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Types of Protists & Their Characteristics

Instructor: Sarah Phenix
In this lesson, we will learn about protists and the different types of these microorganisms, categorized by either their mode of nutrient acquisition or life-stage characteristic. Afterward, test your knowledge with a brief quiz.

What is a Protist?

So what is a protist? That's actually a lot more of a difficult question to answer than one might think. Protists are defined as eukaryotic (having membrane-bound organelles within their cell membrane) microorganisms that can live in either solitary or colonies. They comprise the kingdom Protista, and a sort-of catch-all kingdom that includes organisms that can't be classified under Animalia, Plantae, or Fungi. That may sound pretty vague and, truth be told, biologists agree. The reason why protists are so difficult to define is because they resist universal classification.

This resistance stems from the fact that protists are polyphyletic, meaning that if you were to trace their lineages, you would find that protists as a group don't share a common ancestor. In fact, these ancestors actually overlap other kingdoms. In other words, geneticists have found that protist species are more closely genetically related to organisms within other kingdoms than they are to one another.

So why aren't protists just divvied up and classified within the kingdoms with which they are most closely genetically related? Well, that's simply because they don't posses the 'defining characteristics' (or the unique features, such as all mammals possessing hair, for example) that all organisms within that particular group universally posses. In other words, the presence of these characteristics 'define' organisms as belonging to one group and not another. Kind of a bit a of a predicament, huh? So what you'll find as we talk more about these little critters is that they're quite different from one another and yet, they're all joined by their eukaryotic quality as well as their incompatibility with other kingdoms.

How Are Protists Grouped?

There are actually a few different ways that biologists talk about protists. Some talk about them in terms of their mode of locomotion (cilia, flagellum, or pseudopodia), while others talk about them in terms of where they live (soil, freshwater, marine, or parasitic). But here we are going to discuss them in terms of their feeding behaviors because we so readily categorize animals based on whether they feed on other organisms (animals), photosynthesize (plants), or break down organic matter (fungi).

Animal-like Protists

When we use the term 'animal-like protists' we mean that these protists are motile and heterotrophic (feed by consuming bacteria or other protists). These protists are essentially hunters of prey, which is a more 'animal-like' trait.

Animal-like protists
Animal-like Protists

Protists belonging to this group include a vast array of organisms, like ciliated paramecium, amorphous Amoeba species, zooplankton, such as marine radiolarians and fresh/marine heliozoans, as well as some flagellated organisms like symbiotic parabasalids, which live in the guts of cockroaches. Often, this group of protists is referred to as protozoans, meaning 'first animals', which is not to say that all protozoans are related to the animal kingdom, but only that organisms within this group share the 'animal-like' behavior of hunting.

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