Swallowed by the Sun Animacule

I've long been fascinated by heliozoans aka 'sun animacules' and have posted on them before, but thought I would again because I've captured some pictures of one without squashing it under a coverslip - and also photographed it digesting its last meal. Briefly, heliozoans are single-celled amoebae which, instead of flowing around and engulfing their food with cytoplasmic arms called pseudopodia, radiate spines of cytoplasm supported by an internal scaffold of microtubules. They roll through the water like a one-tenth-of-a-millimetre-in-diameter sea mine, bringing dead to anything small that contacts those spines. This is a through-focus sequence of this microscopic protist menace, so in the image above we are looking down on the upper surface, onto what looks like a sphere of cytoplasm radiating needles in every dimension... 

.... and here the focus has moved down a bit, onto the surface of that cytoplasm, which is broadly divided up into a pattern of hexagonal blisters....

.... and now we are looking at the contents of its central food vacuole - and this heliozoan has somehow managed to ingest a testate rhizopod - a form of amoeba that lives in a shell shaped like a hot air balloon. Incidentally, all the pictures on this post will be clearer if you double-click on them to enlarge them in a separate window. Below is the same sequence, but this time at x400 magnification instead of x100......

..... so here are the surface hexagons, with the base of those lethal radiating spines....

..... and here is the blistered pattern - which looks like cells but this is a single-celled protist - so they must be formed with the aid of microtubules, I guess ......

.... and here's its food vacuole, with ingested testate rhizopod. It will digest the cytoplasm of its prey and spit out the empty shell.

It's easy to loose a sense of scale when peering down the microscope at small organisms - a sense of their place in the universe. So the picture above shows the place where this 100 micron predator came from - from the moss around the edge of this small pool on bleak moorland in Weardale, County Durham . The pool is about a metre long, so if you lined up 10,000 heliozoans spine-tip-to-spine-tip they'd stretch from one end to the other ....

..... and that tiny pool is somewhere in the middle distance in this landscape ......... and beyond that the solar system and beyond that the universe, home to an exquisite tiny life form that has been rolling through our planet's water, ingesting whatever it touches, for well over a billion years. Time travel is possible if you have a microscope....

Microscopic Living Do-nut

Here's another interesting organism that lives in the temporary pools of water, known as phytotelmata, that accumulate in cavities in the surface roots of beech trees. It's a testate rhizopod - an amoeba that lives in a shell that it secretes - and is called Arcella. It's about a tenth of a millimetre in diameter. The shell in old specimens tends to be brown but this one is nicely translucent, revealing the exquisite sculptured pattern that decorates its surface.

Here's the flip-side, showing the hole in the underside of the hollow do-nut shaped shell and...

... here, with a shift in focus, you can see the amoeba inside, with one arm or pseudopodium protruding out of the hole. The dark structure that you can see in the cytoplasm is a vacuole filled with carbon dioxide - the organism produces one of these to increase its buoyancy when it needs to float up to the surface.

Here you can see the whole organism inside the shell in side view, poised above the hole. The circular structure in the cytoplasm, top left at about 10 o'clock, is a nucleus.... the control centre of the cell.

There are always two nuclei in the cytoplasm of each Arcella.... so which one is in control? Who knows?

And finally, here it is on the move, extending a pseudopodium that it uses for locomotion and food capture.

This is probably Arcella discoides. If you'd like to see some wonderful illustrations of more testate rhizopods take a look at this page, where you'll need to double-click on the images to enlarge them.

A Living Jewel

This exquisite object, nature's equivalent of a Fabergé egg but only about one tenth of a millimetre long, is a testate rhizopod - a species of amoeba that lives inside a balloon-shaped shell. Testate rhizopods either secrete their shells or they cement minute sand grains together to create one. When you think about it, that's a remarkable feat of construction for one of the lowest forms of life that, superficially, is little more than a slithering blob of cytoplasm. You can see some more examples here. Testate rhizopds that assemble a shell from sand grains are often assigned to the genus Difflugia and scores of 'species' have been described, based on the components and construction of their shell, although it's not clear to what extent these are really distinct 'species'. You can download a guide to identification and where to find them here. I found this specimen when I was screening a sample of water from amongst the waterweeds on the edge of a pond in Durham. I have to admit that the image above has involved a bit of optical trickery because......... 

... this is what I saw when I first examined the organism under normal bright field microscopy, revealing the translucent quartz grains that formed its case. Switching to dark field microscopy....

... where the image is formed from light diffracted by the translucent grains showed them in a new light. But it was only when I switched to polarised light microscopy, which reveals the interference colours formed by the birefringent grains, that the ultimate beauty of this tiny organism's case was revealed.

Jewellery on a microscopic scale...

Testate Rhizopods

Amoeba come in a wide variety of shapes and sizes. The bog-standard, free-range amoeba glides wherever its pseudopodia take it, going with the flow of its protoplasm (see movie). Heliozoans – amoeba with spines (see earlier post) – roll trough the water. A third form, testate rhizopods pictured here, secrete a glassy shell for themselves, that reminds me of a hot-air balloon or a Greek amphora, and send their pseudopodium ('false feet') out from the opening to engulf anything that’s smaller than themslves and within reach. The upper testate rhizopod here is alive, with an amoeba inside the shell. The yellowish objects within are partially digested small algae or diatoms that it’s grabbed. The image below is of an empty testa, this one with spines and showing the surface patterning on its glassy surface. Different species can be recognised by their testa pattern. They are both about one tenth of a millimetre long and were photographed with a simple microscope, without any fancy optical tricks. They came from a boggy spring in Teesdale.