New molecular evidence on hexacorals of the genus Palythoa and Protopalythoa suggests that the two are actually all members of a single genus.
The taxonomic status of the zoanthid polyp genera Palythoa and Protopalythoa has been under debate by scientists for nearly a century, but now experts believe they are a little closer to unravelling the mysteries behind their taxonomy.
Most previous studies have differentiated the two genera on the basis of their polyp and colony formations, the size of their stinging cells, or nematocysts, and the number of septa they possess. Polyps of the Palythoa genus are typically embedded in a thick coenenchyme, which those of Protopalythoa are not embedded deeply within the coenenchyme.
To determine the relationships of the genera in greater detail, a team of scientists from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) undertook a molecular study and have recently reported their findings in the journal Zoological Science.
The study, which sequenced two mitochondrial regions from Palythoa and Protopalythoa species found around the world, produced an evolutionary family tree called a phylogeny that showed how closely the two were related to each other.
The phylogenetic trees produced for each of the sequenced parts of the corals' genomes showed just 0.0-0.1% divergence, a level similar to that seen among the species of the genus Zoanthus.
The team says that the Protopalythoa genus can't be differentiated from Palythoa genetically, so in the future all of the species from both genera may be lumped together in Palythoa:
"Surprisingly, sequence differences among Palythoa tuberculosa, Palythoa mutuki 1, and Palythoa mutuki 2 were negligible (0.0~0.2% for both COI and 16S rDNA), potentially indicating relationships below the species level.
"Our sequences align well with the few Palythoa and Protopalythoa sequences reported to date. These findings strongly indicate that our samples represent a minimum of two and possibly up to four species (the Palythoa tuberculosa - P. mutuki 1 - P. mutuki 2 group, and P. heliodiscus) within the genus Palythoa, and that the genus Protopalythoa is erroneous nomenclature."
For more information see the paper: Reimer JD, Ono S, Takishita K, Tsukahara J, Maruyama T (2006) - Molecular evidence suggesting species in the zoanthid genera palythoa and protopalythoa (Anthozoa: Hexacorallia) are congeneric. Zoolog Sci. 2006 Jan ; 23(1): 87-94.