An annotated checklist of free-living Copepoda (Crustacea) of Iraq: diversity, distribution, and biogeographical affinities.

Copepods are a major component of aquatic zooplankton and play essential roles in ecosystem functioning, yet their diversity in Iraq has remained poorly documented and taxonomically fragmented. This study presents the first comprehensive checklist of free-living Copepoda in Iraq, based on published records spanning 1921-2025. Only free-living taxa of Calanoida, Cyclopoida, and Harpacticoida were included, excluding parasitic forms and unidentified larval stages. A total of 51 confirmed species were compiled, comprising 26 Calanoida, 20 Cyclopoida, and 5 Harpacticoida. These include 22 freshwater-associated taxa (43.1%) and 29 marine-origin taxa (56.9%), the latter largely associated with Arabian Gulf estuarine and coastal plankton communities. Calanoida represent the most diverse order, comprising both freshwater taxa from inland waters and numerous marine-origin taxa associated with southern Iraq, whereas Cyclopoida dominate inland freshwater rivers and marshes. Harpacticoida remain markedly underrepresented, indicating substantial undersampling of benthic and meiofaunal habitats. The fauna reflects the coexistence of freshwater taxa associated with the Tigris-Euphrates Basin and marine taxa associated with the northwestern Arabian Gulf. Notable records include the endemic Phyllodiaptomus irakiensis and the invasive estuarine cyclopoid Limnoithona tetraspina, highlighting both regional endemism and emerging anthropogenic pressures. This checklist establishes the first modern national baseline for Iraqi copepod biodiversity and demonstrates that actual diversity remains substantially underestimated, particularly in estuarine, coastal, groundwater, and benthic ecosystems.