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Stentor protists are relatively large freshwater protozoans; their size makes them a popular laboratory specimen for students to study. Read this lesson to learn more about where you can find Stentor protists, how they reproduce, and what makes them so intriguing.

What Are Protists?

Protists are a diverse group of eukaryotic organisms. A eukaryotic organism has cells with a defined nucleus surrounded by a membrane.

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They share characteristics with plants, animals and fungi, but are still different enough to warrant their own classification. Most protists are unicellular, but some are multicellular or colonial. We can group protists into four subgroups: protozoa, algae, slime molds, and water molds.

Stentor Protists

Stentor protists fall into the protozoa subgroup. (Stentor refers to the genus name.) These organisms are most often found in lentic (non-flowing) freshwater environments. Interestingly, Stentor organisms are made up of only a single cell (unicellular), but some of them are large enough to be seen by the naked eye.

In fact, some grow up to 2 millimeters in length. This is unusual as typically protozoa are microscopic in size and can’t be seen without the aid of a microscope.

Stentor Anatomy

Though they are made up of only one cell, that cell is capable of carrying out all of the processes necessary for eating, digestion, excretion, respiration, and reproduction.

That’s a lot of work for one cell! There are a few critical pieces of anatomy that are important to understand why Stentor critters are so fascinating.First, Stentor organisms are shaped like little trumpets when they are attached to other organisms, making them easy to identify. They often anchor themselves to plants or dead plant matter found in the water. When they are disturbed or free-swimming, they appear to be pear-shaped and globular instead of trumpet-shaped.Stentor organisms also come in a variety of colors, and move through the use of small, hair-like extensions found all over the body.

These hair-like extensions are called cilia and provide two main functions. First, as previously stated, they move to propel the cell through the water. Second, they move in unison to push water across the mouth area (or gullet). Tiny bacterial food particles are found in the water, so forcing water over the mouth area enable them to eat by picking the bacteria out.

When attached to an organism, Stentor cells are trumpet-shaped. The cilia are also visible around the outside of hte cell.
This magnification shows the macronucleus winding through the cell.

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Stentor Habitat

We’ve already learned a few clues about where Stentor organisms live. They are found in freshwater bodies, like lakes and ponds, and tend to avoid flowing water found in streams and rivers.

If we think about their food preference, this makes sense. Stentor eat bacteria, and bacteria tend to be found around decomposing organic material, like dead leaves or animals. Flowing water washes this organic matter away, so it would be harder for Stentor to find food in this kind of environment. Still or stagnant water means the decomposing matter occurs calmly on the bottom of the lake or pond, making a veritable feasting opportunity for the Stentor. If you want to find Stentor cells, a good place to start is to take floating vegetation and rinse it off into a dish. When that rinsed material is viewed under a microscope, there’s a good chance you will see Stentor organisms floating in it.

Stentor Reproduction

The last part of this lesson focuses on the reproductive habits of Stentor organisms.

This is one of the most fascinating features of this group because they can reproduce in two ways. First, new organisms can generate from segments of the parent. Even a small fragment can regenerate into a new, independent, fully functioning organism. As such, Stentor can rapidly reproduce and repopulate through asexual reproduction.

They can also reproduce through collisions with other Stentor cells. As we learned, Stentor organisms have a macronucleus, but when the cell is ready to reproduce, the macronucleus breaks down into a micronucleus (micro means small, or at least smaller than the macronucleus). This might sound confusing, but we can think of the macronucleus as the control station of the cell, and the micronucleus as the part responsible for passing DNA on to the next generation. Before reproduction, the micronucleus copies itself to prepare for the merge.

When two organisms with micronuclei collide, one copy of the micronuclei from each of the two cells swap with each other and merge with the original micronuclei. The two parent cells separate from each other and the new micronuclei develop into a macronucleus. At this point, the parent cell can divide itself multiple times to create new organisms through asexual reproduction. This mixing of genetic material prevents all individuals from being strict clones of the parent organism.

Lesson Summary

For an organism that many of us aren’t too familiar with, Stentor is pretty fascinating.

As a unicellular protozoa, Stentor can be up to 2 millimeters in size, making them visible to the naked eye. They live in stagnant freshwater environments and feed on bacteria. They move and eat through the use of cilia, and they maintain their water balance with the use of a contractile vacuole. Most of the time, Stentor have a macronucleus, which acts as the main control station of the cell. Before reproduction, the macronucleus may reform into a micronucleus and its copies. The micronucleus contains the DNA for the cell and can be exchanged with micronuclei from other organisms.

After recombination, the parent cell then divides asexually, producing new cells. Stentor also regenerate into new organisms from small fragments of the parent cell.

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