Moments ago, you made a decision to click on this video.

Maybe YouTube has been gently poking you with suggestions to watch this. Or maybe it’s part of your routine.

Whatever the reason, we hope the decision was an easy one to make. But even if the decision was easy, the steps it took to execute that choice, the information you needed to gather, were not simple. From reading the title to physically clicking the link to settling back into a comfortable position, your body had to coordinate a ton of processes using chemical and electrical signals to quickly send instructions all over the place.

These signals are rooted in your nervous system, a reflection of your multicellular complexity. Life in the microcosmos comes with its own choices though. And while they may not have to coordinate multiple limbs, single-celled eukaryotes, or protists, still have decisions to make.

They've got places to be, things to do--and they don't have a brain to help them. All they've got is the one cell to deal with all of life's complications. Microbiologists have been fascinated by the way protists respond to their environment for more than a century.

And so naturally, that has led some scientists to try and provoke a response, like James--our master of microscopes--using a piece of hair to poke these stentors. In 1906, a zoologist named Herbert Spencer Jennings published Behavior of the Lower Organisms, a 366 page book containing sections with titles like "The Daily Life of Paramecium" and "Reactions of Infusoria to the Electrical Current". Nestled among descriptions of the many organisms and behaviors he observed was a section documenting the way stentors responded when he injected carmine into the water around them.

Carmine is a red dye and apparently it’s something that Stentors don’t like very much. Like our stentors, the subjects of Jennings' experiment did not particularly revel in this irritant. In our case, you can see the stentors trying to act like everything is normal.

But with repeated poking, they start to contract, evading the troublesome hair but also missing out on opportunities to gather passing food. They stay contracted for a short period, but eventually they emerge and extend back to their original length. In Jennings' case, the stentors also attempted to ride out this irritant.

But when that failed, those organisms then began to contort and twist to the side. And in that way, the stentors could attempt to evade the carmine while still focusing on what's truly important: food. This avoidance reaction is a common one among protists.

In the same text, Jennings describes the tendency of some organisms to avoid light, similar to this trachelophyllum here. Those changes in brightness that you see are coming from the microscope as the diaphragm on the condenser opens up, allowing more light to enter the slide. As the slide quickly gets brighter, you can see the trachelophyllum trying to swim away.

This organism isn't that well-studied, so we don't know exactly why it's trying to avoid the light. Some microbes definitely seek out dark areas to avoid predators, so that's a possible explanation. The light itself may also be harmful to the organism.

Whatever the reason, this trachelophyllum isn't alone among protists. One of Stentor coeruleus' many compelling traits is its photosensitivity. Inside its unicellular, trumpet-shaped body are photosensitive pigment granules called stentorin that, in the presence of light, drive electrical changes in the plasma membrane and reverse the movements of its cilia, transporting the organism away.

Of course, there are also protists that seek out light, like the photosynthetic Euglena that rely on sunlight to make their food. Even then, there are subtleties in their response, which Jennings observed and described. While the Euglena in his experiments sought out light as expected, if the light was too strong or brought on too suddenly, the euglena sought shadier refuge.

Another protist that captured Jennings' attention was the paramecium, that seemingly simple organism that has captivated many scientists because of its hidden complexities. Here, you can see the paramecium on this slide in their own series of avoidance reactions. And the main thing they seem to be trying to avoid is each other.

That might sound fairly familiar to some of us these days. As they navigate the crowd, you can see the paramecium actually turn around, which is coordinated by the movement of their cilia. By changing the direction that the cilia move in, the organism is able to reverse their direction.

They can also spin, turn, or just come to a stop. If they do a number of these reactions in quick succession, the paramecium can even swim backwards. These movements are a joy to watch, and it is astounding to consider how a unicellular organism is able to change its behavior based on its surroundings.

But Jennings' findings extended beyond just the notion that an organism will react to its surroundings. As he continued to prod his stentors with carmine powder, he noticed that the organism began to explore other strategies to deal with the irritant. When avoiding the carmine failed to stop the provocation, the organism would temporarily reverse the movement of its cilia, driving the water current away from the stentor so it could essentially "spit" water out of its mouth.

And when that didn't work, the stentor tried to contract away from the stimulus. And when that didn't work, well, then the stentor just up and left, detaching themselves from their substrate and swimming away in search of a quieter, less carmine-filled home. There's a pretty key implication to Jennings' observations here: the stentors he observed didn't just react to stimulus, they worked through a hierarchy of possible responses.

Think of it the way a cat deals with the common irritant of a loving human who wants to cuddle: they start with simple evasion tactics, but they might then escalate to swiping and hissing before just picking up and running to a corner where they can't be bothered. In the case of Jennings' stentors, which fortunately did not have sharp claws, there seemed to be a ranked preference to the measures they were going to take. There was just one problem with Jennings' results: no one could replicate them.

And so for more than a century, his observations were set aside until a scientist named Jeremy. Gunawardena decided to dig deeper and found that these follow-up studies used a different. Stentor species than Jennings had used.

When Gunawardena and his team gathered the original species, Stentor roeselii, they found that their subjects exhibited those same behaviors. Interestingly though, different specimens of the same species seemed to have their own ranked preferences. Some might only contract, while others would perform a mix of bending and cilia-reversal.

We think the stentors we've been poking here are Stentor roeselii. And if they are, they seem to have preferred the contraction approach themselves, until they gave in and swam away. For the past few minutes, we've been following one of them as it tries to find a new home, brushing up against possible substrates with its oral cilia.

We think this is part of their inspection process, and if they don't like what they feel, they swim backwards and away. This little guy has tried a few locations, but here, it seems to have found something intriguing. Even as other stentors come in and interfere, this one persists, returning to inspect its newfound spot again and again like it’s about to make an expensive purchase.

And when it's satisfied with its decision, the stentor releases an adhesive substance and moves in, ready to eat until the next decision needs to be made. And if that isn’t life, I don’t know what is. It is remarkable to think that just choosing the wrong species of Stentor can give such different results, that decision-making--even for the simplest of organisms--is such a complicated endeavor for us to untangle.

We're still only at the early stages of understanding what these actions and reactions require from different organisms, and for some, it is likely to be much simpler than for others. What's most remarkable, as always seems the case with the microcosmos, is just how many ways there are to do anything at all. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

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