Amoebas are the main representatives of sarcodynes and their name derives from the Greek word amoibe, which means “modification”, as their forms are constantly changing. A typical example is provided by Amoeba proteus, whose cell reaches 0.5 mm in diameter.
As already mentioned, the locomotion of the amoebas occurs through the emission of pseudopods (“false feet”), which are also used to capture food. Amoebas feed by ingesting small protozoa and algae, as well as dead protoplasm. When they perceive the presence of food, they move towards it, encompassing it with pseudopods. This process of ingesting food is called phagocytosis.
Food is incorporated by the cell, together with a little water, and remains inside a food vacuole (phagosome). This receives digestive enzymes stored in organelles called lysosomes, becoming called the digestive vacuole, where the food will be digested. Due to cytoplasmic currents, the digestive vacuole is moved inside the cell, distributing the digested food. The unusable remains of digestion are stored in the vacuole, which is now called the residual vacuole, until they are eliminated from the cell through clasmocytosis or “cell defecation” (the elimination of slag).
Freshwater protozoan cells are hypertonic with respect to the external environment. In this case, water enters the cell by osmosis. As the difference between the concentration of cell juice and fresh water is sufficient to saturate the cell and break it down, amoebas and other freshwater protozoa have developed cytoplasmic organelles called pulsatile or contractile vacuoles that, from time to time, eliminate the excess of water that penetrates the cell.
In saltwater protozoa there are usually no pulsatile vacuoles, as the concentration of the external medium is similar to that of the cell cytoplasm.
The most common type of reproduction among sarcodynes is binary division. In this type of asexual reproduction, the cell divides in half, giving rise to two daughter cells with the same genetic information as the mother cell.
The most common representatives of sarcodynes belong to two main groups of organisms: the amoeba and the foraminifera.
The group of amoebae includes free-living species, there are some that have a carapace covering the outside of the cell: these are called thecamebas. Thecamebas occur in fresh water and moist soils and their shells can be secreted by the cell itself or formed by small particles that coalesce around its cells.
Among the species that can occur in the human body, some are parasites, others, however, are not. This is the case of Entamoeba gingivalis, which occurs in the gums, and Entamoeba coli, which occurs in the intestine.
The foraminifera are mainly marine sarcodines, which have a secreted or agglutinated carapace, formed by several chambers. The carapace can be formed from calcium carbonate secreted by the body or from grains of sand that coalesce around the cell. The shape of the carapace varies a lot from species to species, and many resemble small snails. Foramina are very important in research on oil prospecting, as they constitute a group with many fossil representatives, good indicators of the presence of fossil fuel.
Parasitic Sarcodines of Man
Among the sarcodynes, there are parasitic species only in the amoeba group, with two that parasitize the human intestine: Entamoeba histolytica and Endolimax nana.
Entamoeba histolytica causes amoebic dysentery (amoebiasis), characterized by mucous diarrhea and bloody stools.
Individuals of this species have the ability to encyst, being released in this form by man’s feces. These cysts are resistant, allowing the parasite to remain outside the host’s body for long periods. When eating poorly washed fruits or vegetables or drinking contaminated water, man can ingest these cysts, which then break up, releasing the protozoa and which settle in the intestinal mucosa causing lesions. The prophylaxis of this disease must be done through basic sanitation and hygiene measures.
Endolimax nana is an intestinal parasite that causes abdominal pain, diarrhea, vomiting and fatigue. The parasite is acquired by eating contaminated food.
Mercer, E. H. “An electron microscopic study of Amoeba proteus.” Proceedings of the Royal Society of London. Series B-Biological Sciences 150, no. 939 (1959): 216-232.
Hardoim, E. L., & Heckman, C. W. (1996). The Seasonal Succession of Biotic Communities in Wetlands of the Tropical Wet‐and‐Dry Climatic Zone: IV. The Free‐Living Sarcodines and Ciliates of the Pantanal of Mato Grosso, Brazil. Internationale Revue der Gesamten Hydrobiologie und Hydrographie, 81(3), 367-384.