Flagellates are protists that move or eat using small numbers of undulating flagella. Eukaryotic flagella rely on a motile protein, dynein, to create tension between microtubules that form a column (axoneme) so that the flagellum flexes. They are not the same as the flagella of bacteria. Flagella appeared early in the evolutionary history of eukaryotic cells, and they seem to have been very useful, as they are retained in many evolutionary lineages – including ourselves. The ‘flagellates’ is a term used for many different kinds of protist that have flagella, about a dozen or so of which are shown here.
Six organisms near the top of the panel are euglenids. Some eugleniods have chloroplast and there is more about them on another page, but others are heterotrophs which means that they absorb or ingest their food, rather than make food by photosynthesis. The majority of euglenids have two flagella. The euglenids are shown first because their flagella are very thick. Unlike most other organisms there is a second rod inside the flagellum which makes it much easier to see. Two flagella can be seen easily in Metanema, Heteronema, and Ploeotia. The second flagellum sticks to the body of Peranema, so it just be seen as a delicate line running along the length of the body. You can see jaws in the cells of Heteronema, Peranema, and Jenningsia – they are used to grab particles of food and draw them into the cell.
Below the euglenids and to the right are two kinetoplastids – Neobodo and Rhynchomonas. They are moderately closely related to the Euglenids, and also have slightly thickened flagella. They too eat particles of food, mostly bacteria. We can see the lips of the mouth in Neobodo, while in Rhynchomonas the mouth is in the bulbous ‘nose’. Kinetoplastids such as these are found in all kinds of watery habitats around the world – including in some that are very extreme. Second down on the left is Mastigamoeba. In this case the flagellum is unthickened and is delicate. The base is connected to the nucleus which sits near the center of the cell.
Next down is a cell that is about 10 µm long (where 1 µm = 1/1000 millimeter), much smaller than the euglenids. It is called Jakoba, and has one anterior flagellum and a second lying in a groove in the body. It is named after Jakoba Ruinen who travelled around Australia over 100 years ago, looking for microbes. The pink lake was one of the places she studied (it is at Underbool in Victoria, Australia). To the right of Jakoba are three cercomonad flagellates, small, common and contributing to the processing of microbial matter. Cercomonas has a very fluid body that often trails behind the gliding cell, while Massisteria lies on the ground extending its fine arms to help intercept possible. Heteromita is especially common in soils, moving in the thin film of water that coats soil particles.
Cafeteria, to the left is one of the stramenopiles – flagellates that have stiff hairs attached to the flagella. The hairs help drive water towards the body where particles of food can be extracted. Cafeteria is exceedingly widespread and common in oceans and coastal water. It is one of the ‘heterotrophic nanoflagellates’ that along with the bodonids and cercomonads, are a key part of microbial food webs – processing microbial biomass to make it available to small animals and releasing nutrients that keep the ecosystem active. A very different stramenopile is the large flat Opalina. Opalinids may grow to be almost 1 mm, and live in the guts of amphibia.
Cryptomonas paramecium (it used to be called Chilomonas paramecium) – on the left – is a cryptomonad flagellate. Like other cryptomonads, it has two flagella of normal thickness anchored in a pouch in the front of the cell. Distantly related to any other flagellates shown here, cryptomonads are to be found in freshwater and marine ecosystems.
The flagellates of Spongomonas and Rhipidodendron form large colonies, producing a mucus that provides the matrix within which cells are embedded. Rhipidodendron forms fan-like colonies, whereas Spongomonas forms branching cylindrical colonies.
The remaining three pictures are of relatively large flagellates that live in the guts of termites. Termites, as with other animals, are unable to digest the cellulose that makes up much of the food of herbivores and so they, like cows and other herbivorous mammals, rely on a community of microbes to supply the digestive enzymes. All of these flagellates have many flagella. Dinenympha, to the left, has an almost checkered sculpting of the surface. The trichonymphid has an array of flagella at the conical front end of the cell – but the filaments projecting from the back end of the cell are bacteria – spirochaetes. Finally, a detail of the anterior of Joenia, with a massive skeletal rod that runs through the cell, wraps itself around the nucleus, and ends under the cap of flagella.