth live in the oceans. Why do you think the oceans can support large animals?

Marine animals breathe air or extract oxygen from the water. Some float on the surface and others dive into the ocean’s depths. There are animals that eat other animals, and plants generate food from sunlight. A few bizarre creatures break down chemicals to make food! The following section divides ocean life into seven basic groups.


Plankton are organisms that cannot swim but that float along with the current. The word "plankton" comes from the Greek for wanderer. Most plankton are microscopic, but some are visible to the naked eye (Figure below).

Phytoplankton are tiny plants that make food by photosynthesis. Because they need sunlight, phytoplankton live in the photic zone. Phytoplankton are responsible for about half of the total primary productivity (food energy) on Earth. Like other plants, phytoplankton release oxygen as a waste product.

Microscopic diatoms are a type of phytoplankton

Microscopic diatoms are a type of phytoplankton.

Zooplankton, or animal plankton, eat phytoplankton as their source of food (Figure below). Some zooplankton live as plankton all their lives and others are juvenile forms of animals that will attach to the bottom as adults. Some small invertebrates live as zooplankton.

Picture of a copepod, an example food source for larger animals

Copepods are abundant and so are an important food source for larger animals.

Plants and Algae

The few true plants found in the oceans include salt marsh grasses and mangrove trees. Although they are not true plants, large algae, which are called seaweed, also use photosynthesis to make food. Plants and seaweeds are found in the neritic zone, where the light they need penetrates so that they can photosynthesize (Figure below).

Picture of a kelp forest, which grows in the neritic zone

Kelp grows in forests in the neritic zone. Otters and other organisms depend on the kelp-forest ecosystem.

Marine Invertebrates

The variety and number of invertebrates, animals without a backbone, is truly remarkable (Figure below). Marine invertebrates include sea slugs, sea anemones, starfish, octopuses, clams, sponges, sea worms, crabs, and lobsters. Most of these animals are found close to the shore, but they can be found throughout the ocean.

Picture of mussels, a sea star, moon jelly, and squid

(a) Mussels; (b) Crown of thorns sea star; (c) Moon jelly; (d) A squid.

Jellies are otherworldly creatures that glow in the dark, without brains or bones, some more than 100 feet long. Along with many other ocean areas, they live just off California's coast.


Fish are vertebrates; they have a backbone. What are some of the features fish have that allows them to live in the oceans? All fish have most or all of these traits:

  • Fins with which to move and steer.
  • Scales for protection.
  • Gills for extracting oxygen from the water.
  • A swim bladder that lets them rise and sink to different depths.
  • Ectothermy (cold-bloodedness), so that their bodies are the same temperature as the surrounding water.
  • Bioluminescence, or light created from a chemical reaction that can attract prey or mates in the dark ocean.

Included among the fish are sardines, salmon, and eels, as well as the sharks and rays (which lack swim bladders) (Figure below).

Picture of a Great White Shark

The Great White Shark is a fish that preys on other fish and marine mammals.


Only a few types of reptiles live in the oceans and they live in warm water. Why are reptiles so restricted in their ability to live in the sea? Sea turtles, sea snakes, saltwater crocodiles, and marine iguana that are found only at the Galapagos Islands sum up the marine reptile groups (Figure below). Sea snakes bear live young in the ocean, but turtles, crocodiles, and marine iguanas all lay their eggs on land.

Picture of a sea turtle

Sea turtles are found all over the oceans, but their numbers are diminishing.


Many types of birds are adapted to living in the sea or on the shore. With their long legs for wading and long bills for digging in sand for food, shorebirds are well adapted for the intertidal zone. Many seabirds live on land but go to sea to fish, such as gulls, pelicans, and frigate birds. Some birds, like albatross, spend months at sea and only come on shore to raise chicks (Figure below).

Picture of shorebirds, seabirds, and an albatross

(a) Shorebirds; (b) Seabirds; (c) Albatross.

Marine Mammals

What are the common traits of mammals? Mammals are endothermic (warm-blooded) vertebrates that give birth to live young, feed them with milk, and have hair, ears, and a jaw bone with teeth.

What traits might mammals have to be adapted to life in the ocean?

  • For swimming: streamlined bodies, slippery skin or hair, fins.
  • For warmth: fur, fat, high metabolic rate, small surface area to volume, specialized blood system.
  • For salinity: kidneys that excrete salt, impervious skin.

The five types of marine mammals are pictured here: (Figure below).

Picture of the five types of marine mammals: cetaceans, sirenians, mustelids, pinnipeds, and polar bears

(a) Cetaceans: whales, dolphins, and porpoises. (b) Sirenians: manatee and the dugong. (c) Mustelids: Sea otters (terrestrial members are skunks, badgers and weasels). (d) Pinnipeds: Seals, sea lions, and walruses. (e) Polar bear.


  • Plankton are tiny organisms that are swept along on currents. Phytoplankton are tiny photosynthesizers and zooplankton are tiny animals.
  • Fish have gills for breathing, swim bladders for rising and sinking, and other adaptations for life in the oceans.
  • Shorebirds live at the interface of land and sea; some birds live on land but fish at sea, and some birds spend most of their time at sea and only come to shore to nest.


  1. How are phytoplankton different from plant-like algae?
  2. Describe how fish are adapted to life in the oceans.
  3. Describe how marine mammals are adapted to life in the oceans. How are these adaptations different from those of fish?

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Use this resource to answer the questions that follow.

  1. What are kelp? Where are kelp forests found?
  2. Why are kelp forests important to their ecosystem?
  3. How do kelp withstand the forces of waves and currents?
  4. How are the kelp blades kept in the zone for photosynthesis?
  5. What does the canopy of the kelp forest provide to organisms and how?
  6. what does the ocean provide?

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    Tiny plants that photosynthesize and create food energy and oxygen.


    A diverse group of tiny aquatic organisms that cannot move on their own and instead drift freely in the water.

    primary productivity

    The creation of food energy.

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