What if the claws are reduced? Morphological and molecular phylogenetic relationships of the genus Haplomacrobiotus May, 1948 (Eutardigrada, Parachela)

Eutardigrada systematics relies mainly on the morphology of the sclerified structures of the animals. In particular, the main division of Parachela into four superfamilies relies heavily upon claw morphology; however, this character, alone, may be either inadequate or useless for tardigrades with no claws, or when secondary claw branches are reduced or absent, as in the case of species belonging to the genus Haplomacrobiotus. This is a very uncommon genus, so far reported only in North America. The systematic position of this genus has been debated since its description, having first been placed in the family Macrobiotidae (Macrobiotoidea) and then in the family Calohypsibiidae (Hypsibioidea). Currently, the position of the supposedly related genus Hexapodibius is still debated, being attributed to Isohypsibiidae (Isohypsibioidea) or to Calohypsibiidae (Hypsibioidea), i.e. to two different superfamilies. The morphological (light and electron microscopy), chemical (X–ray spectroscopy), and molecular (18S and 28S genes) analyses of a population of Haplomacrobiotus utahensis Pilato & Beasley, 2005 from Snow Canyon State Park (Utah, USA) allowed us to validate the position of this genus within Isohypsibioidea. Our integrated findings placed Haplomacrobiotus in a close relationship with the genus Hexapodibius, and allowed us to erect the new family Hexapodibiidae, comprising the genera Hexapodibius, Parhexapodibius, Haplomacrobiotus, and Haplohexapodibius.