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,
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