Underestimated species diversity and hidden habitat preference in Moina (Crustacea, Cladocera) revealed by integrative taxonomy

In Japan, farm ponds retain water throughout the year while rice fields are continuously inundated for no more than 2 months, usually from spring to early summer. Although the zooplankton fauna in artificial water bodies would be expected to vary according to differences in inundation periods, scientific confirmation of this relation is largely lacking. Due to its ubiquity, the cladoceran Moina is a suitable target for studying habitat-related differences in the planktonic fauna, but its taxonomy remains unresolved, making morphological identifications potentially uncertain. We thus applied integrative taxonomy with both morphological and genetic evaluations for reliable species delimitation to Moina samples collected primarily in Japan (with smaller collections from Taiwan). This approach increased the alpha diversity of Moina species in Japan from three (in previous studies) to seven. It also revealed different habitat preferences among Moina species, with the smaller species being distributed mostly in farm ponds (followed by natural lakes), and the larger species mostly in rice fields. We argue that the phenological match/mismatch with inundation period of rice fields was a major factor for this strong trend of spatial species turnover, with differing degrees of fish predation pressure among the habitat types being another factor.