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I can't understand what multicellularity is. Wikipedia states that any organism having many cells is multicellular. By this definition bacteria can also be multicellular. For example, cyanobacteria can form filaments made up of specialised cells like spore cells and vegetative cells.

But aren't bacteria supposed to be unicellular only? I would appreciate it if someone could clear up my confusion.

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Ooh, good question. Bacteria are, as a general rule, unicellular. However, there are some, like the cyanobacteria which you referenced, which are kind of border line. Wikipedia's definition of multicellularity is a very broad sweeping definition, and its statement that bacteria are unicellular is also a broad sweeping statement.

The fact is, that as a general rule, bacteria are unicellular, but at times, like you said, they can undergo filamentation. When they do, they end up with several connected cells which are generally considered functionally independent. In certain circumstances, however, the cells can be dependent on each other for survival, and because they didn't separate completely, it's hard to define whether we're looking at multicellularity or symbiosis.

It is generally accepted that cyaonbacteria are truly multicellular. In order to be multicellular, an organism generally has to fulfill 5 basic requirements:

  1. More than one cell
  2. Cells stick together
  3. Cells communicate
  4. Cells are dependent on each other
  5. Cells are differentiated

By these requirements, some cyaonbacteria technically are multicellular bacteria, defying frequent claims that all bacteria are unicellular. That said, wikipedia isn't totally wrong, since as a general rule, bacteria are unicellular. Cyanobacteria are an odd (debatably) exception to the rule.


Sources:

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    $\begingroup$ At least on the page "multicellular organism" Wikipedia does not claim that all bacteria are unicellular in contrast, wikipedia specifically mentions cyanobacteria as being multicellular. $\endgroup$ – Thawn Mar 14 '18 at 12:36
  • $\begingroup$ Yes, fair enough. I just wanted to go into why the definition at the top didn't apply for cyanobacteria. $\endgroup$ – rotaredom Mar 14 '18 at 12:38
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    $\begingroup$ Another species near that border line is the trichoplax that has differentiated cells but can survive being split up into single cells, and can reassemble $\endgroup$ – trichoplax Mar 15 '18 at 1:07
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The Wikipedia article you mention actually gives cyanobacteria as an example of a multicellular organisms. So yes, there are some (primitive as they may be) multicellular species in the bacterial domain. This should not come as a surprise since a domain in biology is the highest taxonomic rank meaning that it encompasses a huge variety of different species. However, the misconception that all bacteria are single-celled is quite common because it is true for the vast majority of bacterial species.

Furthermore, it is quite common in evolution that similar traits evolve separately (for example flight evolved independently in bats and birds and the claw of the mole and the mole cricket look similar but evolved independently) this mechanism is called convergent evolution. In fact, multicellularity evolved independently at least 46 times in eukaryotes.

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