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I read about the new discovered creature call Myxozoa and it say that Myxozoa is animal because of it possess cnidocysts like other cnidarian. But why doesn't it is a converge evolution ?

Myxozoa doesn't fit one of the most important characteristic of animal kingdom, "Multicellular". I know that all multicellular creatures aren't animals but all animals are multicellular until I read about Myxozoa.

Characteristic is like a rule without even a single exception for any specific group. Something that most of the group have in common, for example, "true tissue"(except sponge), "has limit life span" (except immortal jelly fish) etc. are important property that most of them have in common but not characteristic since some of them don't have.

I know it is hard to find any thing that all animals have in common without even a single exception for any specific group but it is important. We do need a check list to check if any creature is animal or not and to separate it from other eukaryote.

Here is my check list for animals which is from many source and my knowledge of all animal group which I try to separate animal from other eukaryote.

  • Eukaryotic
  • Multicellular in all part of the life cycle except zygote state
  • Heterotroph (No chloroplast but can be symbiosis with other creatures that have chloroplast.)
  • Motile (relocate entire body) in at least one part of the life cycle
  • No cell wall
  • Aerobic respiration (has mitochondria)
  • Has tissue - Has basic tissue (like sponge) or true tissue
  • Have at least 5 different type of cell that do different job in at least one part of the life cycle (Sponges, comb jellies, Placozoa, Cnidaria and Xenacoelomorpha have more than 5 different type of cell. All more advanced bilateria animal obviously have more than 5 different type of cell. All Choanoflagellate have less than 5 different type of cell.) and at least 5 different type of cell that can communicate with each other.

I know I might be wrong but we really a check list to separate animal from other eukaryote. This is the best check list that I can make.

Myxozoa will not fit three of these list, "Multicellular", "Have at least 5 different type of cell that do different job in at least one part of the life cycle" and "Aerobic respiration" (A Myxozoa species call Henneguya zschokkei doesn't have mitochondria.). And if these characteristic is removed, we will not unable to separate all other eukaryote from animal.

Myxozoa should be another protist with its own group but not animal. I think it can possess cnidocysts due to the converge evolution or obtain cnidaria cnidocysts gene through horizontal gene transfer in some way in hundreds million years ago.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – AliceD ♦
    Sep 2, 2022 at 11:03
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    $\begingroup$ Using the traditional taxonomic approach that contain the familiar kingdoms rank (e.g.animalia), has grown increasingly unpopular due to known inconsistencies in relatedness of lumped species when observed from a genetic and then phylogenetic approach. $\endgroup$
    – theforestecologist ♦
    Sep 2, 2022 at 11:08
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    $\begingroup$ A cladistics approach (using Phylogenetic nomenclature) lacks categorical ranks such as "kingdoms" & instead uses hierarchical group names based on nested ancestral synapomorphies. Generally ~6 informal supergroups (sometimes called "suprakingdoms") have been recognized based on phylogenetic research. 1 such group (i.e., clade) is the Opisthokonts that include animals + fungi. Not so simple anymore... $\endgroup$
    – theforestecologist ♦
    Sep 2, 2022 at 11:09
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    $\begingroup$ @theforestecologist please continue in chat. $\endgroup$
    – AliceD ♦
    Sep 2, 2022 at 12:08

1 Answer 1

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First multicellular is not always a defining characteristic of animalia, protozoa used to be included in animalia.

Second you are falling for a issue a lot of young biology students run into, Taxon separations are subjective historical artifacts, and don't really make a lot of sense. Taxon do not need to be monophyletic although it is encouraged. Taxonomic rank is becoming more a way of organizing names and less a reflection of evolutionary relationship. This is one of many problems with trying to keep archaic names and taxonomic approaches, it does not fit well with the current understandings of biology and the system is creaking under the weight of historical baggage.

This is one of many reasons cladistic approaches to taxonomy is being used more and more, it is at least internally consistent and reflects the complexity of life.

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  • $\begingroup$ Yes, the protozoa and very small used to be call animalcule. But once we know more about them, we can separate the protozoa from the animal and get the better characteristic of animalia. I thought that the more knowledge we have, the better characteristic we get. Apparently, I am wrong. It is weird that at some point, the more we know about animal, the fuzzier the boundary between animal and protist is. And at some point, the characteristic doesn't work any more. $\endgroup$
    – Ro Theory
    Sep 3, 2022 at 7:49
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    $\begingroup$ @NawatNakhon actually that makes complete sense when you think about evolutionary history, as you go back in time the line between every group is eventually just the fuzzy line between two closely related species. At one time protists and animals actually WERE the same thing they had not diverged yet. $\endgroup$
    – John
    Sep 3, 2022 at 12:20

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