Polyplacotoma mediterranea is a new ramified placozoan species

The enigmatic phylum Placozoa is harboring an unknown number of cryptic species and has become a challenge for modern systematics. Only recently, a second species has been described [1], while the presence of more than a hundred additional species has been suggested [2]. The original placozoan species Trichoplax adhaerens[3], the second species Hoilungia hongkongensis[1] and all yet undescribed species are morphologically indistinguishable (i.e. no species diagnostic characters are available [4]). Here, we report on a new placozoan species, Polyplacotoma mediterranea gen. nov., spec. nov., which differs from other placozoans in its completely different morphological habitus, including long polytomous body branches and a maximum body length of more than 10 mm. Polyplacotoma mediterranea also necessitates a different view of placozoan mitochondrial genetics. P. mediterranea harbors a highly compact mitochondrial genome with overlapping mitochondrial tRNA and protein coding genes. Furthermore, the new species lacks typical placozoan features, including the cox1 micro exon and cox1 barcode intron. As phylogenetic analyses suggest a sister group relationship of P. mediterranea to all other placozoans, this new species may also be relevant for studies addressing the relationships at the base of the metazoan tree of life.