Three Phyla of Sarcodina
These protists may seem foreign at first, but they are fairly familiar under another name, amoebas. They are similar to their next of kin being Actinopoda and Foraminifera, but have three distinct differences. First they have no form of meiosis or sexual reproduction, but do reproduce mitotically. Second, they have an absence of a siliceous/carbonate shell or exoskeleton. Third, their physical appearance is ever changing as cytoplasmic pseudopods (discussed earlier) appear and disappear as the amoeba moves.
Actinopoda (aka Radiolarians):
Actinopoda are very similar to Foraminifera (some researchers feel that the Actinopoda classification should not exist due to its high DNA proximity with Foraminifera, and should be considered a small subset of the foram group (15)). They are distinct in that they have the formation of an exoskeleton through silica secretions. These exoskeletons are fairly tough and serve as protection against environmental stressors. Actinopoda often have radial symmetry and look quite different from amoebas. They look like stars due to the many pseudopods that form around microtubule projections around the cell membranes circumference.
Forams for short, these protists more likely resemble Actinopoda than Rhizopoda. This is because Foraminifera also have exoskeletons, often called tests, which contribute to their definite shapes. However, these exoskeletons are primarily made of calcium carbonate rather than silica. Also, forams have many large and highly motile pseudopods that aid in movement through aquatic environments and the uptake of food. Unlike amoebas and Actinopoda, Foraminifera have a unique life cycle in the form of alternation of generation. In this life cycle the organism oscillates between haploid (1n) and diploid (2n) stages (2). There are two main types of forams: planktic and benthic. Planktic forams usually reside in upper zones of the ocean and are adapted to more temperate conditions. Benthic forams, in contrast, live on the ocean floor and are better suited to colder and darker waters (10). They thrive and survive from depths of 163 meters as far as 2907 meters (16).
*It is interesting to note that some planktic forams have been observed expressing benthic characteristics (e.g. crawling and burrowing) and that these individuals are a “hybrid” of the two (18).
All protists share one common aspect: they are not plants, fungi, or animal. Also mostly all protists are unicellular with some preferring to live in colonies. Sarcodina further share with other protists the ability to form spores and be heterotrophic (receive energy from chemical break down). Other than that, they vary greatly from one another. One of the most apparent differences between sarcondina and the rest of the protist family is that they have an absence of a permanent locomotor apparatus (e.g. flagellum, cilia). Further, Sarcodina are food based heterotrophs while some protists are photosynthetic autotrophs (4). Additionally, Sarcodina differ amongst themselves in structure and means of reproduction as well as with other protist phyla. They vary in shape, exoskeleton presence/absence, life cycles, and reproductive process such mitotic fission and various meiosis forms. To put it bluntly, protists are a conglomeration of unclassifiable organism.