Pelobiont

The pelobionts (Caryoblastea) are a small group of amoeboid protists lacking both mitochondria and dictyosomes. The most notable member is Pelomyxa pelustris, called the giant amoeba because of its size: usually 500-800 μm, but occasionally passing 3 mm in length. Several other species of Pelomyxa have been described but they may be synonyms.

Pelomyxa are found in the mud at the bottom of freshwater streams. A moving cell is cylindrical in shape, with a single hemispherical pseudopod at the front and a semipermanent bulb called the uroid at the back, which is usually covered in thin non-motile extensions. The cytoplasm streams forward through the center of the organism and back along the outside, allowing the creature to slide along the substratum. There are anywhere from two to several hundred nuclei, which undergo mitosis independently of cell division. Pelomyxa are not picky eaters, and are full of vacuoles containing whatever food they happened across along with sand and other debris. Symbiotic bacteria take the place of mitochondria.

The other pelobionts are monoflagellate amoebae, which share various ultrastructural characteristics with Pelomyxa. These comprise the genera Mastigamoeba, Mastigella, and Mastigina. The group does not appear to have any other especially close relatives, and probably diverged from the other eukaryotes early on.