Hoilungia hongkongensis in the context of "Trichoplax adhaerens"

Placozoa (/ˌplækəˈzoʊə/ PLAK-ə-ZOH-ə; lit. 'flat animals') is a phylum of free-living (non-parasitic) marine invertebrates. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth". Structural and molecular analyses have supported them as among the most basal animals, thus constituting a primitive metazoan phylum.

The first known placozoan, Trichoplax adhaerens, was discovered in 1883 by the German zoologist Franz Eilhard Schulze (1840–1921). Describing the uniqueness, another German, Karl Gottlieb Grell (1912–1994), erected a new phylum, Placozoa, for it in 1971. Remaining a monotypic phylum for over a century, new species began to be added since 2018. So far, three other extant species have been described, in two distinct classes: Uniplacotomia (Hoilungia hongkongensis in 2018 and Cladtertia collaboinventa in 2022) and Polyplacotomia (Polyplacotoma mediterranea, the most basal, in 2019). A single putative fossil species is known, the Middle Triassic Maculicorpus microbialis.

Cladtertia is a genus of placozoan discovered in 2022. The genus contains a single described species, Cladtertia collaboinventa, although several other undescribed lineages are known. Its closest described relative is Hoilungia hongkongensis, with whom it forms the order Hoilungea.

Cladtertia is similar in morphology to most other placozoans, but genetically distinct from them. It has been found in warm tropical waters with low seasonal changes, ranging from 26°N to 25°S, where it occupies a distinct ecological niche compared to other placozoan lineages. Sexual reproduction through cross-fertilization has been observed in a strain of Cladtertia.