DNA evidence suggests that the genera Palythoa and Protopalythoa are actually all members of a single genus.
The taxonomic status of the Zoanthid genera Palythoa and Protopalythoa has been under question by biologists for almost a century. Previous morphological and histological studies have separated these two genera on the basis of the shape of their polyp and colony, the size of their nematocysts and the number and position of septa in their body cavity.
During the last decade, Dr Jame Davis Reimer who is one of the best specialist about zoanthid systematics in the world has published important genetic studies on Zoanthids. To determine the phylogenetic relationships of the Palythoa and Protopalythoa Zoanthids at the molecular level, Dr Reimer and his team from the Japan Agency for Marine-Earth Science and Technology sequenced two mitochondrial genes (coding for the cytochrome oxydase I and small-subunit ribosomal RNA) from different specimens belonging to these genera.
The phylogenetic analyses carried out by the team showed only 0.0-0.1% divergence among the Palythoa and Protopalythoa species, a level similar to that seen among the species of the genus Zoanthus. The authors concluded that Protopalythoa specimens cannot be differentiated from Palythoa genetically.
“The phylogenetic trees for both COI and 16S rDNA from Palythoa and Protopalythoa show four monophyletic groups (designated Palythoa tuberculosa, Palythoa heliodiscus, Palythoa mutuki 1, and Palythoa mutuki 2), with levels of sequence divergence (COI and 16S rDNA divergence approximately 0.0 similar to 1.1%) similar to or lower than that previously found among congeneric species within the closely related genus Zoanthus”.
In conclusion, the molecular genetic evidence provided by the Japanese Team strongly suggests that Palythoa and Protopalythoa are not truly separate genera. Protopalythoa is likely erroneous nomenclature.
After Reimer JD, Ono S, Takishita K, Tsukahara J, Maruyama T. Molecular evidence suggesting species in the zoanthid genera Palythoa and Protopalythoa (Anthozoa: Hexacorallia) are congeneric. Zoolog Sci. 2006 Jan;23(1):87-94.