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Stentor Protist: Reproduction, Anatomy & Habitat

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

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. 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.
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The different colors of Stentor often come from the unicellular algae they consume. They don't consume the algae for food, but instead the two work together as a form of symbiosis, meaning the two organisms benefit from interacting with one another. The algae consumes waste material from the Stentor organism and uses it in photosynthesis. This produces nutrients that Stentor can then benefit from. It's a win-win!

Another important feature of Stentor is the use of a contractile vacuole. Because these organisms only live in freshwater, the salt concentration inside their cell is higher than that of the surrounding water. This requires osmosis to take place, meaning that water from outside the cell is being drawn into the cell to maintain a balance. As a single-celled organism, they can only hold so much excess water, which is stored in the contractile vacuole. Once the vacuole is filled, it contracts (hence, the name), forcing this excess water back out of the cell.

Finally, as we learned in the previous section, all protists are eukaryotic, meaning they have a defined nucleus. Because the Stentor organisms are relatively large for unicellular beings, they actually have a macronucleus that stretches the length of the body. (This is easy to remember because the term macro just means large.) This macronucleus looks like a string of beads inside the 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.

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