Host species-driven genetic structure of respiratory mites in two sympatric marine mammal species

Understanding the factors shaping parasite population structure is key to unravelling host–parasite coevolution and transmission dynamics. Respiratory mites of the genus Orthohalarachne (Acari: Halarachnidae) are obligate parasites infecting the respiratory tract of otariids, yet their population genetics remain largely unexplored. Here, we examined the genetic structure of O. attenuata and O. diminuata infecting two sympatric hosts, the South American fur seal (Arctocephalus australis; SAFS) and the South American sea lion (Otaria byronia; SASL), across their distribution ranges. Genetic analyses revealed strong host-associated divergence for both mite species, independent of geographic origin. Variation among host species explained more than 65% of total genetic structure in O. attenuata and more than 57% in O. diminuata, with no haplotypes shared between SAFS- and SASL-associated mites. Despite overlapping distributions and potential interspecific contact, gene flow between mite populations is highly constrained, suggesting prolonged co-divergence with their hosts and limited opportunities for cross-species transmission. Ecological segregation, reproductive timing, and behavioural barriers likely reinforce host specificity. Our findings highlight Orthohalarachne mites as a model system to study parasite diversification and reveal potential cryptic speciation within morphologically indistinguishable lineages. This work underscores the importance of multi-host, spatially wide approaches for understanding the evolutionary and epidemiological dynamics of parasites in marine ecosystems.