An international team of researchers led by Hokkaido University has characterized the unique mechanics that enable Arcella, a shelled, single-celled amoeba, to move skillfully across different surfaces. Their findings, published in the Proceedings of the Japan Academy, Series B, have shed light on how this tiny microorganism maintains mechanical balance during movement, supporting its shell through the coordinated use of its many pseudopodia.

Commonly found in peatlands and freshwater environments, Arcella belongs to a group called testate amoeba, which are characterized by having their bodies enclosed, almost entirely, within a dome-shaped shell, or “test,” that is thought to protect them from predators, parasites, and environmental stress like desiccation.

Through an opening on its underside, Arcella can mold its cell body to create finger-like projections called pseudopodia to capture prey and move around. This type of amoeboid motility, which is a type of cell movement using pseudopodia, is common in the living world. In humans, it can be seen in immune cells during tissue repair and in cancer cells during metastasis. But Arcella’s crawling is different. “Because of its large, rigid shell, its locomotion doesn’t follow the typical amoeboid movement observed in many cells. Instead, it moves through the coordinated use of multiple pseudopodia at once, almost like an octopus,” explains lead author Associate Professor Yukinori Nishigami.

Researchers discovered that Arcella samples collected from the Tomakomai Experimental Forest of Hokkaido University generate mechanical forces, known as traction stress, on the surface they crawl on, enabling them to grip and migrate. To quantify these forces, the team embedded fluorescent beads within the underlying surface and tracked their displacement. This allowed them to measure the traction stress and understand how the organism uses it to orient itself in the direction of travel. They found that differences in stress distribution between the front and back control straight-line movement, while differences between the left and right sides cause sideways movement.

What researchers found surprising was that Arcella’s locomotion varied depending on the stiffness of the surface. On a hard surface such as glass or firm gel, the amoeba moved more actively, covering up to three times the distance compared with a soft gel surface in the same time period. On a hard surface, Arcella was also more likely to extend pseudopodia mainly in the direction of movement, while on a soft gel its movement was more random, with researchers noticing pseudopodia extension in directions other than the direction of movement.

This surface-dependent locomotion strategy could be how Arcella adapts to different environments in nature and could be crucial for its survival. On hard surfaces, it migrates quickly and efficiently, while on soft surfaces it shifts to slower, more exploratory movements. This ability to shift and prioritize between speed and caution could be how it thrives in the unpredictable conditions of the natural world.

“I was impressed by how a single-celled amoeba can skillfully coordinate the movement of several pseudopodia at once. Its ability to change behavior depending on the environment demonstrates the remarkable adaptability of life, even at the microscopic scale,” Nishigami notes.

Shelled amoebae play an important ecological role in food webs, feeding on bacteria and serving as prey for larger organisms. Understanding their behavior could help us better understand their ecological functions, environmental responses, and biodiversity. Additionally, pseudopodial strategies could inspire innovations in soft microrobotics, where cellular mechanics provides natural design principles.

“Natural environments are far more complex than the flat, two-dimensional surfaces we often use in laboratories. I once observed Arcella ‘tightrope walking’ along a speck of dust in culture. Understanding how an organism less than 100 micrometers in size, without eyes or a brain, can perform such feats is an exciting frontier in research,” Nishigami reflects.