I wrote some posts about Ciliates, the protists I
studied directly for my research work. Then I wrote about Dinoflagellates and
Foraminifera. All these protists have a well defined shape and their genus,
often even species, can be identified by a simple observation at the light
microscope (obviously with a
well-trained eye).
Amoebae are
different. Indeed these protists are characterized by an
amazing variability in shape. For this reason the first specimens observed at microscope
were called “little Protei” remembering the multiform Greek god Proteus.
Amoebae belong to the class Sarcodina and lack specialized structures for locomotion and
sensation like cilia or flagella. Moreover their cell membrane is not
reinforced by cuticles or other structures. Their locomotion is trained by extending
and retracting pseudopodia, i.e. temporary extension of the protoplasm. In this
way Amoebae change continuously their shape. Pseudopodia act as locomotor organelles adhering to
the substrate and pulling the body itself. This locomotion is called indeed amoeboid.
Pseudopodia are the key feature of the organisms of Sarcodina.
Indeed also Foraminifera that are included in this class have pseudopodia but their are slim and elongate, to be let out trough the shell foramina and may split and rejoin each other. Moreover they contain a rigid internal structure. For this reason are called Actinopodia. Amoebae pseudopodia are instead various in shape and size, lack of internal rigid structures, and extend from every part of the cellular body. They are called rizopodia
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ameba rizopodia |

Foraminifera actinopodia
Amoebae are heterotrophic this means that to live and
reproduce they must eat.
They haven’t a cytostoma
like ciliates; they traps food
particles with the help of pseudopodia that encircle them. After that, the food
particle along with water is taken in and digested. This process is called
phagocytosis . .
Like other characteristics described in protists amoeboid motion and phagocytosis were maintained during evolution and utilized in specialized cells of pluricellular organisms, even in human beings!
For example in Macrophages, a type of white blood cell of our immune system, that engulf and digest pathogens, such as cancer cells, microbes, cellular debris.
Engulfment of bacteria by macrophages
Free living amoebae are very common: they live in the sea, in fresh water and in damp soil. Their size varies from a few micrometers to millimetres according to the species. They can be “naked” that is without any recovering structure or covered by a rigid shell consisting of different materials (Calcium, Silicium or a conglomeration of environmental debris). These are “thecate”
Arcella vulgaris tecamoeba
· Amoebae feed on bacteria, other protists and organic debris. They are primary or secondary consumers small in size that can be eaten by bigger consumers: thus they fit in the food chains.
Some amoeba species can be pathogenic, causing disease in humans and other organisms. I willingly leave the study of those species to parasitologists.
My expertise concerns only free living
protists. Those I treated in the course PROTIST ECOLOGY I held for many years
at
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