Integrative taxonomy of a new hyporheic ostracod species of the genus Candonopsis (Ostracoda: Candonidae) from Nouvelle-Aquitaine, France, with first evidence of Cardinium bacterial infection in groundwater crustaceans

Abstract The non-marine ostracod subfamily Candonopsinae is typically defined by the absence of a posterior seta on the caudal ramus. However, due to frequent reductions in chaetotaxy across ostracod lineages, this character must be evaluated cautiously and alongside other morphological traits, ideally supported by molecular data within an integrative taxonomic framework. Candonopsis , the type genus of Candonopsinae, comprises 23 extant species distributed worldwide, mostly in subtropical and tropical regions of the Southern Hemisphere. Of the six European species, two are epigean and four hypogean. Here, we describe a new hypogean species Candonopsis danilucae sp. nov., based on male and female specimens collected from the hyporheic zones of three rivers in the Dordogne catchment, department of Corrèze, central France. Morphologically, the new species is distinguished by a unique carapace shape and the structure of the male copulatory organ, particularly the inner lobe of the hemipenis. Molecular comparisons based on mitochondrial COI and nuclear 28S rDNA markers further support its distinctiveness and indicate that its closest genetic affinities among the taxa included in the analysis are with Candonopsis kingsleii (Brady & Robertson, 1870), the type species of the genus. Notably, this study, using newly designed primers, also reports the first molecular evidence of intracellular Cardinium infection in a groundwater crustacean, suggesting that the female-biased sex ratio observed in C. danilucae sp. nov. may result from sex manipulation by this endosymbiont. Our findings highlight the importance of integrative taxonomy, combining carapace morphology, limb structure, habitat preferences, and molecular data, to resolve species boundaries and understand symbiotic associations in subterranean ostracods.