Amoebae

 

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

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 Pisa University.