Motility is the ability of an organism to move on its own by using metabolic energy. This capacity for self-propelled movement is a defining feature of many life forms, from microscopic bacteria to the largest animals, and is genetically determined but can also be influenced by the environment.
In contrast, sessile organisms lack the means for self-locomotion and remain in one place. Many plants and fungi are sessile, anchoring themselves to a substrate. While the organism may be immobile, it might produce motile components, like seeds or spores, that are dispersed by external forces. Some organisms are also sedentary, meaning they move infrequently.
How Organisms Move
The mechanisms that power movement are incredibly diverse, scaling from the microscopic to the macroscopic. At the cellular level, one tool is the flagellum, a whip-like appendage that rotates or undulates to propel the cell forward. Many bacteria and some single-celled algae employ flagella for locomotion.
Another common method involves cilia, which are short, hair-like structures that cover a cell’s surface. Cilia beat in coordinated waves to move the organism through a fluid medium. Other microorganisms, like the amoeba, use temporary projections of their cytoplasm called pseudopods, or “false feet.” The cell extends a pseudopod and then flows its internal contents into this extension, effectively crawling forward.
In larger, multicellular animals, movement is achieved through the action of muscle tissues. These tissues contract and relax, generating force that is applied against a skeletal system. This can be an internal framework of bones (endoskeleton), an external casing like a shell (exoskeleton), or a fluid-filled cavity (hydrostatic skeleton). The coordinated contraction of muscles pulls on the skeleton, resulting in the movement of limbs and the body.
The Reasons for Movement
Movement is a behavior driven by the needs for survival and reproduction. A primary reason for motility is the pursuit of resources, which includes searching for food, locating water, or positioning for optimal light exposure. The ability to move allows an organism to leave depleted areas and explore new territories with more abundant resources.
Another major driver of motility is the avoidance of threats. This can involve escaping from predators or moving away from unfavorable environmental conditions. For instance, an animal might flee from a hunter, or a microorganism might navigate away from a toxic chemical, a response known as chemotaxis. Organisms may also move to escape extreme temperatures or droughts.
Finally, motility plays a role in reproduction for many species. In some cases, movement is necessary for finding a suitable mate. For others, motility is about the dispersal of offspring. By moving away from their parents, young organisms can reduce competition for resources and colonize new areas.
A Spectrum of Motile Organisms
The ability to move is found across a vast and diverse array of life. Microscopic organisms like bacteria and protozoa exhibit motility to explore their worlds, find nutrients, and react to their surroundings.
Moving up in complexity, simple animals also display various forms of motility. For example, jellyfish, while often carried by ocean currents, can propel themselves through rhythmic contractions of their bell-shaped bodies. This pulsing motion allows them to hunt for prey and adjust their position in the water column.
More complex animals have evolved specialized systems for movement in water, on land, and in the air. Fish utilize powerful muscles and fins to swim through aquatic environments, while insects and birds have developed wings for flight. On land, mammals showcase a wide range of gaits, from the walking and running of terrestrial animals to specialized forms of crawling.