Science News reveals the strange habits of little single-celled swimming organisms called “choanoflagellates” that tend to gather together in clusters for specialized tasks – which was probably how the first multi-celled organisms started out:
[W]hen cell biologist Nicole King, a Howard Hughes Medical Institute investigator at the University of California, Berkeley and her colleagues discovered hundreds of these organisms locked together in a sample taken from a splash pool along the coast of the Caribbean island of Curaçao, they were surprised. The cells formed a concave sheet, with their tail-like flagella extending from the cupped side.
What happened next stunned the scientists. In unison, the organisms making up the sheet inverted into a ball-like shape, tiny flagella flailing outward like tiny oars, allowing the organisms to swim much more swiftly. Accordingly, the team dubbed the new species Choanoeca flexa.
“It was this crazy behavior unlike anything we’d ever heard of in choanoflagellates,” King says, “We just had to figure out how they pulled it off.”
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Each individual resembles a sort of smooshed sphere. From one end, many tiny, tentacle-like protrusions form a collar that’s accented with a single, longer flagellum that extends beyond the collar.
Individual choanoflagellates join together by touching these collars. In the concave form, the flagella all point inward, “which aids feeding on bacteria,” King says. When the organisms flip into a more of a sphere, the flagella all point outward, becoming hundreds of tiny paddles that help with swimming.
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Exactly what triggered C. flexa’s transformation remained a mystery until the researchers noticed that the flipping stopped when the organisms were exposed to a microscope’s light for too long. On a whim, King tried turning off the lights then turning them back on. In the dark, C. flexa inverted into a ball shape. “And then we did it again, and did it again, and did it again, and every time we changed the illumination, they flipped.”
The researchers haven’t fleshed out the full mechanism, but they’ve confirmed that a light-sensitive protein known as rhodopsin plays a role.
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King isn’t sure why changes in light trigger this response. But she notes that a consequence of swimming faster in darkness and staying put in light is that C. flexa tends to move toward well-lit areas that might have more food. Individual cells can’t effectively swim toward light; groups of C. flexa can.