Notodendrodes antarctikos, is a single celled amoeba-like creature that lives in sediment on the floor of the Arctic Ocean. Some scientists believe that Foraminifera may represent a 'missing link' between naked cells and shelled protists (tiny tiny animal-like creatures). This particular Foraminifera is named because its shape is strongly convergent with a tree, it has root-like projections into the sediment which absorb dissolved nutrients, and intricate branches rising a few millimeters above the sea floor which capture suspended food such as algae. Its closest living relative is found in the tree form and the more 'traditional' sphere form.
I stumbled across these while watching "Encounters at the End of the World" which is available for instant view on Netflix. By far one of my favorite documentaries. In it I learned that N. antarctikos has been central to the debate on intelligence. These single cells apparently pick out only particular size grains in the building of their shells, which makes them eligible for all definitions of 'biological intelligence' proposed.
Darwin apparently had this to say:
"The case of the three species of protozoan (I forget the names) which apparently select differently sized grains of sand, etc., is almost the most wonderful fact I ever heard of. One cannot believe that they have mental power enough to do so, and how any structure or kind of viscidity can lead to this result passes all understanding."
The picture below (this is one cell...) comes from Dr. Sam Bowser (who was in the film) courtesy of Shawn Haper.