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If I were to ask you to just do a little sketch of what you think of when you hear
the phrase “moss animals,” you might imagine something like this tardigrade—a waddling,
slightly fantastical creature that we did in fact find living within the minuscule greenery of moss.
But depending on who you are and what you know, maybe you draw something completely different.
Maybe you draw something more like this. These don’t really look much like an animal at all,
do they? At first glance, they seem a bit more like plants, with a stalk connecting a
series of flowers with thin, elegant petals. But no, that is indeed an animal. One that
has the dubious honor of being defined largely by its anus.
Before we get there though, let’s start with just explaining what these animals actually
are. These organisms are members of the phylum Bryozoa, a name that translates in Greek to
“moss animals”. But Bryozoa aren’t actually animals that live in moss. They get their
name because of what they look like when all of the elegant individuals you see here congregate
into colonies that look, ya know, mossy. That giant gelatinous mass seems to bear
little resemblance to the tiny winsome creatures that make it up. But Bryozoa are notable in the
animal kingdom because of just how colonial the phyla is. Around the world, in waters salty and
not, are colonies of different shapes and sizes made up of individuals like these.
These colonies can get their start in different ways. Some might be the result
of sexual reproduction, sperm and egg that could have met in the waters away from their makers. Or,
in other cases, one colony may have captured sperm released by another, using it to fertilize its own
eggs, which stay brooding until a larva emerges, ready to set out and develop its own colony.
But Bryozoa have another asexual option for reproduction. Instead of fertilizing an egg,
a colony can amass a bunch of cells into a structure like this one, called a statoblast.
These statoblasts can be so prevalent in our samples that James, our master of microscopes,
has to scoop them out to keep them from being the main focus of our videos. They are thick
and durable, able to lay dormant and protect their contents from cold temperatures and dry
weather. But when the conditions become right again, they grow into a new individual that
will form the basis of a new colony. To do that, the individual Bryozoa,
called a zooid, navigates the waters as a larva, eventually attaching itself to
some kind of solid surface. Perhaps a piece of rock, or some kind of shelf, maybe even
some seaweed. And from there, it will begin to divide, budding off but keeping its descendants
close and connected as they increase in number. Now, the arrangement of these zooids dictates the
shape of their colony, as does the material they secrete to support the colony overall.
Some excrete a gelatinous material that turns them into those masses of jelly we saw earlier. Others
rely on chitinous materials that give them more rigidity. And as they spread over their surface
and even grow up into the water around them, Bryozoa colonies take on many spectacular shapes.
Even though Bryozoa are different from coral, they definitely look similar. And like coral,
Bryozoa colonies don’t move. Instead they remain attached to their surface.
But while they may not be able to move through the world around them to gather resources, Bryozoa
are able to make the most of their surroundings, with individual zooids taking on special functions
that allow the group to accomplish more than the individuals. Some focus on making eggs,
while others form small beaks that allow them to snap at predators. And these zooids are all
dependent on another group of zooids that gather food and share the nutrients with everyone else.
To get that food, the feeding zooids rely on the lophophore, the ring of tentacles crowning
every zooid’s bodies that looks a bit like a bunch of thin petals or a very elegant duster.
Those tentacles are covered in cilia, which circulate the waters around the Bryozoa,
stirring currents like the ones you see here so that it can sweep bits of food like plankton
towards the animal’s mouth. And from certain angles,
you can see how the lophophore and the rest of the zooid’s organs tuck into a sort of casing,
which is a structure called the cystid. And while the Bryozoa has various other
parts that we could talk about, what we’re really here for is the anus,
which is a much bigger deal than you might expect from an animal with such a simple
digestive system. There’s a mouth, an esophagus, and then a u-shaped gut that leads to the anus.
The issue is really rooted in the existential crisis at the heart of microcosmos: how are
things related to each other? And what does that mean for the names that we give to things?
Organisms like these were originally given the name Polyzoa in 1830,
but that was changed a year later to Bryozoa by the German naturalist Christian Ehrenberg.
But as more organisms were added to the group, the people watching them and describing them
realized that they were actually lumping together two very similar groups of organisms that actually
had some key differences between them. And one of those key differences was
where the organism’s anus was located. One group had their anus situated just outside
their lophophore—that tentacle crown we were watching gather food earlier. And one had their
anus situated just inside their lophophore. So to differentiate between the two groups,
scientists gave them very appropriate names. One group was called Ectoprocta, which translates in
Ancient Greek to “outside anus”. And the other were called Entoprocta, which means “inside
anus.” Because you can define anything by its butt, as long as you turn to ancient languages.
In general, Bryozoa typically refers to the Ectoprocts, though for many reasons (..I
can’t possibly imagine why…), Bryozoa seems to be the preferred name. But the history
and relationship between the Ectoprocts and Entoprocts remains an ongoing mystery
that scientists continue to explore. They’re not the only creature out there
to be wrapped up in a phylogenetic mystery. But isn’t it great to know,
especially for such a strange animal like this, that the mystery is built on butts?
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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