Cuticle structure and systematics of the Macrobiotidae (Tardigrada, Eutardigrada)

The cuticle of tardigrades is characterized by three main layers: epicuticle, intracuticle and procuticle. Pillars are present in the epicuticle of almost all heterotardigrades, but these structures are also known in a few species of eutardigrades. The apparent heterogeneity of the cuticular ultrastructure in several species of the Macrobiotidae (Eutardigrada) prompted us to analyse the structure of the cuticle in this family. Eleven species in several genera were investigated with light and/or electron microscopy. All the species of the genera Murrayon and Dactylobiotus showed pillars in the epicuticle, whereas the examined species of Macrobiotus, Richtersius and Xerobiotus completely lacked pillars. Therefore, in the Macrobiotidae, in contrast to what appears with light microscopy, the cuticle is homogeneous within each genus examined at the electron microscopic level. Considering the absence of pillars in the Macrobiotidae a synapomorphy, we propose the erection of two new subfamilies. Macrobiotinae subfam. n. is characterized by the absence of pillars in the epicuticular layer and includes, in addition to the genera Macrobiotus, Xerobiotus and Richtersius, the genera Pseudohexapodibius, Adorybiotus, and probably also Minibiotus, Calcarobiotus and Pseudodiphascon. Murrayinae subfam. n. is characterized by the presence of pillars in the epicuticular layer and includes the genera n Murrayon, Dactylobiotus and, probably, Macroversum.