Revising taxonomy and systematics within the enigmatic genus Xerobiotus (Tardigrada: Eutardigrada: Macrobiotidae)

Three Sicilian populations belonging to the enigmatic genus Xerobiotus were analyzed using integrative taxonomic methods combining detailed morphological and genetic data. A new population of X. inermis was found at its locus typicus in the coastal dunes of Gela, Sicily, while X. euxinus was recorded for the first time in Sicily at Viagrande (Catania) and Serra La Nave (Etna). Both species were examined using phase contrast microscopy (PCM) and scanning electron microscopy (SEM), and sequenced for four molecular markers: 18S rDNA, 28S rDNA, ITS-2, and COI. For X. inermis , the complete mitochondrial genome was also obtained. These data enabled us to redescribe X. inermis and to re-evaluate and amend the description of the widespread and morphologically variable X. euxinus . To explore phenotypic and quantitative variation, we conducted a principal component analysis (PCA) of morphometric traits across Xerobiotus populations and selected Macrobiotus taxa. The analysis revealed extensive character overlap among species, limiting diagnostic resolution and complicating genus-level delimitation. Phylogenetic reconstructions and species delimitation analyses were partially congruent with earlier studies but also challenged the current boundaries of Xerobiotus , especially with Macrobiotus , suggesting potential synonymy or misclassification among genus- and species-level taxa. In this paper, we integrate multiple lines of evidence from our analyses and comparisons in order to synonymize two genera and two species and identify two additional taxa as species in need of urgent revision to test for further potential synonymies.