WALTER  DIONI                                          Durango (Dgo) Mexico

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Homage To Christian Colin


I know that he would have really appreciated this article.

Fig. 1 - an image of a very probable St. muelleri.
Click on the image to view the original
I captured this image, and the others of this species, in a population developed in an aquarium with hard water, at Cancún, Mexico.
Christian Colin (aka stentorbleu) was a Frenchman who 4 years ago did not even have a microscope. At the time of his sad decease some weeks ago he was a remarkable microscopist with a good theoretical knowledge, high manipulative skills, capable of sorting out the trophi from a rotifer's mastax and take high resolution images of it, and to manage to study and photograph difficult species of Stentor. He was also until his death a notable and indefatigable palynologist and an incredible correspondent in the French microscopy forums. Separated as we were by an entire ocean we became good friends. This is my humble homage to a man that doesn't give up to his sickness and converts it into an intellectual stimulus.


    The purpose of this article is to invite the reader, who has never engaged in taxonomic research, to follow me through the maze of data that will finally allow us to develop the key which was the initial objective of this work. I suggest that this article should be read carefully. I hope that at the end of the journey, after having consulted the documents which are suggested, reading the text a few times and carefully observing the images, the microscopist with little or no taxonomic experience will have a better overview of the subject and, especially, that they will be able to really give a name to their specimens.

As I state in the paragraph on methods of observation and documentation, the simple photographs taken by most microscopists do not allow a precise species determination. Names applied to the specimens shown here are compatible with the key. But the absence of any complementary data only allows "probable" identifications. I hope that in the future any microscopist having a Stentor under their objectives (including me, of course, if I have the chance to find them anew) compiles all the relevant information which they will need to do a positive identification.


One genus which readily draws the attention of microscopists who observe the freshwater protists, is Stentor, whose first species was named 231 years ago. Generally of large size, sometimes numerous, and very frequent, the stentor is easy to study even with low powers. The desire to give a denomination (a specific name) to the studied specimen conflicts with the shortage of descriptions in the reference books.

My first intention was to produce a small key of the more common species of the genus. But, aside from the difficulty in deciding which species to exclude, the study of the subject made me more interested in a broader key, to encompass not only the Stentor genus, but also the family which includes it and two other closely related families. This article is the result of all these inquiries.

The heavily annotated key that I add at the end, tries to offer microscopists the best and fastest taxonomic determination of their specimens. One cannot however regard this method as infallible. A few characters from one key, even if they are important, are not enough to replace a complete description of the species. The big task that faces the taxonomist (professional or not) who is interested for the first time in the stentors is clearly illustrated by the number of publications on the genus, which, according to Foissner and Wölfl 1994 exceeds 1300.  Individuals of each species can present great variations, which can require for their definition a somewhat deeper study, with advanced techniques, like silver impregnation for example. The specialists have not even been able to completely agree on the characters that are used to define and to differentiate the Stentor species.

There exists however a series of characters which can be observed without ambiguity with average powers and which make it possible to attempt the primary identification of even live specimens. These characters are, in order of ease of observation:

1 - presence or absence of symbiotic algae,
2 - the shape and size of the nucleus,
3 - the form of the apical sector or peristomial bottom, and the presence or not of one "buccal pouch",
4 - the presence or not of pigmented granules between the longitudinal lines of cilia
5 - obviously the color of the pigment if it is present
6 - the existence or not of a total or partial gelatinous tube (lorica) in which the individual lives

stentor 002
Fig. 2 - description of the anterior end of one Stentor roeselii (silver impregnation): AZM, Adoral Zone of Membranelles. BC, oral cavity. Ma, Macronucleus. PBC, Ciliary lines of the "peristomial bottom", PM, parorale membrane. SC, ciliary somatic lines. Figs. 2-6, different nuclear forms: 2 vermiform, 3 nodular, 4 moniliform (in chain), 5 only one ball, 6 more than one ball. Fig. 7a- peristomial bottom with a "buccal pouch", 7b shows a  peristomial bottom without "buccal pouch". (One of the lines of cilia of the peristomial bottom is drawn for showing better the position and form of the pouch. Figs 1-6 modified from Foissner and Wölfl 1994, Fig. 7 modified from Kumazawa 2002.

stentor 003
stentor 004
stentor 005
Fig. 3 Stentor amethystinum - AZM sinking to the mouth. In this and the following pictures we can see the ectoplasm with its pigment granules and the zoochlorelles.
Fig. 4 Stentor amethystinum - disposition of the ciliary lines (clear lines) and the bands of pigmented granules. The background is strewn with zoochlorelles. Fig. 5 Another species, showing the cytoplasm deprived of zoochlorelles, but with food vacuoles full with chlorelles. See the moniliform nucleus.
above three images are from Christian Colin

stentor 006
stentor 007
Fig. 6 - prob. St. Muelleri from Cancún. We can see the lines of ciliary insertion. Fixed with hot AFA, stained with Fast Green, mounted in NPM. In the right insert the moniliform nucleus of ellipsoidal beds is shown.

