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December 9, 2022

Placozoans, World’s Simplest Animals, Get Their Place in the Tree of Life

The placozoan Trichoplax adherents. Professor Tessler describes placozoans as "the pancake of the animal world." Photo: © B. Schierwater, et al

The group of animals consisting of tiny blob-like life forms having no organs and just a few cell types finally has a fleshed-out family tree, built by a research group led by the American Museum of Natural History (AMNH), St. Francis College (SFC) and the University of Veterinary Medicine Hannover.

Published on December 8 in the journal Frontiers in Ecology and Evolution, the study comes more than 100 years after the discovery of these ameboid animals called placozoans. The research is based on genetic makeup — the presence and absence of genes — rather than outward physical appearance, which is traditionally used to classify organisms.

“Placozoans look like miniscule, shape-shifting disks — basically, they are the pancake of the animal world,” said the study’s co-lead author, Michael Tessler, an assistant professor of biology at SFC and a research associate at the AMNH. “For a taxonomist looking through a microscope, even a powerful one, there are almost no characters to compare and differentiate them. Yet, despite most of them looking almost exactly the same, we know that, on the genetic level, there are very distinct lineages.”

The first placozoan species was described in 1883, and Placozoa remained a “phylum of one” until DNA-based research in the last 20 years revealed that it contains multiple lineages. Most placozoans, which generally live in tropical and subtropical waters across the globe, are about the size of a grain of sand, with hair-like structures that allow them to move.

The researchers used a method called molecular morphology — using differences in DNA sequences and other molecular characters — to make classifications. In doing so, they established a backbone taxonomy: two new classes, four orders, three families, one genus and one species. Their research also suggests that placozoans are most closely related to cnidarians (a group of aquatic animals including jellyfish, corals and sea anemones) and bilaterians (animals that have a left and right side, like insects and humans).

Because it took decades of effort from a great number of people to get to the first classification for the cryptic phylum, the newly described species is called Cladtertia collaboinventa, which means “discovered in collaboration” in Latin.

The study’s authors suggest that it could serve as a template to revisit systematics of other organisms that look very similar, such as bacteria, fungi, protists and parasites. Tessler also is the lead author of a second paper out now in Frontiers in Ecology and Evolution that makes the case for molecular morphology in other groups of organisms that have few distinguishable visual features but are genetically diverse.

“Taxonomic blank slates are problematic. Without names, communication is hampered, and other scientific progress is slowed,” noted Tessler. “We suggest that the morphology of molecules, such as proteins — which have distinctive structures — should not be considered as anything less than traditional morphology.”

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