Diverse eggs, diverse species? Production of two egg morphotypes in Paramacrobiotus bifrons , a new eutardigrade species within the areolatus group

Species within the eutardigrade superfamily Macrobiotoidea are usually differentiated through their eggshell morphology. However, this trait can show very high intraspecific variability and, as consequence, species delimitation is challenging without molecular support. Although eggshell variability usually consists of small details, cases of species with very different egg morphotypes are known. Paramacrobiotus bifrons sp. nov. here described with an integrative approach, represents one of the species with extreme variability in eggshell morphology and it belongs to the areolatus group. The production by this gonochoric species of both areolatus-type and csotiensis-type eggs, the latter being rare in the group but most abundant in this species, was assessed by DNA barcoding, culturing, and inter-morphotype crosses. The reason underlying the production of two different eggshells resulted not related to tested culturing conditions, seasonality, or male's influence. Confocal (with and without staining), Light, and Scanning Electron Microscopies analyses of the two egg morphotypes laid by P. bifrons sp. nov. allowed to redescribe the csotiensis-type egg, shed light on its composition (excluding chitin as a component), and to find shared details supporting the belonging of the two egg morphotypes to the same species.