Fig 7 - Stentor picture in phase contrast. Although the inner details are not very visible, cilia differentiate in short somatic cilia and stereocilia or rigid cilia, more dispersed than the somatic ones.

Fig. 7 reproduced by the kindness of Rick Gillis, PH.D, from his site

HTTP:// Contents/Lab-2b/Stentor/Live_ Stentor_1/live_stentor_1.htm

Some confirmatory characters, better observed with a larger power (400x), are the width, the number and the position of the longitudinal bands of ectoplasmic granules, the presence, the number and distribution of rigid cilia (stereocilia) between the normal ones, the size, and forms of the extended individuals.

Conditions for the identification

Careful live study is essential. The best data at weak enlargement (40x and 100x - objectives 4x and 10x, with an eyepiece 10x) will be obtained on individuals fixed on a substrate and extended in their normal attitude of food gathering. The specimens can be anaesthetized by mixing carefully in the liquid which contains them small drops of a 1%  solution of sodium or potassium iodide, but it is very difficult to obtain a perfect anaesthesia in conditions of total extension.

The use of strong enlargements (400 - 1000 X) can be used only on specimens carefully compressed between coverslip and slide. One can resort for this "to the petroleum jelly compressors". Lubricate the palm of the hand with a very thin layer of solid petroleum jelly (Vaseline) and pass on both opposite borders of a coverslip. With a micropipette select one or several individuals and drop them in the centre of a slide. Apply the coverslip over the very small drop of water containing the protozoa, in such a way that the drop remains in the centre of the cover. By using one or two mounted needles, the small quantity of petroleum jelly will make it possible to apply and vary the pressure on the specimens, while the two open borders will allow water addition, anaesthetics or other reagents with a micropipette.

If one wishes to make longer observations, use a seal with paraffin, or a microaquarium.

Using phase contrast illumination can be useful to define ciliature and even better to identify rigid cilia, but it does not appear suitable for the study of internal organization, because there exists in the cytoplasm too many inclusions of very similar index of refraction. It is advisable to try the darkfield stops and contrast filters. The protozoan must be observed in all possible positions, as it is free, or as adhered to the substrate, if it is sessile.

Many of these details and especially the color of the ectoplasmic bands of granules, can only be interpreted by microscopists, during the direct examination of the live individual.

To highlight in pictures the required identification details, they must be taken at several powers and with a high resolution digital camera (if possible with a 2 or more Mpx), specially focusing on the important details (e.g. ex. stereocilia).

Consulted Bibliography  - As well as Kahl’s 1935 basic and irreplaceable work, we consulted that of Corliss 1961 and  Foissner and Wölfl 1994, which carries out a brief but complete and informative revision of the genus, by treating all the species, almost 50, described after the traditional work of Ehrenberg 1838. They recognize as valid only 18 of those species.

  (Tens of other specific names have been applied to specimens or populations which analysed with the current techniques are considered simply badly studied, or are synonyms of the recognized species.)

Kumazawa 2002, who studies the differential traits between some of the species with moniliform nucleus, recognizes as valid, one species that Foissner and Wölfl considers invalid, and adds a new species from Japan.

The  Lynn 2002 publication on line, which summarizes the genera and the type species of ciliates is also very useful to see in detail the position of Stentor in the traditional taxonomic system of Ciliatea.

Lobban et al. 2002 and Song and Wilbert 2002 describe two new genus (and species) of Stentoridae.

I thank very much Professor Daniel Nardin, whose kindness and effort allowed me to consult the article of Fauré-Fremiet 1936, on Condylostoma auriculata which will be much commented on subsequently.

Systematic position of the Family

I consider it worthwhile to include in the key, the 21 species currently recognizable of the genus Stentor, even if the microscopist probably does not find frequently more than 4 or 5 species. 

Below are links providing an introduction to the taxonomic categories and traditional hierarchies. If the reader does not have any background on taxonomy I suggest they must be your first reading (at least the first two). 

 Classic classification scheme:

to understand the modern systematics more deeply:


Parts of a specific name

Stentor multiformis (O F Müller, 1786) Ehrenberg 1838

                                                 1              2                          3                          4

1 - name of the genus (it is always written with a capital letter)
2 - name of the species (
one always writes it with small letters)
3 - name of the researcher who described the species for the first time,
but probably in a different genus, and dates of the description.
4 - name of the researcher who assigned the species to the correct genus, and year of the revision work

When there  have not been modifications of the name since its description one only uses 4, ex:

Maristentor dinoferus  Lobban et al 2002

"et al." (and others), indicates that the authors of the specific name are more than two (see the complete title of the work of Lobban and his associates,  in the references). 


Taxonomic situation and its relations according to Lynn 2002 (see the references).

