Hokkaido University and University of Toyama clarify spatial awareness in eyeless unicellular Stentor

A research group including Specially Appointed Assistant Professor Syun Echigoya, Assistant Professor Takuya Ohmura, Professor Toshiyuki Nakagaki, and Associate Professor Yukinori Nishigami of the Research Institute for Electronic Science at Hokkaido University, along with Specially Appointed Professor Katsuhiko Sato of the University of Toyama, has discovered that the " Stentor coeruleus," a unicellular organism living in aquatic environments, selects its attachment site based on subtle differences in the surrounding micro-shapes and prefers to anchor itself in "corner" spaces. This suggests that the shapes of micro-scale extracellular structures may influence ecosystems. The results were published in PNAS on February 25.

Mammals and insects rely on visual information to use structures in the environment as landmarks for spatial understanding. In contrast, the spatial recognition methods of unicellular organisms, which lack visual information, have been studied within the framework of "taxis," the tendency to be attracted to or repelled by certain stimuli.

In this study, the research group focused on the behavior of the 1-mm-long Stentor coeruleus and investigated how it finds important attachment sites for surviving in complex environments involving predators and currents. These organisms swim through the water by creating currents with their cilia, but they occasionally attach themselves to structures and use those same water currents to gather nearby food.

First, the researchers created a "diorama environment," an observation vessel that mimics the complexity of shapes found in nature. In this vessel, they provided various choices for attachment sites by varying geometric properties such as angle and depth.

As a result of the observations, they found that the organisms selected attachment sites based on differences in the surrounding shapes. They preferred to attach themselves to "corners" with angles of 45 degrees or less, corners deeper than their own body length, and areas around sharply pointed corners.

Normally, Stentor coeruleus swims in a straight line, repeatedly changing direction whenever it hits a wall. On the other hand, before attaching, they switched to a behavioral mode of moving along the walls, allowing them to find corners with a high probability. This wall-following behavior is achieved by asymmetrically contracting their bodies and tilting the structure of their ciliary bundles at an angle.

By conducting mechanical simulations of their behavior based on the tilt of their locomotive apparatus, the researchers were able to physically explain the wall-following movement through the tilt of that apparatus.

Echigoya stated, "It took two years just to successfully breed them. However, through that process of trial and error, we've come to understand a little bit about the way of life of the Stentor coeruleus. I think many people are unfamiliar with unicellular organisms, but they are living diligently even now in places invisible to the eye. I would be happy if this could be an opportunity for people to think about such a microscopic world."

Journal Information
Publication: PNAS
Title: Geometrical preference of anchoring sites in the unicellular organism Stentor coeruleus
DOI: 10.1073/pnas.2518816123