Diversity and distribution of freshwater copepods (Calanoida, Cyclopoida, Harpacticoida) in Thailand

Freshwater copepods are an important component of aquatic biodiversity in Thailand, but information on their diversity and distribution is still scattered across taxonomic and ecological studies. We compiled and reviewed published records of freshwater copepods from Thailand, revised nomenclature and taxonomic status and analysed species richness, habitat associations, geographical and zoogeographical distributions and inventory completeness. The dataset included 21,505 occurrence records from 115 publications, describing 187 species in 60 genera. Of these, 67 species (35.8%) were originally described from Thailand and 58 species (31.0%) have so far been recorded only from Thailand according to the available literature. The most species-rich families were Cyclopidae (82 species, 43.9%), Diaptomidae (47 species, 25.1%) and Canthocamptidae (28 species, 15.0%). Species richness varied markedly amongst habitats and geographical regions. Ponds supported the highest richness (75 species), closely followed by cave pools (70 species) and north-eastern Thailand had the highest regional richness (103 species). Cave pools also contained the most habitat-restricted species (36 species). Only 11 species (5.9%) were recorded from all six geographical regions, whereas 102 species were restricted to a single geographical region and 94 species (50.3%) were restricted to the Oriental Region. The species accumulation curve did not reach an asymptote and non-parametric estimators predicted 220–346 species, with estimated sampling completeness ranging from 54.1% to 85.0%. These findings indicate that Thailand's freshwater copepod fauna remains incompletely documented despite extensive research. Continued surveys, particularly in underexplored geographical regions and in habitats that yielded few species, such as groundwater and cave streams, are needed to improve estimates of copepod diversity and distribution in Thailand.