Placozoa: a phylum of one?

In the phylum Placozoa, only two species have been described, Trichoplax adhaerens Schulze (1883) and Treptoplax reptans Monticelli (1893). Whereas the former has been reported from tropical and subtropical localities around the world, the latter has not been observed since Monticelli's original description, and its validity is generally doubted. Thus, the high level of genetic diversity in our samples from a relatively small number of localities is quite surprising. Four separate molecular markers (nuclear 18S, ITS1-5.8S-ITS2, and 28S, and mitochondrial 16S)indicate that Placozoa comprises at least four distinct, deeply divided clades, all of which fit the gross morphological description of T. adhaerens. Whereas molecular differences within the four groups are modest, differences between the four clades appear large (for example, partial 16S lengths range from 544 to 689 bp). The deep divisions between the clades are comparable to those seen between different families or orders of cnidarians, suggesting a similarly long history. So far, no straightforward relationship has emerged between placozoan clades and biogeography or habitat. For instance, multiple groups are present on both the Pacific and Caribbean sides of Panama, in two cases at the same site, but a single group dominates our samples from both regions. Continued sampling from diverse localities and habitats from around the world promises to provide a more accurate picture of biodiversity in this supposedly monotypic phylum, and scrutiny of the ultrastructure of placozoans in the four clades may reveal diagnostic differences in morphology. Indeed, the discovery of a large complex of nested placozoan taxa now seems inevitable.