This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
I have a confession to make -- even though I work with ciliates at the moment, I have a bit of an unhealthy obsession with amoebae. I love them to the point that I get offended whenever anyone within earshot insults them as 'formless' or 'shapeless blobs'. Amoebae might be fairly squishy, but one cannot accuse them of lacking morphology: every group has its own way of moving about and its own shapes of pseudopodia, sometimes altering them between life stages, environments or just for fun. But given that one can, with practice, group amoebae into distinctive types based on form, they clearly must have some sort of it. The issue with amoebae is that their shape is highly dynamic, and difficult for our brains to analyse -- leading us to the delusion that it's absent altogether. What would be a particularly fascinating avenue for research is studying what it is about the cell biology of these amoebae that makes them exhibit squishy-ness in such different ways -- but alas, not only do we hardly know the ways of motility for classical Amoeba proteus and Dictyostelium, much of what is said about 'amoeboid motility' is actually based on mammalian tissue culture cells. Which, by the way, do qualify as amoebae -- but only as somewhat degenerate, non-free-living cousins. Most 'real' amoebae lack the proteins responsible for anchoring our cells -- integrins -- and thus must employ some other way to glue their way across the substrate. I'd be curious to know what it is, and how they differ -- and how the different proteins ultimately lead to some amoebae having fine, branching tubular pseudopodia (filopodia), while others have finger-like protrusions, spikey feet or move as one giant flat disk.
To begin with, this I believe is a Vampyrellid -- a group of voracious predatory amoebae belonging to the Rhizaria supergroup, which is one of the three major groups of amoebae. Other groups include the Amoebozoa (as the name may suggest) and Heteroloboseans in Excavata (of which the so-called "brain-eating" (misnomer alert) Naegleria fowlerii is a proud member). Amoebae or at least some amoeboid characteristics are found in almost all the other groups as well, and it appears that protists as a whole could care less about our yearnings to classify them as 'amoeba' or 'flagellate'. So in the centre of this vampyrellid we see a vesicular nucleus -- vaguely doughnut-shaped, but with a thick core where the hole should go (more like a danish, perhaps?). There is also a have digested diatom sitting around. Above the amoeba is a Stramenopile flagellate, perhaps Spumella sp. or something. Those too can be quite malleable and amoeboidy, despite being 'flagellates' to us.
Staying in Rhizaria, here is a short timelapse of a cercomonad (Cercomonas sp?), a common denizen of soil ecosystems, but here found in a pond sample. Note how stretchy and squishy it is, and how its thin pseudopodia (filopodia) tend to branch a bit. Cercomonads are really fun to watch, and have been reported to gang up on the much larger nematodes and kill them, seemingly for later grazing upon the bacteria that grow on their corpses (Bjørnlund & Rønn 2008). Microbes don't seem to care much for our 'food chain' concept either. (There's also a recent report of marine dinoflagellates ganging up on small crustaceans and devouring them!)
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The next amoeba I will stick somewhere between Rhizaria and Amoebozoa, which is roughly equivalent to be uncertain about whether something is a plant or an animal, as far as evolutionary distances go. Basically, this is a crazy skinny amoeba with lots of thin pseudopodia, some branching and fusing. And I'm serious about the skinny part -- it appears to be around a couple microns thick at most, and I've seen specimens over 100 microns wide. It's extremely dynamic, both in wiggling its pseudopodia but also in circulating the cytoplasm. The contents slosh about rather wildly.
Here's a glimpse of a different, much larger specimen -- which I am fairly confident should be the same thing, but may well be wrong. Note how the pseudopodia branch and fuse, giving it a rather off shape. The pseudopodia on the right are freshly retracted, giving them a thicker, bubblier appearance as there is a fair bit of extra membrane material there. The behaviour is a bit Rhizarian-like, but there is this somewhat annoying group of Amoebozoans, the Varioseans, that can basically do whatever they want -- including mimicking Rhizarians to make our lives harder. This amoeba might be Biomyxa sp. (Rhizarian), or Leptomyxa sp. (Variosean) -- probably can't say without sequence, or at least a healthy dose of experience.
Now you may wonder... where is it's, you know, stuff, given how thin it is. Surely, it must have some sort of nuclei and vacuoles and various other things a eukaryote must have. I think one can barely make out its nuclei, with a healthy dose of imagination: indicated here by arrowheads:
This amoeba appears to have another morphology, perhaps more common in nature -- as there aren't too many cover slips floating around in the wild to provide flat surfaces (water surface may work though -- the water side of the air-water interface is covered in numerous 'benthic' species, including amoebae, because life -- and physics -- is very different on the scale of microns). Here this amoeba (or something different altogether, for all I know) is seen gathering up clumps of diatoms and generally being thicker as it floats around in the water, as opposed to being stuck to glass. I hope it eats those diatoms.
By the way, this particular group of amoebae comes from a 5 month old sample from a Chicago pond (Jackson Park Lagoon) that was sitting around on my bench, amongst a metropolis of other seemingly-abandoned petri dish condominiums. Yes, I can pretend my bench mess has a purpose now -- take that, labmates!
Now for something more normal -- an amoebozoan amoeba that has gathered up a clump of empty diatom shells around like a test. This may be an arcellinid (group of amoebae known for building elaborate shells), but the way it moves would make it a rather awkward one. In any case, I think it's pretty!
A random small amoebozoan amoeba. Can't really say what this is without sequence data. I think you can barely make out a nucleus-like structure towards the right, between the clear cytoplasm (ectoplasm) and the more granular/filled stuff (endoplasm). If that is the nucleus, it's definitely in the endoplasm, for the record. There are tons of tiny amoebae crawling around in practically everything, and many of them are completely ignored because of their tendency to look like slide gunk. One of the most common marine amoebae, Parvamoeba, hasn't been described until the early 90's, even though it has probably been unknowingly seen by hundreds before that. It's so small and inconspicuous that most have probably dismissed it as slide gunk. I wonder if some of them might harbour substantially less-inconspicuous genomic and cellular madness, if only somebody looked...
And lastly, a gratuitous spirotrich ciliate, that has nothing to do with amoebae. Next to it is a small flagellate of the cryptomonad affinity -- Chilomonas sp. Its group is photosynthetic, but this one lost photosynthetic abilities and is left with colourless plastids. The ciliate has a couple macronuclei paired with micronuclei, the latter being actually visible as clear roundish things in the top left image. Note that this poor ciliate is a bit squished, and it would normally be less blobby in shape.