Description of a new genus and species of Isohypsibioidea (Tardigrada), together with its mitochondrial genome sequence

Abstract A new tardigrade taxon, Thulyphoribius melitense gen. nov. et sp. nov. is described from a population of limnic tardigrades collected in the sediment of a temporary pond in Malta. Those habitats, characterized by fluctuating environmental conditions and ephemeral water availability, provide a particularly challenging setting that can drive morphological and genetic diversity in aquatic microfauna. The new genus shows a unique combination of morphological characters, including distinctive morphologies of the peribuccal region, a short and wide ventral lamina and Pseudobiotus -type claws. Although it shares certain traits with some extant genera, the presence of unique characters combinations, precludes its assignment to any of the previously established genera. A comprehensive investigation was conducted, including morphological (using Phase-Contrast Light Microscopy, Scanning Electron Microscopy), morphometric, and molecular analysis. In the latter context, the molecular markers 18S, 28S, COI, and ITS2 were sequenced, and the complete mitochondrial genome was obtained and characterized, offering important insights for future molecular studies of tardigrades. Phylogenetic analyses based on Maximum Likelihood and Bayesian methods, incorporating the four aforementioned molecular markers, indicate that the new genus belongs to the superfamily Isohypsibioidea, yet it does not cluster within any currently recognized extant family of tardigrades. Collectively, these findings underscore the relevance of investigating underexplored habitats and highlight the potential for discovering novel evolutionary lineages among limnic microinvertebrates that inhabit temporary ponds.