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One day, James—our master of microscopes—was cleaning the marine tanks that some of his organisms live in when he noticed this creature. It was hard to miss given that it was visible to the naked eye, thanks to both its bright red color and large size.

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Before we get into today’s episode I wanted to let you know that we’re going to be taking a short break from uploading videos for the next couple of weeks, but we will be back soon with brand new episodes.

And you won’t have to miss us the entire time because some of the Microcosmos team will be doing a livestream here on the channel during the break, and you can find more information about that on the community tab here on YouTube or on our twitter @journeytomicro. One day, James—our master of microscopes—was cleaning the marine tanks that some of his organisms live in when he noticed this creature.

It was hard to miss given that it was visible to the naked eye, thanks to both its bright red color and large size. What he had found was a species of foraminifera, which are single-celled amoeboid creatures that spend their lives poking their false feet—or pseudopodia—through holes in their tests, which is what we call their shells. But foraminifera are perhaps best known for what they reveal to us in death, their tests are preserved as fossils that record our planet’s history in their own chemical composition.

And among the various things that make foraminifera so helpful is one very simple fact: foraminifera fossils are large. For comparison, here is a one Euro coin sitting next to an array of foraminifera fossils. The forams are not large the way a T. rex is large, but they are still big enough to substitute for an object that people use every day.

When you zoom in to our giant red foraminifera, you can see the contents of its cytoplasm streaming in both directions. We think that this particular foram is a member of a group called xenophyophores, which are deep-sea organisms that have tiny crystals in their cytoplasm called granellae that are usually made from barium sulfite absorbed like a glittery souvenir from the places the foram once lived. Usually when we look at slides under the microscope, we’re on the hunt for creatures that are smaller than a speck of dust, creatures for whom the whole microscopy slide is a vast world they may never see the entirety of.

But this foraminifera is large. Yes, we are aware that we have said that multiple times in this video, but it's really worth emphasizing. The coverslip that James used for this footage was about 22 millimeters in length on each side, and our mysterious red giant managed to extend its web of pseudopodia across practically the entire surface.

And yet, we don’t want you to think that all foraminifera are like this. Over 500 million years, there have been tens of thousands of foraminifera species both extinct and still surviving that we know of. There some are far less than a millimeter in diameter, while others are centimeters in diameter.

The larger of these foraminifera are known by the extremely technical name of “larger foraminifera.” It’s a name that hints at a question we’ve sometimes wondered during our journey through the microcosmos: If life works so well at such tiny scales, why bother with size? What is the point of being large? Of course this question is too big for any single organism to answer.

But scientists have spent a considerable amount of time studying foraminifera, which has, yes, led to the publication of papers with relatable titles like "Why are larger foraminifera large?” Larger foraminifera are typically found in warm waters where conditions are stable, but food is usually scarce. That might sound surprising: it seems like if you’re living in an area without much food, it would be difficult to grow so large. But one of the reasons these particular foraminifera are able to grow so large is because they have help.

Within their tests are chambers that house algal symbionts like the foraminifera’s own personal greenhouse that they can harvest photosynthetic products from. These symbionts compensate for the lack of nutrition in the foraminifera’s environment. And yet symbiosis alone is not enough to explain the larger foraminifera’s size.

There are smaller species with algae symbionts, and larger species without them. So while the symbionts help the organisms survive in these nutrient-poor areas—and it certainly doesn’t hurt to have the size and test complexity to house them—they aren’t the main factor that decides that a foram species will become larger. What ultimately drives that shift is the choice foraminifera make on when to reproduce.

In an environment where food is plentiful, forams will reproduce at an earlier age. But reproducing earlier means smaller baby forams that are vulnerable to predators or currents that take them to less ideal environments. So these smaller foraminifera are playing a numbers game, their survival as a species is contingent on proliferating enough that these threats to the individual forams can’t take them all out.

But larger foraminifera aren’t living in those kinds of places. They brave more restricted conditions, ones where it makes more sense for them to wait a little longer to reproduce so they can focus their resources and energy on producing offspring that will be more likely to survive than a large population of tiny foraminifera. These larger forams spend years expanding their single-celled bodies, so that by the time they reproduce, their offspring will be larger and hardier compared to their smaller counterparts.

Their survival as a species is driven by the survival of these individual young. And over millions of generations, this strategy means the evolution of a larger species. These adaptations have made the large foraminifera very specialized in terms of the kind of conditions they thrive in.

And that’s great for survival in the moment, but it creates problems for them in the very, very, very long term. In the hundreds of millions of years that foraminifera have been around, mass extinctions have wiped out countless species—including, it turns out, larger foraminifera. Again and again, their adaptation to such a narrow, specialized environment has rendered them unable to contend with the changes around them fast enough.

And on the flip side of the extinction coin, their smaller, more rapidly reproducing counterparts have had the numbers and adaptability to survive. They may not be as well-adapted to the same niche that larger foraminifera are, but what good is being well adapted to a niche when that niche is changing? Through the fossil record left behind by foraminifera, we can see this history play out.

Over and over again, the fossils show, mass extinctions have obliterated larger foraminifera. But the fact that it happens again and again also reveals something else: the larger foraminifera may find themselves knocked down with each extinction, but…well, they get back up again. It seems like evolution is never gonna keep them down.

Because that’s what happens: evolution. Life rebounds, and when it does, the larger foraminifera evolve once again, in the same kinds of conditions we see them living in today. Over and over again, this form appears, almost as if the larger foraminifera is inevitable.

But of course, nothing is inevitable. Life has gone through so many extinctions, and much of what has emerged on the other side is different from what came before. We live with the shadows of dinosaurs and giant mammals, and maybe the future could some day hold some shadow form of us.

And in that future, foraminifera fossils will record the world as we lived it, a world that the larger foraminifera probably did not survive, but that it will still have managed to return to, eventually. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. We wanted to let you know that we still have some Journey to the Microcosmos calendars available in stock at Each spectacular image is accompanied by a little blurb explaining what you're looking at, and each photo showcases both James’ work as an artist, and of course the beauty of the microcosmos.

So if you love this little world that we almost never get to see. How about stick it on your wall, for all of 2023. You’re going to need a wall calendar, so why not get one that helps to support your favorite microscopic YouTube channel?

You can find it at, where some of the other shows from Complexly, the company that produces this show, also have calendars available. And you can find all of them at the link in the description. The people whose names are on the screen right now, they are our patrons on Patreon.

Thank you so much to all of you for supporting what we do, so that we can continue to explore all of the weird and wonderful ways that life on this planet functions. We couldn't do it without that support and so, we're very grateful and so are all the other people who enjoy what we do here. If you would like to join these people, you can do that at

If you want to see more from our master of microscopes, James Weiss, you can check out Jam & Germs on Instagram, and if you want to see more from us, there's probably a subscribe button somewhere nearby.