Cryptic Species in Macrobiotus (Tardigrada): A Morphological and Molecular Approach

In the last years, morphological analysis of Tardigrada has been carried out even more in depth, leading to recognise many new species, thus reconsidering the concept of intraspecific variability. In addition, molecular approach has very recently identified a more complex scenario within some taxa, supposing or evidencing the presence of cryptic species. Our previous studies already identified a complex situation bound to karyology and reproductive biology within single morphospecies. For these reasons we have reconsidered populations belonging to the morphospecies Macrobiotus richtersi and to Macrobiotus “hufelandi group” analysing their morphology and mtDNA COI (cytochrome oxidase I) gene sequences. The presence of both diploid amphimictic and triploid apomictic populations has been confirmed in both taxa. In M. richtersi all animals are very similar, while limited differences have been evidenced in the egg shell ornamentations of some populations. DNA sequences have shown that populations with different reproductive modes (and ploidy degree) have always decidedly different haplotypes. Within parthenogens equal or very similar haplotypes are retrieved, while within amphimictics both equal and different haplotypes are found. In M. “hufelandi group” molecular analysis confirms the morphological diagnosis of 3 populations of Macrobiotus macrocalix, while it evidences differences between 2 populations attributed in references to Macrobiotus terminalis and M. cf. terminalis. A general conclusion is that in tardigrades it is important to integrate morphological and molecular analyses. In fact, in some instances the morphological approach is adequate to recognize the species, but sometimes the molecular approach is indispensable to identify cryptic species.