The ostracode fauna of the neartic-neotropical transitional zone has remained poorly known until this study. Ten ostracode species inhabit ten highland lakes (five maar lakes (phreatic/phreato-magmatic explosion origin), one volcanic-tectonic lake, three natural dams and one man-made dam) in East-central Mexico. Surface sediments from the deepest part and the littoral zone from all studied lakes were collected. Environmental variables (pH, temperature, dissolved oxygen, conductivity, TDS) were measured in situ and parallel water samples for chemical analysis were collected for habitat description. Widely distributed species in the study area (≥5 lakes) include Cypridopsis vidua, Darwinula stevensoni and Eucandona cf. patzcuaro. Limnocytherina axalapasco is an endemic species and was collected in three maar lakes and in one man-made dam. Rare species included: Chlamydotheca arcuata?, Fabaeformiscandona acuminata?, Ilyocypris gibba?, Limnocythere friabilis?, Potamocypris smaragdina? and Potamocypris unicaudata?. Highest species richness (6 spp.) was found in the large and shallow Lake Metztitlán (2.6km2, 5.5m deep), with the lake water type HCO3->>SO42->Cl- -- Ca2+>Na+>Mg2+. The rest of studied lakes (<63m, <27km2) had not more than three species. For instance, only two ostracode species were collected in Lake Alchichica, which is the largest, deepest and most saline studied maar lake.
Keywords: freshwater ostracodes, maar lakes, natural dams, Central Mexico highlands, nearctic-neotropical transitional zone