Robust phylogenetic position of the enigmatic hydrozoan, Margelopsis haeckelii revealed within the family Corymorphidae

Abstract

The life-cycle and polyp morphology of representatives of Margelopsidae are very different from all other species in the hydrozoan clade Aplanulata. Their evolutionary origin and phylogenetic position has been the subject of significant speculation. A recent molecular study based only on COI data placed Margelopsidae as a sister group to all Aplanulata, an unexpected result because margelopsid morphology suggests affiliation with Tubulariidae or Corymorphidae. Here we used multigene analyses, including nuclear (18S rRNA and 28S rRNA) and mitochondrial (16S rRNA and COI) markers of the hydroid stage of the margelopsid species Margelopsis haeckelii Hartlaub, 1897 and the medusa stage of Margelopsis hartlaubii Browne, 1903 to resolve their phylogenetic position with respect to other hydrozoans. Our data provide strong evidence that M. haeckelii, the type species of Margelopsis, is a member of the family Corymorphydae. In contrast, M. hartlaubii Browne, 1903 is sister to Plotocnide borealis Wagner, 1885, a member of Boreohydridae. These results invalidate the family Margelopsidae. The phylogenetic signal of polyp and medusa stages is discussed in light of concept of inconsistent evolution and molecular phylogenetic analysis.