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As animals, we owe a lot to the single-celled organisms that came before us. These are the organisms that laid the chemical groundwork for how we live, from the DNA and proteins within them to the molecules they released into the environment. There’s something humbling about looking at our hands or feet and imagining the mixture of cells within them, and realizing the lessons that keep those cells bound together physically and biologically are rooted in a very ancient study in cooperation.

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SOURCES:
https://astrobiology.nasa.gov/news/how-did-multicellular-life-evolve/
https://hgic.clemson.edu/factsheet/nostoc/
https://pubmed.ncbi.nlm.nih.gov/21708762/
https://link.springer.com/protocol/10.1007/978-1-0716-2172-1_8
https://www.nhbs.com/rotifera-part-1-biology-ecology-and-systematics-book
https://www.jstor.org/stable/pdf/1535550.pdf

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As animals, we owe a lot to the  single-celled organisms that came before us.   These are the organisms that laid the  chemical groundwork for how we live,   from the DNA and proteins within them to the  molecules they released into the environment.  And eventually, some of those early single-celled  organisms had an evolutionary epiphany that laid   the groundwork for multicellularity: if  they banded together into a colony and   worked together—blending their lives and  identities until the lines between them   began to blur—then maybe they could accomplish  more together than they could on their own.  There’s something humbling about looking at  our hands or feet and imagining the mixture   of cells within them, and realizing  the lessons that keep those cells   bound together physically and biologically are  rooted in a very ancient study in cooperation.  And colonies persist to this day.  Single-celled organisms like Nostoc link   themselves together into chain-like colonies  that allow individual cells to be assigned   specific tasks like nitrogen-fixation  that benefit the colony overall.  But colonies aren’t restricted to single-celled  organisms.

Animals have gotten in on this behavior   too, stacking their multicellular selves  together for the same reason the Nostoc does:   because they can, and because it’s good for them. Two particularly notable examples of this are the   colonies of mossy bryozoa found  in waters throughout the world,   as well as coral that live in the ocean - and  also, as we’ve seen before, in coral farms.  But there’s one animal colony that’s  a little bit…unusual.

And of course,   that animal is the rotifer.  To be fair to rotifers, this is not a typical  behavior for all of them. There are more than   2,000 species of rotifers that we know of,  and only around 25 of them are known to   glue themselves together into a colony. You can see this arrangement a little   more clearly in this Sinantherina colony.

The  rotifers are all glued to some kind of plant,   bound together through the cement-like substance  they secrete from their foot. They radiate outward   from there, like petals on a flower, but like  petals that have a crown of cilia at their end   to stir up water and draw in food. Some of these colonies are large,   holding around 200 rotifers in one  group.

Others are smaller, containing   only five or so individuals connected together. And what makes rotifer colonies so different from,   say, Bryozoa or coral colonies comes down to  differences in how those colonies are made.  Let’s take Bryozoa, for example. The process  starts with an individual Bryozoa larva that   attaches itself to some kind of surface,  like a rock.

And once it has latched on,   it’ll split into two in a process called  budding, creating a clone that can divide   again and again to produce a whole colony. We have a whole episode on Bryozoa that you   can watch if you want to know more about  how these strange-looking organisms live,   but the main thing you need to know right now is  that as these clones accumulate into a colony,   their bodies are still connected in a way  that allows them to channel nutrients to   each other. And coral colonies are similar,  surviving as a connected collective of clones.  But rotifer colonies form in a few different  ways depending on the species, and none of them   bear much resemblance to the asexual budding  that drives coral and Bryozoa colony growth.  In some species, colonies are formed through  what’s called autorecruitive colony formation,   where a colony breaks apart, and creates  multiple factions that then grow in size   as new rotifers are born and added to the colony.

Then there is what seems to us like the strangest   method: some species use allorecruitive colony  formation, where young rotifers swim over to a   sedentary adult rotifer and stick themselves  to the adult. The result can involve 50 or so   young rotifers using a grown-up as a convenient  branch to live on. And I imagine it must be odd   to go from life on your own to suddenly having  a rotifer daycare attached to you, but maybe   it all makes more sense when you’re a rotifer.

And in the last route, called geminative colony   formation, a bunch of baby rotifers swim to a  spot and gather into a colony. It sounds simple   enough, but it’s also my favorite one because  for some species, this process is kind of a mess.  In 1906, a scientist named Frank M. Surface  documented how one species creates a new colony,   which begins when all the adults of a previous  colony lay eggs.

It’s a coordinated and carefully   timed procedure that results a few days  later in a brood of baby rotifers connected   to their parent colony by adhesive threads . But then the mess begins. As the babies move,   those threads tangle together, creating a little  ball of baby rotifers that are still wriggling   and jerking around until the ball breaks away  from its old colony.

From there, the babies are   on their own, swimming as a ball until they find  a place to settle down into a permanent colony.  However these colonies are formed, the  important—and unusual—thing is that they   don’t form in a way that allows the rotifers  to share with each other. They don’t seem to   be exchanging nutrition or anything the way  that corals and Bryozoa seem to be doing.  So why? Why go through all the  trouble of forming a colony?  There are a few possibilities.

While  the rotifers may not be sharing food,   gathering in these formations means that  they’re all generating currents around   each other with their crown of cilia. So  perhaps—as scientists have considered—the   joint movement of these rotifers in this  formation helps them all gather more food.  Or perhaps it makes them safer. A single  rotifer is an easy target for a predator,   but a group—while conspicuous—is too big  to eat, making it an excellent hiding   spot for the individuals that live within it.

Or it could be that there is adhesive strength in   numbers. As more rotifers attach themselves to a  surface, their glue reinforces each other, keeping   them more securely locked in place and less  vulnerable to forces that might sweep them away.  It could be one of these reasons, or multiple  of them, or perhaps something else altogether   that we haven’t yet imagined. Whatever the  reasons, they belong to the rotifer, and to   the microcosmos that tumbles around them and 50 of  their closest friends, glued together at the foot.  Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.  Since we’ve been talking about  rotifers this whole time,   we felt that it was probably a good idea to  let you know that over at microcosmos.store   you can actually get an enamel pin of one  of our favorite rotifers, the Brachionus.

Now, the Brachionus is not a traditionally a colonial  rotifer, but you could always get like 20 of these   pins and create your own Brachionus colony.  Although… that might be a bit expensive and   also extremely heavy to walk around with.  So, maybe actually, you should just stick with one. But we’ll leave that choice up to you.  Whether you want 1 or 20 brachionus pins,   you can find those at microcosmos.store, along  with all of our other Microcosmos merchandise. And speaking of a group of individuals coming  together, the names you’re seeing on the screen   right now are our wonderful patreon patrons.  The people who come together to support this   channel and keep these videos showing up in your  subscription feed every week.

We can never thank   them enough, and if you’d like to join them  you can sign up at patreon.com/journeytomicro  If you want to see more from  our master of microscopes,   James Weiss, and why wouldn’t you,  check out Jam & Germs on Instagram,   and if you want to see more from us, there is  always a subscribe button somewhere nearby.