Individual cells have only cellular level organization, well obviously. Sponges are multicellular organisms, that, by the book anyway, have only cellular level organization. However, when we encounter them on the rock or underneath the dock, they are not single cells. How is it that we consider them not to have tissue level organization, or higher?
There is experiment that can be done with sponges that indicates this, that is, why we think of them as not having tissues. A sponge can be shifted through a mesh. The organism’s cells, those that survive the mesh, aggregate and form a new sponge. The structure of the whole sponge is now different, usually flatter, but a living sponge goes on. Of course you can destroy too many cells when pushing it through the mesh and so the new sponge may be a shadow of its former self. This process, through the mesh, cannot be done with tissues. A jelly (jellyfish if you prefer) has tissue level organization. If you pushed a jelly through the mesh you would end up with, well, a pile of jelly, who’s cells are, or soon will be, dead.
What a beautiful creature this dangerous Man of War is: Look, but don't touch, those tentacles.
Can we devise a test of this colonialism similar to the sponge test? We don’t want to push it through a mesh screen, that would certainly kill it. How about we divide zooids more carefully and see it they either come back together or, this would be cool, become a few separate, surviving, organisms. This would be a a form of asexual reproduction.
This is the way. This can be done with another colonial organism group, the Bryozoans:
Cut the colony in half and you have two bryozoan colonies.This group of organisms has some other aspects that point to its colonialism; it has some genera (just a few) that are single zooids.
So corals and bryozoans fit the bill, they are colonial. Can this be done with Physalia physali, the Portuguese man of war? The zooids or polyps if you wish, are more specialized in the man of war; some act as floats, some feed, some have tentacles (defense and sensing), and some produce offspring. After it is specialized, during development, an individual zooid probably cannot survive on its own. How about cutting the whole colony in two so that each side has plenty of specialized zooids?
I cannot find an answer. Anyone?
Let me back up just a little. How do we distinguish an organism with great regenerative abilities from a colonial organism? For example, sea stars can be cut in half and two sea stars result. There needs to be enough nerves structure in each half for this regeneration to take place. Thus, if you cut off one sea star arm, the arm will die, but the rest of the seastar can grow a new arm. And, wait for it, some jellys are able to do this, if cut in sections, separate jellys can result. This is likely due to there non-specialized makeup. But can the man of war, with its specialized zooids do this? I doubt it, but
I cannot find an answer. Anyone?
This makes the man of war of interesting to study. It’s somewhere between an organism with tissue level organization and organ level specialization. It doesn’t quite have organs though, but many specialized zooids that do these functions. Or does it have lots organs? Each gastrozooid in this case being a digestive system.
Here is my conclusion based on my skimpy science, Physalia physalis, the Portuguese man of war, is not colonial but instead has many tiny organs scattered throughout its body. The precursor, if you will, to full organ systems.
Note:
Bryozoan genera that are solitary (Loxocorone, Loxosomella, Aethozooides and Monobryozoon).
Sources and Further Readings:
Dunn CW. Siphonophores.
http://www.siphonophores.org/SiphOrganization.php
Mackie, GO, Pugh, PR, Purcell JE. 1988. Siphonophore Biology. Advances in Marine Biology 24: 97-262.
Munro C, Siebert S, Zapata F, Howison M, Damian-Serrano A, Church. SH, Goetz FE, Pugh PR, Haddock SHD, Dunn CW. 2018. Improved phylogenetic resolution within Siphonophora (Cnidaria) with implications for trait evolution. Molecular Phylogenetics and Evolution 127: 823-833. ISSN 1055-7903.
Munro C, Vue Z, Behringer RR, Dunn CW. 2019. Morphology and development of the portuguese man of war, Physalia physalis. Nature Scientific Reports 9: 15522.