Saturday, May 23, 2009

Porcupines of the Pond

This amazing creature is a heliozoan (literally ‘sun animal’), which is an amoeba that surrounds itself with a formidable battery of needle-sharp spines, radiating out from its core like sunbeams. Heliozoans are between a tenth and a quarter of a millimetre in diameter and drift through the water, slowly spinning. Each spine has a thin film of cytoplasm that flows up to its tip and them back down into the core, and any small food particles that the tip of the spine contacts are carried downwards on this conveyor and engulfed into the food vacuoles of the organism. The size of the core varies – normally it’s quite compact but it can expand by forming a mass of vacuoles (second and third photos down) and if it’s seriously stressed, like the heliozoan in the bottom picture, it flattens its spines and crawls across surfaces like a normal amoeba. These specimens came from water squeezed from moss on the edge of a pond. I used a microscope equipped with differential interference contract (DIC) optics to show the internal structure and needle-like spines. DIC optics generate a three dimensional image of transparent objects, which are otherwise difficult to see with conventional microscope optics.

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