The unexpected relationships within Haliclystus (Cnidaria: Staurozoa)

Staurozoa is a unique group of stalked jellyfish, whose taxonomy has been significantly revised over the past two decades. However, phylogenetic relationships within the genera remain poorly understood, and the most numerous and problematic genus is Haliclystus, with 13 species, some of which have questionable validity. The current study aims to investigate the diversity and phylogeny of Haliclystus species in the northern seas, using both morphological and molecular methods.

The specimens of Haliclystus were collected in the White, Barents, Okhotsk, and Japan Seas in 2022-2024. Their morphometric characteristics, rhopalioid shape and size, and cnidocysts arrangement were analyzed for morphological analysis. For the molecular analysis, COI, ITS, and 16S gene regions were sequenced.

As a result of morphological analysis, we identified two species with confidence: Haliclystus auricula (White Sea and Barents Sea) and H. tenuis (Sea of Japan). Three specimens from the Okhotsk Sea and the Sea of Japan have not been identified.

The resulting phylogenetic trees generally correspond to morphological identifications. Specimens of H. auricula from the White and Barents Seas form a single clade with species of H. auricula from the North Sea. The closest sister branch to the H. auricula clade is presented by the H. antarcticus species from the Southern Atlantic Ocean. Specimens of H. tenuis from the Sea of Japan are combined with specimens of H. tenuis collected in the North Sea and on the Pacific coast of Hokkaido. Unidentified samples from the Okhotsk and Japan Seas form two separate clusters that do not overlap with the available sequences from GenBank, and therefore may potentially be undescribed species. Our findings suggest two major groups, Atlantic-oriented and Pacific-oriented, with some specimens challenging this geographical classification. To establish phylogeographic relationships within the genus Haliclystus, it is necessary to extend studies using more samples.