What can walk about on surfaces, swim as needed, gobble up mouthfuls of relatively large cells while on the run, or hang out, drawing in food from a distance, and do it all in a package about 150 microns in length? The answer -- one of the most amazing single cell creatures found in pond water -- the ciliated protist, Euplotes.

There are several common species in the genus Euplotes, each showing small differences in structure, length of cirri, size and behavior, but all have the characteristic four caudal (tail end) cirri. The four long cirri, which often project at different angles making them hard to see at the same time without a change of focus, sets the genus Euplotes apart from another common organism with which it is sometimes confused, Stylonychia, which has three caudal cirri.

However, even at a glance, the two can be distinguished by their very different behavior. Stylonychia (far right) goes into a zigzag dance when entering the brightly lit microscope field while Euplotes tends to remain relatively calm.

But Euplotes does something that no other protist does as well -- it walks about on surfaces.

Walking, and to a degree swimming, as well as hanging on to objects while feeding is achieved using the clumps of cirri. Cirri are tendril-like structures composed of fused cilia (notice how the tips of the caudal cirri flair out into component cilia). Cirri are a characteristic of a very successful evolutionary line of ciliates, usually called the Spriotrichs. Euplotes and Stylonychia are the most commonly found spirotrich genera in pond water samples.

Viewed from the front, Euplotes has a cup-shaped buccal (mouth) region . Along the margin of the cup is a row of short tufts of cilia that help direct food items into the cup, which acts as a holding and sorting area. At the end of the cup is the actual cytostome, where food is engulfed in food vacuoles. The selection, rejection, and engulfment process appears to be quite complex and must involve some sort of sensory apparatus lining the buccal cavity. Digestion proceeds as in all ciliates with lysosomes delivering hydrolytic enzymes to the food vacuoles along with hydrogen ions, creating the acid condition conducive to effective digestion. Undigested wastes are eliminated through a pore at the rear of the cell.

Euplotes has a U-shaped macronucleus and divides in typical ciliate fashion by transverse fission with duplication of the cell organelles and surface structures. So, next time you are exploring drops of pond water, look for a ciliate strolling about on fragments of plant material, creating a stir with its feeding cilia, and exhibiting such structural complexity that it’s hard to believe that you are viewing a single cell.

For a colorful and revealing instructional video / DVD about protist microlife see Branches on the Tree of Life: Protists