Placozoans

Placozoans are a phylum of simple, free-living marine animals. The best-known placozoan is Trichoplax adhaerens, the only described species in the phylum, though molecular studies have revealed substantial diversity and likely additional species.

Physically, placozoans are tiny, flat, plate-like organisms typically 1–3 mm across. They have a two-layered body plan with an upper and a lower epithelium and a loose middle layer of fiber-like cells. They lack true tissues and organs, nerves, and a digestive cavity. There is no mouth or gut; feeding occurs by external digestion and uptake of particles by endocytosis across the lower epithelium.Cell types include upper epithelial cells, lower epithelial and gland cells, and the contractile fibers in the middle layer.

Reproduction occurs mainly by asexual budding and fission. Sexual reproduction is rarely observed in culture, but some meiosis-related genes have been identified in their genome.

Genome and development: The genome of Trichoplax adhaerens is compact, roughly 90–100 million bases, encoding about 8,000–12,000 protein-coding genes. The gene content shows many conserved metazoan families despite the organism’s simplicity and reflects the evolution of multicellularity and development.

Ecology and evolution: Placozoans inhabit marine surfaces worldwide, on rocks, algae, or substrates in shallow to moderately deep waters. Their simplicity has made them important for questions about early animal evolution. Phylogenetically, their position within Metazoa is debated; many analyses place Placozoa as sister to Cnidaria and Bilateria (Planulozoa), while others place a more basal position.