Occurrence of <i>Geehydrosoma</i> cf. <i>intermedia</i> (Chislenko, 1978), a harpacticoid copepod species in the family Cletodidae assessed by DNA barcoding in Omura Bay, a seasonally hypoxic enclosed bay, in Japan

Benthic copepods, the second most abundant meiofaunal taxa, are mainly composed of harpacticoids, which are generally sensitive to environmental stressors, such as oxygen depletion. However, harpacticoid copepods in the family Cletodidae are known to cope with hypoxic conditions. Tolerance of Cletodidae to oxygen depletion has been reported in a previous study in Omura Bay, a semi-enclosed, seasonally hypoxic bay in Nagasaki, western Kyushu, Japan. To gain insights into the genetic diversity of the copepods in the bay, we initiated DNA barcoding of selected specimens obtained from April through October 2022. Based on genetic comparison of a portion of the mitochondrial cytochrome c oxidase subunit I gene (COI), we found that all specimens of the family Cletodidae represented Geehydrosoma cf. intermedia, a member of a recently established genus. Morphological examination of their exoskeletons that had been secured after the DNA extraction provided good support for that tentative species assignment. Furthermore, it was suggested that G. cf. intermedia in Omura Bay is genetically differentiated from its counterparts in the eastern margin of Eurasia. Continued research, including a broader sampling of G. cf. intermedia specimens, is clearly required to clarify the genetic diversity of this harpacticoid copepod.