Stentor: A Trumpet-Shaped Organism With Interesting Behaviour
An Interesting Predator
Stentor is a single-celled organism that's shaped like a trumpet when it's extended. It's interesting to observe, especially when it's catching its prey. The organism has some impressive features. Researchers have discovered that Stentor roeselii seems to make relatively complex decisions with respect to avoiding harm. It can "change its mind" about its behaviour as a dangerous stimulus continues. Understanding the biology of this process might help us understand the behaviour of our cells.
Stentor is found in ponds and other bodies of still water. It's between one and two millimetres long and can be seen with the naked eye. A hand lens provides a better view. A microscope is required in order to see details of the organism's structure and behaviour. If a microscope is available, watching a living Stentor can be a very absorbing activity.
Phylum Ciliophora (or Ciliata)
Terminology: Ciliates, Protists, and Protozoa
Stentor is a member of the phylum Ciliophora. The organisms in this phylum are commonly known as ciliates and live in aquatic environments. They are unicellular and bear hair-like structures called cilia on at least some part of their body. The cilia beat and move the surrounding fluid. In some organisms, they move the cell itself.
Stentor, other ciliates, and some additional organisms are sometimes referred to as protists. Protista is the name of a biological kingdom. It contains unicellular or unicellular-colonial organisms, including Stentor, as well as some multicellular ones The kingdom system is often used to classify organisms in schools. Scientists prefer to use the cladistic system of biological classification.
Ciliates and some other unicellular organisms are sometimes referred to as protozoa. This is an old term that comes from the Ancient Greek words proto (meaning first) and zoa (meaning animal).
The word "Stentor" is a genus name as well as a common name. Multiple species exist in the genus. S. coeruleus is named for the blue-green pigment called stentorin that it contains. S. roeselii was used in the recent experiment concerning the organism's "change of mind" behaviour. S. polymorphus contains living algae.
Stentor was named after a Greek herald in the Trojan War who is mentioned in Homer's Iliad. In the story, Stentor had a voice as loud as fifty men. The organism lives in bodies of fresh water such as ponds, slow-moving streams, and lakes. It spends some of its time swimming through the water and the rest attached to submerged items such as algae and debris.
When it's swimming, Stentor has an oval or a pear shape. When it's attached to an item and feeding, it has a trumpet or horn shape. Its covered by short, hair-like cilia. The edge of the trumpet opening bears much longer cilia. These beat, creating a vortex that pulls in prey.
Stentor is attached to the substrate by a slightly expanded region known as the holdfast. It has the ability to contract into a ball when it's joined to a substrate. In some individuals, a covering called a lorica surrounds the holdfast end of the cell. The lorica is mucilaginous and contains debris and material excreted by the Stentor.
Stentor has organelles found in other ciliates. It contains two nuclei—a large macronucleus and a small micronucleus. The macronucleus looks like a beaded necklace. Vacuoles (sacs surrounded by membrane) form as needed. Ingested food enters a food vacuole, where enzymes digest it. Stentor also has a contractile vacuole, which absorbs water that enters the organism and expels it to the outer environment when it's full. The water is released through a temporary pore in the cell membrane.
Life of a Stentor
Stentor can stretch its body far beyond the substrate as it feeds. It eats bacteria, more advanced single-celled organisms, and rotifers. Rotifers are also interesting creatures. They are multicellular, but they are smaller than many unicellular ones and much smaller than a Stentor.
Stentor polymorphus and a few other species contain a single-celled green alga named Chlorella, which survives in the ciliate and performs photosynthesis. Stentor uses some of the food that the alga produces. The alga is protected inside the ciliate and absorbs substances that it needs from its host.
The Stentor species that have been studied reproduce primarily by splitting in half, a process known as binary fission. They also reproduce by attaching to one another and exchanging genetic material, which is known as conjugation.
The video below is interesting and well worth watching. As one of the creators says in a YouTube comment, however, the commentary contains an error. Tardigrades contain eight legs, not six.
The Genetic Code
Researchers are discovering that Stentor has multiple features of special interest. Three of these features are its genetic code, its ability to regenerate, and the polyploidy in its macronucleus.
Stentor primarily uses the standard genetic code, which we use. Other ciliates whose genome has been studied have a non-standard code. The genetic code determines many of an organism's characteristics. It's created by the order of specific chemicals in the nucleic acid (DNA and RNA) of a cell. The chemicals are called nitrogenous bases and are often represented by their initial letter.
Each sequence of three nitrogenous bases has a particular meaning, which is why the code is referred to as a triplet code. The sequence is known as a codon. Many codons contain instructions related to the manufacture of polypeptides, which are the chains of amino acids used to make protein molecules.
In the standard genetic code, which humans possess, UAA and UAG are called stop codons because they signal the end of a polypeptide. They tell the cell to stop adding amino acids to the polypeptide that is being made. The two triplets are an example of codons with a different meaning in other ciliates and in Stentor coeruleus and us. In most ciliates, they code for an amino acid named glutamine. In Stentor and us, they are stop codons.
