Many this lesson, you will learn about

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.

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In this lesson, you will learn about the varied and somewhat bizarre organisms that inhabit the world under the sea.

Plankton

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.

Invertebrates

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.

Fish

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.

Marine Mammals

Ocean waters can get mighty cold, so it’s surprising to learn about just how many warm-blooded mammals make their homes in the oceans. Marine mammals, including whales, dolphins, manatees, sea otters, seals and walruses share many characteristics with land-dwelling mammals, including you and me. For instance, all mammals, whether they live on land or under the water, give birth to live young and feed their young with milk.

But, marine mammals have some adaptations that allow them to live in the harsh ocean environment. These adaptations include fur and a high fat content for warmth. They also have skin that is impervious to the salty seawater.

Reptiles and Sea Birds

A discussion of marine organisms would not be complete without mentioning reptiles and seabirds. Although few reptiles and seabirds live in or spend long periods in the oceans, many rely on oceans for food.

Seabirds, such as albatross and gulls, develop adaptations that help them thrive in marine environments, such as long legs that help them wade along the shallow water’s edge and long bills for digging up shoreline creatures to eat. Reptiles, such as sea turtles, crocodiles and marine iguana, feed in marine areas and also lay their eggs on the ocean shores.

Lesson Summary

Let’s review:

  • Marine organisms are ocean-dwelling organisms. They live in different underwater habitats that are based on properties such as the salt content of the water, temperature, light penetration and ocean currents.
  • Some of the tiniest marine organisms are plankton, which are small or microscopic organisms that float in collective groups near the surface of the water. Some plankton are tiny plants and use sunlight for photosynthesis, while other plankton are tiny animals or immature stages of larger marine animals.
  • Plants and marine algae, such as seaweed, also capture sunlight for photosynthesis.

    Plants such as grasses and mangroves can be found in shallow ocean waters.

  • Invertebrates are ‘lacking a backbone’ and can be found in abundance in the world’s oceans. Marine invertebrates include sea worms, jellyfish, sea anemones, shellfish, squid, octopus and starfish.
  • Fish are vertebrates that are found in marine environments. Examples of ocean fish include sardines, anchovies, sharks and barracudas. Fish have features that allow them to live under water, such as fins, gills, scales and swim bladders.
  • Ocean waters are also home to a variety of marine mammals, including whales, dolphins, manatees, sea otters, seals and walruses.

    Marine mammals have some adaptations that allow them to live in the harsh ocean environment, including fur, a high fat content and skin that is impervious to the salty seawater.

  • Seabirds, such as albatross and gulls, have adaptations such as long legs and long bills to help them feed from ocean environments. Reptiles, such as sea turtles, crocodiles and marine iguana, feed in marine areas and also lay their eggs on the ocean shores.

Learning Outcomes

After this lesson, you’ll be able to:

  • Identify different properties of underwater habitats that affect marine organisms
  • Describe the categories of marine organisms and list examples of each category
  • Explain adaptations that allow fish, mammals and seabirds to live in ocean environments
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