Pelobiont

The pelobionts are a small group of amoebae that lack mitochondria. The most notable member is Pelomyxa palustris, called the giant amoeba because of its size: usually 500-800 μm, but occasionally passing 3 mm in length. Several other species 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. They 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.

Because it lacks mitochondria, dictyosomess, and flagella, Pelomyxa has been considered one of the most primitive living eukaryotes, and is sometimes given its own phylum (called Caryoblastea). However, the monoflagellate amoebae Mastigamoeba, Mastigella, and Mastigina are now considered pelobionts as well, and the absence of mitochondria is secondary. The entamoebae, parasitic amoeba which also lack mitochondria, are close relatives of the pelobionts, and the two may be derived from the Amoebozoa.