Monday, February 22, 2010

A Living Jewel

This exquisite object, nature's equivalent of a FabergĂ© 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...

Wednesday, February 17, 2010

Cyclops with Hitch-hikers

This little crustacean, about a millimetre long, is Cyclops, with a single red eye-spot in the centre of its head - a minute freshwater counterpart of the monocular monster of Greek mythology. My garden pond is swarming with them at present, even though the ice has barely thawed. If you take a close look at the top end of the tail, near the body, you can just make out clusters of short-stalked objects attached to the animal's exoskeleton. At higher magnification these turn out to be....

.......... Vorticellids - single celled protists with beating cilia around their mouth, creating a whirlpool current that sucks in foot particles. You can see a movie of Vorticella in action here. At even higher magnification.......... can see their cilia and the contractile vacuoles that they use to expel waste (double-click for a larger image). Vorticellids attach themselves to all sorts of small pond animals, hitching a ride.

As Jonathan Swift (1677-1745)  noted,

So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite ’em;
And so proceed ad infinitum.