Even tiny fragments (of Stentor), 1/64th the size of the original cell, are able to regenerate into a small but normally proportioned cell, and then grow to the full size.— Athena Lin et al, Journal of Visualized Experiments
Regeneration and Polyploidy
Stentor is known for its amazing ability to regenerate. If its body is cut into many small pieces (anywhere from 64 to 100 segments, according to different sources), each piece can produce an entire Stentor. The piece must contain a portion of the macronucleus and the cell membrane in order to regenerate. This is not as unlikely a condition as it may sound. The macronucleus extends through the whole length of the cell and a membrane covers the entire cell.
The macronucleus exhibits polyploidy. The term “ploidy” means the number of sets of chromosomes in a cell. Human cells are diploid because they have two sets. Each of our chromosomes contains a partner bearing genes for the same characteristics. The Stentor macronucleus contains so many copies of chromosomes or segments of chromosomes (tens of thousands or higher, according to various researchers) that it's highly likely that a small piece will contain the necessary genetic information to create a new individual.
Scientists have also observed that a Stentor has an amazing ability to repair damage to the cell membrane. The organism survives wounds that would most likely kill other ciliates and single-celled organisms. The cell membrane is often repaired and life appears to go on as normal for an injured Stentor, even when it has lost some of its internal contents through a wound.
Changing a Response to a Stimulus
Stentor consists of just one cell, so many people likely have the impression that its behaviour must be very simple. There are two problems with this assumption. One is that researchers are discovering that the activity in cells—including our own—is far from simple. The second is that scientists at the Harvard Medical School have discovered that at least one species of Stentor can change its behaviour based on the circumstances.
The Harvard research was based on an experiment performed in 1906 by a scientist named Herbert Spencer Jennings. Stentor roeselii was the subject in his experiment. Jennings added carmine powder to the water by the trumpet shaped openings of the ciliate. Carmine is a red dye. The powder was an irritant.
The scientist noticed that at first Stentor bent its body to avoid the powder. If the powder kept on appearing, the ciliate reversed the direction of its cilia movement, which would normally have pushed the powder away from its body. If this action didn't work, it contracted its body into its holdfast. If this failed to protect it from the irritant, it detached its body from the substrate and swum away.
The results of the experiment attracted the attention of other scientists. A 1967 attempt to repeat the experiment couldn't replicate the discoveries, however. Jennings' work was discredited and ignored. Recently, a Harvard scientist became interested in the experiment and by the fact that its results were refuted. After investigating the situation, he found that the 1967 experiment had used Stentor coeruleus, not Stentor roeselii, because the researchers couldn't find the latter species. The two species have slightly different behaviour.
The Harvard researchers tried using carmine powder as an irritant for S. roeselii but didn't see much response. They discovered that microplastic beads were an irritant, however. They were able to replicate all of Jennings' observations by using the beads. They also made some new discoveries.
They do the simple things first, but if you keep stimulating, they 'decide' to try something else. S. roeselii has no brain, but there seems to be some mechanism that, in effect, lets it 'change its mind' once it feels like the irritation has gone on too long.— Jeremy Gunawardena, Harvard Medical School
The Harvard researchers found that some individuals had a slightly different set of behaviours from others and in a few an orderly sequence wasn't observed, but in general a clear sequence of behaviours was observed in response to the continuous presence of the irritation.
Most of the time, the individual Stentors first bent away from the stimulus and reversed the direction of their cilia. These behaviours were often performed simultaneously. As the irritation continued, the Stentors contracted and then in some cases detached from the substrate and swam away.
It might be wondered why scientists at a medical school are interested in the behaviour of a ciliate. They believe that the behaviour shown by Stentor might apply to the development of a human embryo, the behaviour of our immune system, and even cancer.
Nobody is suggesting that Stentor has a mind, despite the use of the phrase "change its mind". Nevertheless, the discovery of its reaction to a harmful stimulus and its more autonomous behaviour compared to that of other cells could be important with respect to our biology. As the researchers in the second referenced article below say, Stentor challenges our assumptions about what a cell can or cannot do.
Stentor hasn't been as well studied as other ciliates, though this may be about to change. Until recently, researchers were unable to create a large population of the organism in captivity, even by binary fission. The ciliate also has a low mating frequency, at least under captive conditions. The situation seems to be improving as scientists are becoming interested in Stentor and are learning more about its behaviour and requirements.
The researchers who are studying the organism have discovered some intriguing facts, but there are still many unanswered questions about its life. It will be very interesting to discover whether any of our cells behave in ways similar to Stentor. Studying its cell may teach us more about the ciliate and perhaps more about our cells as well.
- Ciliata morphology from UCMP (University of California Museum of Paleontology)
- Stentor coeruleus information from Current Biology/US National Library of Medicine
- The study of regeneration in Stentor from Journal of Visualized Experiments/US National Library of Medicine
- The macronuclear genome in Stentor coeruleus from Current Biology
- Complex decision making in a single-cell organism from the ScienceDaily news service
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© 2020 Linda Crampton