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Rhizarians: Radiolarians, Forams & Cercozoans

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  • 0:01 Rhizaria
  • 1:25 Forams
  • 2:28 Radiolarians
  • 4:02 Cercozoans
  • 5:07 Lesson Summary
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Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

There is an entire world of microscopic organisms living all around us, and they can make a pretty big impact on their ecosystems. In this lesson we will check out the group called the rhizarians, and explore their role in the world around us.

Rhizaria

When you think of the beach, what images come to mind? Sand? Hotdogs? Sharks? How about rhizarians? Yes, that's right, rhizarians! What are they? Well, amongst the underappreciated microorganisms on this planet is a major group called protists, which is a very diverse assortment of generally unicellular creatures that cannot quite be categorized as animals, plants or fungi, but do have membrane-bound organelles like these other eukaryotes.

Within the protists are several subgroups, one of which is rhizaria. Rhizarians are defined by a few characteristics. They are generally amoeboid, meaning they can alter their shape. How? With pseudopodia, projections of cell membranes.

Rhizarians are categorized by a specific kind of pseudopodia that are thin and needle-like. What is their purpose? Partly to catch food and partly for locomotion. These things help the rhizarians get around. So, what do you say? Want to see some rhizarians upclose and personal? We're at the right place, but to experience microorganisms we need to get down to their scale. This is going to be one trip to the beach you'll never forget!

Forams

So, this is what the ocean looks like at a microscope level. Many of the tiny creatures all around us are rhizarians, which actually make up a substantial part of the marine food chain. Here's one right now!

This is a type of rhizarian called a foraminifera, or foram for short. Forams are by definition unicellular heterotrophs with porous shells. What's that mean? Well a heterotroph is something that consumes food rather than creating food the way that plants do. For example, the forams eat other microorganisms, but what really sets them apart are their shells, called tests. The foram test is covered in pores, and this is where those pseudopodia that characterize all rhizarians pop out. Foram tests are made up of organic matter solidified with calcium carbonates. Many forams also maintain colonies of algae on their test, which they can eat. It's sort of like growing your own garden.

Radiolarians

Now, the forams are only one kind of rhizarian. Another groups is the radiolarians, characterized by glassy, symmetrical silica shells. Like all rhizarians, the radiolarians use pseudopodia to get around and to catch food and, as with the forams, these needle-like arms stick out through the shell. Radiolarians are a very old group of microorganisms, and are actually very important to researchers.

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