There’s a
chance that before you had an idea of what an amoeba was, you had an image of them in
your head, built on headlines describing invisible brain-eaters or pop culture references to
their simple, amorphous bodies. Tardigrades and hydras and stentors are magnificent,
but amoebas…they’re classic. When asked how he named his iconic California
record store Amoeba Music, co-owner Marc Weinstein said, “The word "amoeba" came up because
it sounded good with ‘music.’ We just wanted it to be a psychedelic kind
of name that students would relate to.” Record stores aren’t alone in finding familiarity
with amoebas. They show up often in writing, like a cultural
ambassador of the microcosmos that serves as a handy reference for anything that is
blobby, that splits, that is undeveloped. But as is the truth of many a muse, the reality
of amoebas is so much more complicated and weird than the flattened image of a formless
ancestor that we project our literary needs onto. The very idea of “amoeba” as a category
is a relic from the 19th century classification of single-celled eukaryotes, or protists,
which grouped the organisms into one of four types that seemed to largely correspond to
how they move: sporozoa, flagellates, ciliates, and amoeba. According to this classification, amoebas
are the ones with pseudopodia, extensions of the cytoplasm that help the amoeba eat
and move around. Amoeboid protists were largely grouped together
into a class or phylum called Sarcodina, a name derived from the word “sarcode,”
which was in turn a French version of the Greek word for “fleshy.” These classifications were built on the tools
of the time: namely, microscopes and what scientists could observe through them. And so the pseudopodia that defined amoebas
were also used to differentiate them from one another. Some of these pseudopodia are likely what
you would imagine, loose and spreading. But heliozoa are also amoebas, their bodies
extending outward in distinct, spiking, rays that are actually pseudopodia held rigid by
inner microtubules. And the bodies of amoebas, despite the collective
blob we often envision, show the same range. Some amoebas, called testate amoebas, actually
build shells around themselves, they’re single-celled constructors. These shells vary across species in how they’re
made and what they look like. This young Arcella is colorless, but as it
ages, the storage of iron and manganese in its shell will turn it brown. Unfortunately, while the shells of testate
amoeba can be stunning, they can also be difficult to record because the weight of the glass
coverslip on the slide can break those shells. In terms of size, amoebas can range from microns
to centimeters, making them sometimes visible to our naked eyes. The amoeba Pelomyxa is somewhere in between. But even though they usually measure in a
little under 1 millimeter, they’re still considered a “giant amoeba”. To get a sense of just how big a giant is
in the microcosmos, here’s a Pelomyxa…and here’s a tardigrade. Remember, tardigrades have thousands of cells,
and that Pelomyxa has just the one. Inside of that one giant cell is a body lacking
in many organelles we associate with eukaryotes: there is no mitochondria powering the cell,
no Golgi body moving things around. It does, however, house endosymbiotic bacteria,
and multiple nuclei. While some Pelomyxa might have only a few
nuclei, others have been found with several thousand. The simplicity of the Pelomyxa body was for
a long time the basis of the belief that it is primitive, predating the advent of those
other eukaryotic parts. The truth, as we will see later, gets murkier
as we try to piece together two billion years of eukaryotic history. Amoebas move in a way that is, very helpfully,
described as amoeboid locomotion, a combination of extending pseudopodia and altered shape. And as this single-celled creature makes its
way around in that particularly amoeboid way, you should know that this kind of movement
is going on inside of you right now. It’s driving your white blood cells around,
for example, as they monitor for any external threats. White blood cells share another amoeboid feature:
phagocytosis. Amoebas don’t have mouths, so when it comes
time to eat, they use their pseudopodia to trap their prey and engulf it, drawing the
food into its own compartment called a vacuole, where it will then be digested. And yes, some species will feed on nerve tissue,
including your brain, if the opportunity presents itself. But the meal of choice for many amoebas is
not necessarily so terrifying. This Vampyrella, for instance, sounds menacing
because of its name. And sure, when it wants to feed, the Vampyrella
pokes a hole into its prey and begins to suck out the contents. But its primary food is algae and sometimes
fungi, even the occasional microscopic worm. When they’re done eating, they form a cyst,
creating a wall around themselves as they digest their meal and then divide. Given how important pseudopodia are to the
way amoebas live, it’s easy to understand why they were used in our early attempts to
make sense of the microcosmos. But in more recent decades, as we’ve developed
the tools to investigate life based on DNA, our categorization of amoebas has changed
as well. No longer lumped together in the Sarcodina,
most amoeboid protists are now divided between two supergroups. One is Amoebozoa, which includes the Pelomyxa. The other group is Rhizaria, which includes
Heliozoa. But there are some lineages of amoebas that
weave their way out of these groups. As these changing evolutionary trees shift
our understanding of amoebas, they raise questions about just what we call primitive. Pelomyxa, for example, was considered to be
among the earliest eukaryotes because they lacked mitochondria. More recent evidence, however, indicates that
they’re actually the descendants of eukaryotes that had mitochondria. It’s just that for some reason, Pelomyxa
seems to have lost theirs. And if someone would like to do some research
to try and determine why Pelomyxa is better off without mitochondria, please do it, because
we are very curious. Even our understanding of how amoebas reproduce
has had to be corrected, our vision of a simple organism simply dividing is now complicated
by interwoven lineages that mix sexual and asexual reproduction. At the risk of getting too circular with our
references, our understanding of amoebas is…it’s well…a lot like amoebas themselves: shifting,
full of splits and divisions, and yet somehow encompassing. Amoebas are scattered now: left, right, up,
and down the evolutionary branches, sometimes as single cells, sometimes as parts of other
organisms, even us. It’s fitting then that we see amoebas as
useful touchstones or metaphors in books or music or wherever else they show up; our bodies,
after all, are built out of the lessons that nature learned with them. Thank you for coming on this journey with
us as we explore the unseen world that surrounds us. And thank you especially for all of our patrons
on Patreon who let us do this wonderful thing. We hope that you enjoy it as much as we do. If you want to see more from our Master of Microscopes, James Weiss. Check our Jam and Germs on Instagram. And if you want to see more from us, there's always a subscribe button somewhere nearby.