                         Phylum Ciliophora Doflein, 1901

                                      Class Heterotrichea Stein, 1859

                                                Order Heterotrichida Stein, 1859

                        Family Stentoridae Carus, 1863

Having been established almost 140 years ago the Family Stentoridae has a long and complex history. In the times of the monumental work of Kahl 1935, it included also the genus Fabrea and Climacostomum. In 1961 Corliss considered valid the genus Stentoropsis created by Dogiel and Bychowsky in 1934. This same year (1961) Vuxanovici included a genus, Parastentor, with only one species, which raised to 5 the genera of the family. In 1972 Fabrea and Climacostomum were shifted by Repak to one new family: Climacostomidae. Vuxanovici’s genus was questioned, and consequently Stentor and Stentoropsis only remained, until 1978 in which year Jankowski (in an article written in Russian) proposed to separate the species Stentor auriculatus  Kahl 1935 in a new genus Condylostentor. 

However in 1936 Fauré-Fremiet, had already proposed this species, which he described and illustrated in detail from specimens collected at Concarneau (Bretagne, France), is regarded as a member of the genus Condylostoma, corresponding to the family Condylostomatidae, and its criterion has enjoyed more acceptance from the protozoologists, who rejected Condylostentor, created without bringing a valid criticism to the work of Fauré-Fremiet. Consequently Stentoridae continued to be a family with only two genera (one of them very poorly known) until 2002 when almost at the same time two new genera Maristentor and Heterostentor, were described and appear to be accepted for the moment.

On Parastentor, Stentoropsis and Heterostentor we will make some comments below.

and Climacostomidae

are consequently the families closely related to the Stentoridae. I want to propose a key for the reader to recognize the three families. Read carefully, because I include extended comments and some images between the various options.


The key is based on dichotomous selections. There are only two options, one is true, the other not. You compare your specimens with the options and decide which is in conformity with your material. The option gives you a denomination, or sends you to another dichotomous option.

Read the key as follows:

A(B) means that if option A is not true you must pass to the option B.
C at the end of the line means that if your material is in agreement with A you must continue to seek in the option C.

A(B) Peristomial bottom ciliated, without undulating membrane or with a rudimentary one ………….......................................................................................................................................................C

B (A) Body large, (700-900 microns) contractile; with one undulating membrane very visible on the right* anterior end; one Adoral Zone of Membranelles (AZM) delimits a peristomial bottom without cilia ................................................................................................... CONDYLOSTOMATIDAE


            With one vast peristome, V shaped; macronucleus moniliform                                              …………………………..................................................... Condylostoma Bory de St. Vincent, 1824


Some species of Condylostoma can be seen at

Stentor auriculatus
was included in Condylostoma as C. auriculata, since it has the characteristics of the family and the genus (undulating membrane, V shaped peristome, peristomial bottom without cilia, moniliform nucleus.), (Fauré-Fremiet 1936).

Note: Kahl presents and illustrates moreover S. auricula Kent 1881. In the illustrations of Kahl, both species are fixed at the substrate, showing the stentor's aspect. Other species of Condylostoma are free, and the only image which we can ascribe to C. auriculata (a picture taken by Jean Marie Cavanihac from the Mediterranean shore, see it below) shows a free individual.

S. auricula according to Fauré-Fremiet 1936, and Foissner and Wölfl 1994 is most probably a species of Condylostoma, however Kahl allots a single and oval nucleus to it. Obviously this species (if it exists) needs to be newly found and well described. It was found only in 1881 among bryozoa in the aquarium at Westminter.

It is a notable fact that these two species (at least in the documents that I could find on the Internet) "disappeared", being quoted neither as Stentor, nor as Condylostentor, nor as Condylostoma.

stentor 008
Condylostoma  prob. auriculata - pictures from Marseilles, France, on the Mediterranean shore by Jean Marie Cavanihac. I believe that this is the only available picture of this species and the first citation for the Mediterranean.

C(D) Large body, generally with ovoid shape with a truncated former end. More or less flat body; a large and visible AZM, in the left edge* of the peristome, spiralling into cytostome, uniform somatic cilia. A contractile posterior vacuole ……….........................................................................................................…….CLIMACOSTOMIDAE


Included here is Fabrea Henneguy 1910, a genus found in brackish water (up to 20% of salinity), often found in saltworks, and Climacostomum, Stein 1859, from fresh and brackish waters. Species of the two genera are relatively small (around 300 microns).

Image of Fabrea salina HTTP ://
Image of Climacostomum

D(C) Form of long reversed cone (shaped as a "trumpet") adhesive posterior end, with one Adoral Zone of Membranelles forming a spiral around one vast and ciliated peristomial bottom. Anterior contractile vacuole, with thin channels, one with the length of the cell and another around the base of the peristome


* To understand the orientation of a subject in a drawing or a picture, it is necessary to consider how it is presented with respect to the observer. On this assumption, what is at our left in the drawing or photograph, corresponds to the right in the organism. If no special statement is made it is assumed that the organism is drawn by the ventral face.

Now we can go to the Stentor species key

Comments to the author, Walter Dioni , are welcomed.


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