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

Studies of geographic variation in anhydrobiosis may increase our understanding of the population dynamics of terrestrial meiofauna and the relative importance of local adaptation and microhabitat niche separation. Alltough anhydrobiosis in tardigrades has been studied intensively, few studies have dealt with intraspecific variation in survival and none of these included genetic data, which are necessary when working with meiofauna as cryptic species are common. We analyzed the anhydrobiotic survival and genetic variation in COI of two eutardigrades (Richtersius coronifer and Ramazzottius oberhaeuseri) from Italy and Sweden to detect possible local adaptation. Survival was analyzed as a multidimensional contingency table and showed that anhydrobiotic survival was higher in Sweden for R. oberhaeuseri whereas no geographic variation was found for R. coronifer. Our genetic analysis indicated the coexistence of two cryptic species of R. oberhaeuseri in Italy, of which only one was found in Sweden. It could not be determined if 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. Our analysis showed indications of long term isolation of the individual populations of R. coronifer but recent dispersal in one of the cryptic species of Ramazzottius. We found higher survival in R. oberhaeuseri than in R. coronifer and suggest that these results can be explained by a coupling between anhydrobiotic survival and dispersal rate.