In the enormous tree of the evolution of life on earth there are very diverse beings. Among animals, the group considered to be the most primitive is that of sponges, called Porifera, whose fossil record goes back at least 760 million years ago. Traditionally, it has been considered that the simplest organisms are the most primitive, although there are cases that show that this is not always true; the pentastomidsfor example, are a group of extraordinarily simple crustaceans, which evolved from much more complex organisms.
So although sponges are extraordinarily simple, they are not necessarily the simplest. And in fact, they are not. The simplest known group of animals is the placozoos.
Placozoo (‘Trichoplax aherens’) under an optical microscope. – (Eitel et al., 2013)
The discovery of placozoans
The first placozoan It was discovered by the German zoologist Franz Eilhard Schulze in 1883; The curious thing is that it was not discovered in the natural environment, but attached to the inside of the glass of a seawater aquarium at the Zoological Institute of Graz, in Austria. This new species received the name Adherent trichoplax.
One of the first analyzes of this new organism was carried out by the zoologist Thilo Krumbach, who in 1907 hypothesized that this small organism was actually a a larval form of a group of cnidarians known as hidrozoos. Although this hypothesis was rejected by Schulze, it remained true for decades.
But in 1972, a reappraisal of scientific knowledge in force, carried out by Karl G. Grell, allowed us to discover that, in reality, Adherent trichoplax it was a very different being and was cataloged in a new phylum of animals, separated from the cnidarians; a phylum composed exclusively of a single species, named as Placozoa.
Currently, three more species are known: Hoilungia hongkongensisdiscovered in 2018, Mediterranean polyplacotomaof 2019, and Cladtertia collaboinventain 2022. In addition, there are samples whose species has not yet been determined, and it would not be uncommon for new species of placozoans to be identified in the coming years.
‘Polyplacotoma mediterranea’ instead of disc-shaped, it has a more branched shape. – Hans-Jurgen Osigus
anatomy of a placozoan
The reason why placozoans are considered the simplest animals known It’s in your anatomy. A placozoan is a flattened organism, more or less circular in shape, with a diameter close to 1 mm, and a thickness of 25 µm —25 thousandths of a millimeter. Your body is made up of a single embryonic layer of cells. ciliated epithelial, which folds on itself, and extends over the upper and lower face of the organism. Inside, an aqueous medium presents free stellate cells of the mesenchymal type. It is, therefore, an organism even simpler than sponges, which have two cell layers, in addition to the mesenchyme, —one outer and one inner.
The placozoan only has six cell types. —a sponge has eight—, it lacks differentiated tissues, and of course, organs. It lacks overt body symmetry. It looks more like a multicellular version of an amoeba..
The animal strongly adheres to surfaces, thanks to the cilia on its underside, and moves through them by moving these structures. It’s a detritus and predator of algae; It is placed on the food, and ingests it by phagocytosis. Reproduction is eminently asexual, by budding or by fission: clones sprout from the adult that break off, or fragment into several new individuals. However, it has been proven that they can develop sexual reproduction under certain conditions.
Placozoo (‘Trichoplax adherent’) reproduces by fission. – Hans-Jürgen Osigus
Origin of placozoans
From its consideration as an independent phylum, the evolutionary origin of the placozoans, and therefore their position in the evolutionary tree of animals, has been in dispute.
Initially, and given its simplicity, it was considered that placozoans should be at the evolutionary basis of animals, with an origin prior to that of sponges. They were also considered as heirs of the Ediacaran fauna.
However, the genetic analysis recent ones have changed that perspective. According to the results of a study carried out by a research team led by Gonzalo Giribet, from Harvard University, Krumbach’s vision of 1907 was not misguided. Placozoans are not cnidarian larvae, but this group, which includes corals, jellyfish, and anemones, is the closest evolutionary relative to placozoans..
The simplicity of these animals, in reality, does not lie in how primitive they are, but it is about a secondary simplification, product of the evolutionary process from more complex animals.
References:
- Eitel, M. et al. 2013. Global Diversity of the Placozoa. PLOS ONE, 8(4), e57131. DOI: 10.1371/journal.pone.0057131
- Grell, KG 1972. Egg formation and cleavage of Trichoplax adhaerens FE Schulze (Placozoa). Journal of Animal Morphology, 73(4), 297-314. DOI: 10.1007/BF00391925
- Laumer, C. E. et al. 2019. Revisiting metazoan phylogeny with genomic sampling of all phyla. Proceedings of the Royal Society B. DOI: 10.1098/rspb.2019.0831
- Tessler, M. et al. 2022. Phylogenomics and the first higher taxonomy of Placozoa, an ancient and enigmatic animal phylum. Frontiers in Ecology and Evolution, 10.
- Ueda, T. et al. 1999. Dynamic patterns in the locomotion and feeding behaviors by the placozoan Trichoplax adhaerence. Biosystems, 54(1), 65-70. DOI: 10.1016/S0303-2647(99)00066-0
