In a shallow (<15 cm) pool of fresh water after rain.
According to Patrick De Deckker, 16 September 2024, species is Heterocypris incongruens (https://www.biwahaku.jp/smith/genera/heterocypris_korea.html). He describes it as "a cosmopolitan animal that is found in muddy pools, road-side ditches etc. Both sexes are found; It is usually pale brown in colour, can swim due to its long antennae".
Note category should not be marine invertebrates" as terrestrial invertebrates also exist. See DeDeckker, P (1983) Terrestrial Ostracods in Australia, The Australian Museum, Sydney. https://journals.australian.museum/media/Uploads/Journals/17620/374_complete.pdf