In this work, staurozoans of two distinct morphotypes are reported in Kitsunezaki (Ishinomaki City, Miyagi, Japan) in the years following the Great East Japan Earthquake and Tsunami. Staurozoa specimens were collected from Eisenia and Gelidium macroalgal beds at the Kitsunezaki survey site (October 2019–July 2021). Morphological observations indicated that the Kitsunezaki staurozoans represented two species, Haliclystus inabai and Calvadosia nagatensis, but molecular analyses of the genetic markers 16S rRNA and COI suggested that the former actually encompasses two distinct lineages, H. inabai and a cryptic as yet unnamed species. Phylogenetic analysis reveals the two H. inabai lineages are separated by significant divergences for both gene markers. H. inabai lineage 1 includes specimens sampled with molecular sequences from Hokkaido (Japan) and Kitsunezaki (Japan), whereas H. inabai lineage 2 includes sequences from Victoria (Australia), Kitsunezaki, as well as populations that appeared in a lab in Germany and aquariums in Tsuruoka and Kagoshima (Japan) and Québec (Canada). Conversely, C. nagatensis from Kitsunezaki appears to be a species distributed only in the temperate NW Pacific. Observations on early life stages of H. inabai lineage 2 within aquarium tanks permitted confirmation of the presence of “microhydrula” settled larva, frustules, and elongated settled larvae. C. nagatensis was collected from the Kitsunezaki survey site in warm months only, and always exhibited gonads, while H. inabai stauromedusae were collected in most months throughout the year, with gonads usually present irrespective of season. An extensive literature review covering more than 100 years and observations in this study revealed seaweed and seagrass as the primary substrates for these two Staurozoa species. Our findings show C. nagatensis is associated with just two types of algal substrates and seagrass, while H. inabai has a much broader substrate preference, consistent with its wider geographic distribution. These findings have contributed to our understanding of Staurozoa epibiotic associations in exposed bays during the recovery period following a major natural disaster.

Mastigias jellyfish (“tako kurage”) have been reported throughout littoral Japan for over a century, historically identified by morphology alone as Mastigias papua. Molecular resolution of the genus in 2018 revealed three geographically structured clades and suggested Japanese populations instead belong to M. albipunctata, but subsequent morphology-only studies of Japanese specimens have continued to apply the M. papua designation, leaving species identity unresolved across much of the species’ Japanese range. Here, we use cytochrome c oxidase subunit I (COI), combining published GenBank sequences with two newly collected specimens from Okinawa, the southernmost extent reported in Japan. Maximum-likelihood and Bayesian phylogenetic reconstruction recovered three reciprocally monophyletic clades consistent with previous molecular work, with all Japanese sequences, from Sukumo Bay (Shikoku Island) to Okinawa Island, falling within the M. albipunctata clade. Morphological comparison against published diagnostic characters showed consistency with this identification. These results indicate that Mastigias populations in Japan are consistently identified as M. albipunctata, while highlighting the value of molecular analysis in tandem with morphology-based identification approaches.