Oribatid mite fauna of three members of the lichen genus Cladonia in Europe – observations on species richness, endophagous juveniles and their morphology

Lichen - mite interactions have generally been acknowledged for a long time and yet, despite their ubiquity they are rarely studied in detail. Oribatid mites occur on and in various lichens and prior research has shown that mites use them as shelter and food source. To gain deeper insights into specific mite-lichen relationships, we investigated the oribatid mite fauna of three lichens from the genus Cladonia, C. coniocraea, C. norvegica and C. rubrotincta, from four European countries. In total, we found 61 oribatid mite species in all samples, from which 28 are common in all investigated lichen species. Most abundant were Carabodes areolatus, C. labyrinthicus, C. marginatus, C. subarcticus, Melanozetes mollicomus and Neobrachychthonius marginatus magnus. The species richness and the overall abundance of the oribatid mite fauna were similar between the investigated lichens and independent of geographic origin. The lichen Cladonia rubrotincta is characterized by red spots on its vegetative structures, and these spots have been suggested to be the result of mite feeding activity. Our investigations of these red spots revealed signs of endophagous mite activity in approx. 75% of the cases; immature mite stages were found tunneling, feeding and living inside the lichens. Molecular genetic identification of these juveniles showed that Carabodes areolatus, C. labyrinthicus, C. marginatus and Mycobates carli use Cladonia rubrotincta as kindergarten. Additionally, we characterize the morphology of Carabodes ssp. and Mycobates carli juveniles in detail and show their adaptations to an endophagous lifestyle, e.g. stocky body, short legs, reduced trichobothria and a subterminally positioned anal orifice. We conclude that lichen-mite interactions are versatile and diverse. Adult mites lay their eggs on the lichen, hatched juveniles complete their development inside the lichen and after their final molt, adults leave the lichen. Based on our results, we suggest that Cladonia rubrotincta reacts with color changes to the feeding activity of immature mites. Mite feeding on lichens is generally common, although it remains to be tested whether all life stages can trigger phenotypic responses in their lichen food sources.