Wednesday, March 2, 2011

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.


  1. Nice shots, Phil, especially the pseudopodium which is so delicate.

  2. Arcellinids are awesome – nice pictures too! A cool thing about them: when dividing, Arcella makes a gas bubble that props up the shell to be vertical, and then splits. As for which nucleus is in control, I'm guessing both, just like in diplomonads (eg. giardia). If they do play different roles (also quite possible, though they look similar), could be a cool case of evolving nuclear dimorphism. Also, apparently Arcella can fuse pseudopodia into a big plasmodium with multiple shells on it. Not sure if anyone knows why it would do that...

  3. Beautifully clear illustrations Phil. A fascinating life form.

  4. Wow. Surely this is the snail of the protist world. All it lacks are retractable eye stalks . . .

  5. It's interesting watching them glide around on the slide with the pseudopodia Wilma - wish I'd shot some video too.

  6. Thanks for the insights Psi Wavefunction - I could get really interesting in these organisms (along with thousands of other protists - for my money evolutions most interesting products)

  7. Hi Jogn, I think it's the most transparent one I've found - mostly they seem to be dark colours, concealing the owner within

  8. Interesting thought Jennifer. Now that's an interesting thought - a perfectly radially symmetrical gastropod that could swivel within the shell through 360 degrees on its axis and head off in any direction.

  9. hai... i want to ask... this organism not phytoplnakton right? because i also found it in my sample. well, im doing my master study on phytoplankton on wetland area, so i found this organism too, i can't identified it until i found that picture from this blog...

    1. No, it's not phytoplankton, it's a relative of Amoeba and is member of the phylum Protista. Good luck wiith your masters...


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