For the first time, researchers have systematically classified an entire animal phylum: Using genetic and molecular analysis, they have created a complete taxonomy of the phylum Placozoa. Since these disc-like creatures, the size of a grain of sand, do not have a fixed shape and the species hardly differ externally, it was not possible for scientists to identify different species based on external characteristics. The new analysis now uncovers hidden relationships and facilitates scientific exchange about the various types of primeval multicellular organisms.
Placozoa are tiny, blob-like creatures with no organs and few cell types. They live mainly in tropical and subtropical waters and can move using hair-like structures. In 1883, the first Placozoa species was described - and remained the only species in the entire animal phylum for more than 100 years. The reason for this: their external appearance is so bulky and changeable that it is not suitable for classification. "For a taxonomist looking through a microscope, even a powerful one, there are almost no features to compare and distinguish between them," explains Michael Tessler of St. Francis College in Brooklyn in New York. It was only in 2010 that scientists using DNA analyzes proved that there are very different lineages of these beings at the genetic level.
Full taxonomy
Tessla and his team have now carried out a systematic classification of the Placozoa for the first time. Instead of relying on physical features, they relied on differences in DNA sequences and other molecular features - a technique called molecular morphology. "We wanted to know how this ancient group of animals is related and where they are in the tree of life," says co-author Johannes Neumann of the American Museum of Natural History in New York. "This has been speculated about for decades, but now that we examine the differences between the placozoans at the molecular level, we can paint a clear picture of how these animals are related."
The researchers created a baseline taxonomy with two new classes, four new orders, three new families, one new genus and one newly discovered species. "Coming more than 100 years after the discovery of these tiny and structurally simplest of all multicellular animals, the study is the first." - and probably only - time in this century that a complete taxonomy for an entire animal tribe will be created," says co-author Bernd Schierwater from the University of Veterinary Medicine Hanover. It was he himself who, together with his team, collected plate animals from various sea coasts around the world in 2010 and proved for the first time that there is more than one species.
New insights into kinship relationships
Because the classification and discovery of the new species was based on decades of international collaboration, the researchers named the new species Cladtertia collaboinventa, which means "the third clade discovered in collaboration." The new species was found in a seawater aquarium. Genetic analysis showed that C. collaboinventa has 11 unique genes not found in other species, while 12 genes found in other species are absent. The research shows not only the relationships within the tribe of plate animals, but also how this primordial group is related to other animals. The closest relationship is therefore to the cnidarians, which include jellyfish, corals and sea anemones, as well as to the large group of Bilateralia. This includes all animals whose body is symmetrically divided into left and right sides - from snails to humans.
The authors propose using the method of molecular morphology for other organisms whose classification based on external characteristics is problematic, including some bacteria, fungi and protists. "Taxonomic gaps are problematic," the researchers explain. "Without names, communication is impeded and further scientific progress slowed down." Tessler and his colleagues believe that molecular analyzes can help: "Since molecules, such as proteins, have a structured shape, we propose that they should not be viewed as anything other than the traditional morphology should be considered. We are convinced that in this way every taxonomic tabula rasa can be filled.”
Source: Michael Tessler (St. Francis College, Brooklyn, New York) et al., Frontiers in Ecology and Evolution, doi: 10.3389/fevo.2022.1016357