Global Diversity of the Placozoa

Abstract

The enigmatic animal phylum Placozoa holds a key position in the metazoan Tree of Life. A simple bauplan makes it appear to be the most basal metazoan known and genetic evidence also points to a position close to the last common metazoan ancestor. Trichoplax adhaerens is the only formally described species in the phylum to date, making the Placozoa the only monotypic phylum in the animal kingdom. However, recent molecular genetic as well as morphological studies have identified a high level of diversity, and hence a potential high level of taxonomic diversity, within this phylum. Different taxa, possibly at different taxonomic levels, are awaiting description. In this review we firstly summarize knowledge on the morphology, phylogenetic position and ecology of the Placozoa. Secondly, we give an overview of placozoan morphological and genetic diversity and finally present an updated distribution of placozoan populations. We conclude that there is great potential and need to erect new taxa and to establish a firm system for this taxonomic tabula rasa.

Never Ending Analysis of a Century Old Evolutionary Debate: “Unringing” the Urmetazoon Bell

“…For instance, most studies have used a single representative for the Placozoa because its taxonomic status was long thought to be that of a monospecific phylum. It is now evident that this phylum is quite diverse (Voigt et al, 2004;Eitel and Schierwater, 2010;Eitel et al, 2013) and addition of as many lineages of this group as possible is needed. Another aspect of taxon sampling that has been a major part of approaches to this problem is the exclusion of taxa from the analysis because of poor "behavior" of the taxon under certain model criteria.…”

The Proto-MHC of Placozoans, a Region Specialized in Cellular Stress and Ubiquitination/Proteasome Pathways

“…T. adhaerens is the only recognized species of the phylum Placozoa, although a greater degree of diversity has been proposed for this phylum (41)(42)(43)(44). T. adhaerens is a small (100-200 mm), discshaped marine organism consisting of two epithelial layers enclosing a layer of multinucleate fiber cells.…”

The most primitive metazoan animals, the placozoans, show high sensitivity to increasing ocean temperatures and acidities

“…Placozoans represent the simplest (not secondarily reduced) metazoan bauplan and have become an emerging model organism for understanding early metazoan evolution (Eitel et al., 2013; Schierwater, de Jong, & DeSalle, 2009; Schierwater, Eitel, et al., 2009; Schierwater et al., 2016; Signorovitch, Dellaporta, & Buss, 2006). …”

“…At present, the phylum Placozoa is the only monotypic phylum in the animal kingdom, with the only formally described species Trichoplax adhaerens (Schulze, 1883, 1891). Placozoans offer unique possibilities for experimental ecophysiological studies because of their small size, simple morphology, and fast vegetative reproduction (Eitel & Schierwater, 2010; Eitel et al., 2011, 2013; Schierwater, 2005). Vegetative reproduction through binary fission or budding is the usual mode of reproduction in the laboratory and in the field.…”

“…H2—“Panama” (warm‐water population): This haplotype culture derived from a single animal collected in 2002 in Bocas del Toro (Panama), hereafter named “H2 pan ” (Eitel et al., 2013). …”