Systematics
Currently, only one species, Trichoplax adhaerens, is acknowledged to belong to the Placozoa. The Italian Francesco Saverio Monticelli described another species in 1893, which he found in the waters around Naples, naming it Treptoplax reptans. However, this has not been observed since 1896, and most zoologists today doubt its existence.
Significant genetic differences have been observed between collected specimens matching the morphological description of T. adhaerens, suggesting that it may be a cryptic species complex.
Because great genetic differences often occur between representatives of Trichoplax adhaerens, differences that in other taxa would result in their being spread among different genera, it is currently unclear whether the single species based on morphological criteria does not actually correspond to a group of cryptospecies, i.e. species that are not outwardly distinguishable from one another. Distribution of the genetic variants is not a function of geography: some variants are found in multiple regions (e.g. Pacific, Caribbean and Red Sea). At the same time, very different genetic variants can be isolated from the same habitat.
Read more about this topic: Trichoplax Adhaerens