Types of Marine Organisms

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  • 0:53 Plankton
  • 2:16 Plants & Algae
  • 2:51 Invertebrates
  • 3:40 Fish
  • 4:25 Marine Mammals
  • 5:12 Reptiles and Seabirds
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Many marine organisms thrive under the surface of the ocean. Learn about the biodiversity of the oceans, from the microscopic plankton to the enormous whales, and how these organisms adapt to their oceanic environment.

Marine Organisms

When you look out over the ocean, it appears to be nothing more than a big basin filled with water, so it's hard to image that under the surface of the water there is a rich, biologically-diverse community of organisms. These ocean-dwelling organisms, known as marine organisms, live in a variety of underwater habitats, and the habitats vary based on the chemical and physical properties of that section of ocean. In fact, properties such as the salt content of the water, temperature, light penetration and ocean currents dictate where different marine organisms live within the vast oceans of planet Earth. In this lesson, you will learn about the varied and somewhat bizarre organisms that inhabit the world under the sea.


Oceans are home to all sizes of creatures, from microscopic organisms to the world's largest animals. Let's start our discussion with some of the tiniest marine organisms there are, called plankton. Now, if you watch a lot of cartoons, you know Plankton as a fictional character that has a recurring role on the TV show SpongeBob SquarePants. But, in marine biology, plankton are described as small or microscopic organisms that float in collective groups near the surface of the water.

Plankton are at the mercy of the ocean currents because they do not have the ability to swim. In fact, it might help you to recall the definition of these tiny organisms by remembering that the word plankton comes from the Greek language and means 'wandering.' Basically, plankton wander or drift along near the water surface and try not to get eaten by bigger marine organisms.

Some plankton are tiny plants, and like plants on land, they use sunlight for photosynthesis. Photosynthesis, you may recall, is the process that uses sunlight to make food from carbon dioxide and water, and generates oxygen as a byproduct. Other plankton are tiny animals or immature stages of larger marine animals.

Plants and Algae

You have likely heard of seaweed before and may have even eaten some wrapped around a sushi roll, but did you know that seaweed is actually a type of marine algae? Seaweed and many other types of marine algae have large flat surfaces that make them well-suited for capturing sunlight, which they use for photosynthesis. There is great diversity of plants and algae that grow in the ocean environments. Non-algae plants, such as grasses and mangroves, can be found growing in shallow ocean waters and provide habitats and foraging sites for coastal animals.


Many of the more unusual creatures of the sea are classified as invertebrates. The prefix 'in' means 'lacking,' and the suffix 'vertebrate' refers to having 'a backbone.' So, invertebrates, by definition, are 'lacking a backbone.' Invertebrates are abundant in the ocean waters, and this might be because the ocean waters buoy them, making a backbone an unneeded feature.

Invertebrates seem to glide and move with fluidity through their watery environment, and they come in many varieties. We see the classification of marine invertebrates include the slithering sea worms, the umbrella-like jellyfish and the colorful sea anemones. Invertebrates also include shellfish, squid, octopus and starfish.


Even though starfish have the word 'fish' in their name, they are not actually fish, because fish have backbones, making them vertebrates. There are many varieties of fish in the marine environment, including the tasty sardines and anchovies, as well as sharks and barracudas. Even though fish are varied, they share many features that allow them to live under water. For example, they have fins that they use for propulsion, gills to extract oxygen from the water and protective scales covering their bodies. Many fish also have swim bladders, which are like internal air balloons that they can use to control buoyancy as they move to different depths within the water.

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