Octorhopalona saltatrix, a New Genus and Species (Hydrozoa, Anthoathecata) from Japanese Waters

SIMPLE SUMMARY: In this study, we describe a new genus and species of hydromedusa belonging to the family Halimedusidae (Hydrozoa, Anthoathecata) that is found off Oarai, Sagami Bay, and Tosa Bay, Japan. This family comprises four species in three genera: Halimedusa, Tiaricodon, and Urashimea. The new genus and species can be distinguished from all other Halimedusidae species by genetic sequences and morphological characteristics. A comparative table of the primary diagnostic characteristics of the genus Halimedusa is provided. In addition, the diagnosis of Halimedusidae was modified. Jellyfish blooms cause serious problems in fishing, industry and public health. This paper contributed to the understanding of ecology and diversity of jellyfish on our planet. ABSTRACT: Approximately 300 species of cnidarian jellyfish have been reported in Japanese waters. However, many specimens remain unidentified. In this study, taxonomic investigations, including morphological observations and molecular 16S phylogenetic analyses, were conducted on unknown specimens collected off Oarai, Sagami Bay, and Tosa Bay, Japan. The specimens have the following morphological characteristics: distinct peaks in jelly above the base of the manubrium, a red band on the manubrium, and cylindrical marginal bulbs, each with an abaxial ocellus that is common to the family Halimedusidae. However, the specimens can be distinguished from other Halimedusidae species by their eight radial canals, eight tentacles with numerous stalked nematocyst knobs, and eight nematocyst tracks on the exumbrella. Moreover, molecular phylogenetic analyses revealed that the Kimura two-parameter distance between the specimens and other Halimedusa species was 0.066–0.099, which is considered to represent intergeneric variability. Based on this result, we described it as a new species and established a new genus for taxonomic stabilization. We also emended the diagnostic characters of the family Halimedusidae owing to the establishment of the new genus. Halimedusidae comprises five species in four genera. This paper provides taxonomic keys for the identification of species in the family Halimedusidae.