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Revealing the mechanisms of life cycle changes is critical for understanding the processes driving hydrozoan evolution. Our analysis of mitochondrial (COI, 16S) and nuclear (ITS1 and ITS2) gene fragments resulted in the discovery of unique polymorphism in the life cycle of Sarsia lovenii from the White Sea. This polymorphic species exhibits two types of gonophores: hydroids produce both free-swimming medusae and attached medusoids (phenotypic polymorphism). Our phylogenetic analysis revealed the intrinsic genetic structure of S. lovenii (genetic polymorphism). Two haplogroups inhabiting the White Sea differ in their reproductive modes. Haplogroup 1 produces attached medusoids, and haplogroup 2 produces free-swimming medusae. Our experiments indicated the possibility of free interbreeding between haplogroups that likely is a rare event in the sea. We propose that inter-haplogroup crossing of S. lovenii in the White Sea may be limited by discordance in periods of spawning or by spatial differences in habitat of spawning specimens. Our finding can be interpreted as a case of nascent speciation that illustrates the patterns of repeated medusa loss in hydrozoan evolution. Life cycle traits of S. lovenii may be useful for elucidating the molecular mechanisms of medusa reduction in hydrozoans.

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Identification of hydrozoans of the family Cytaeididae L. Agassiz, 1862 is confused due to their morphological similarity, to the shortage of morphological characteristics, and to a lack of molecular data. In this study, sequences of nuclear and mitochondrial DNA markers were analyzed from hydroids of Cytaeis Eschscholtz, 1829 collected in the southern Red Sea on shells of Nassarius Duméril, 1805 (Gastropoda: Nassariidae), along with sequences of Cytaeis and Perarella Stechow, 1922 from GenBank. Analyses revealed the paraphyly of the family Cytaeididae, and indicate that the genus Cytaeis may actually belong to the family Bougainvilliidae Lütken, 1850 (Hydrozoa: Anthoathecata). Perarella species grouped within different families and far from Cytaeis species within phylogenetic trees. Additional studies are necessary to confirm the phylogenetic position and formally revise the taxonomy of Perarella. These results demonstrate the need to utilize molecular sequences in re-assessing the taxonomy and phylogeny of filiferan hydrozoans.