The Shelled Amoeba - Arcella sp.

Without a reference for size in the picture, some of our viewers may have wondered if this Unknown was left over from last weeks discarded soup. This organism, a type of amoeba, is about 100 micrometers across. Although somewhat blobish in nature, the amoebas are a remarkable group of single-cell protists with flexible outer membranes that allow them to surge forward by forcing their cytoplasm into an extension of the cell membrane called a pseudopod -- a type of movement called amoeboid movement.

But a naked amoeba cell is extremely vulnerable to predation by other small creatures. Natural selection has favoured the evolution of a variety of shell adaptations in some groups of amoebas. In Diflugia, the shell is made of tiny sand grains cemented together in a kind of miniature upside-down vase. In Arcella, the shell is constructed from a protein-sugar compound called chitin.

To learn more about testate amoebas, see our program, Branches on the Tree of Life: Protists