A paramecium is a single-celled organism that swims through fresh water, pushed along by thousands of tiny hairs called cilia. A large kind, Paramecium caudatum, is about 200 micrometers (0.2 millimeters) long. A micrometer is a thousandth of a millimeter.

Size

Paramecium is a genus, a group of closely related species, and its species differ in size. Measured from end to end, they are about 100–300 micrometers long. The size shown here is a length, since a paramecium is long and narrow: Paramecium tetraurelia, for example, is about 115 micrometers long but only 34 micrometers wide.

Paramecium caudatum is one of the large species, with a pointed rear end. A 2021 review gives it as about 200 micrometers long, and another review from the same year as about 300 micrometers. Paramecium tetraurelia, a species widely used in research, is smaller, about 115–150 micrometers long.

Size comparisons

• A 200-micrometer paramecium is about twice as long as a human hair is wide.
• It is about 1.7 times the width of a human egg cell, or ovum, which is about 120 micrometers across.
• A dust mite, about 0.3 millimeters long, is about one and a half times the length of a paramecium.
• Each cilium is about 10 micrometers long, so the cell is about 20 times as long as the hairs that move it.
• Swimming at about 1 millimeter per second, a paramecium covers five times its own length each second. A person 1.7 meters (5 feet 7 inches) tall moving five body lengths a second would travel at about 31 kilometers per hour (19 miles per hour).

How a paramecium swims

A paramecium is covered in several thousand cilia, each about 0.2 micrometers thick. They beat 10–20 times a second in coordinated waves that pass over the cell. The cell swims forward in a spiral path at about 1 millimeter per second, turning around its long axis about once a second.

When it bumps into an obstacle or meets a chemical it avoids, a paramecium swims backward for a moment, turns, and sets off in a new direction. This is called the avoiding reaction. An electrical signal across the cell's membrane, like the signals nerve cells use, reverses the beat of the cilia. For this reason, some scientists have called the paramecium a "swimming neuron".

Life in a pond

Paramecia live in fresh water all over the world. They feed on smaller organisms such as bacteria, algae and yeast, which cilia sweep into a groove that leads to the mouth. In turn, other microorganisms such as Didinium eat paramecia.

A well-fed paramecium can divide in two every 6 hours, and without food it can survive for several weeks. Paramecia have lived in ponds and lakes for hundreds of millions of years, and a fossil paramecium has been found in amber about 200 million years old.

Sources

• Integrative Neuroscience of Paramecium, a "Swimming Neuron". Brette, eNeuro, 2021.
• Using Paramecium as a Model for Ciliopathies. Valentine and Van Houten, Genes, 2021.
• Just How Small Is "Nano"?. National Nanotechnology Coordination Office.
• Oocyte diameter predicts the maturation rate of human immature oocytes collected ex vivo. Pors et al., Journal of Assisted Reproduction and Genetics, 2022.
• Environmental assessment and exposure control of dust mites: a practice parameter. Portnoy et al., Annals of Allergy, Asthma and Immunology, 2013.