Variation in anhydrobiotic survival of two eutardigrade morphospecies: a story of cryptic species and their dispersal

Studies of geographic variation in anhydrobiotic tolerance may increase our understanding of the population dynamics of terrestrial meiofauna and the relative importance of local adaptation and microhabitat niche separation. Although anhydrobiosis in tardigrades has been studied extensively, few studies have dealt with intraspecific variation in survival and none of these included genetic data to validate the intraspecific nature of the comparisons. Such data are necessary when working with meiofauna as cryptic species are common. We analysed the anhydrobiotic survival and genetic variation in cytochrome oxidase subunit I of two eutardigrades (Richtersius coronifer and Ramazzottius oberhaeuseri) from Italy and Sweden to detect possible local adaptation. Survival was analysed as a multidimensional contingency table and showed that anhydrobiotic survival was higher in Sweden for Ra. oberhaeuseri whereas no significant geographic variation was found for Ri. coronifer. Our genetic analysis indicated the coexistence of two cryptic species of Ra. oberhaeuseri in Italy, only one of which was found in Sweden. It could not be determined whether the variation in Ramazzottius is intra- or interspecific due to the presence of these cryptic species. We suggest that geographic variation in anhydrobiotic survival may be a general phenomenon in tardigrades but further research is necessary to determine the degree of intraspecific variation. The genetic analysis showed indications of long-term isolation of the individual populations of Ri. coronifer but recent dispersal in one of the cryptic species of Ramazzottius. We found higher survival in Ra. oberhaeuseri than in Ri. coronifer. These results indicate a possible coupling between anhydrobiotic survival and dispersal rate.