Gastrotricha: A Marine Sister for a Freshwater Puzzle

Background: Within an evolutionary framework of Gastrotricha Marinellina flagellata and Redudasys fornerise bear specialinterest, as they are the only Macrodasyida that inhabit freshwater ecosystems. Notwithstanding, these rare animals arepoorly known; found only once (Austria and Brazil), they are currently systematised as incertae sedis. Here we report on therediscovery of Redudasys fornerise, provide an account on morphological novelties and present a hypothesis on itsphylogenetic relationship based on molecular data.Methodology/Principal Findings: Specimens were surveyed using DIC microscopy and SEM, and used to obtain the 18 SrRNA gene sequence; molecular data was analyzed cladistically in conjunction with data from 42 additional speciesbelonging to the near complete Macrodasyida taxonomic spectrum. Morphological analysis, while providing newinformation on taxonomically relevant traits (adhesive tubes, protonephridia and sensorial bristles), failed to detectelements of the male system, thus stressing the parthenogenetic nature of the Brazilian species. Phylogenetic analysis,carried out with ML, MP and Bayesian approaches, yielded topologies with strong nodal support and highly congruent witheach other. Among the supported groups is the previously undocumented clade showing the alliance between Redudasysfornerise and Dactylopodola agadasys; other strongly sustained clades include the densely sampled familiesThaumastodermatidae and Turbanellidae and most genera.Conclusions/Significance: A reconsideration of the morphological traits of Dactylopodola agadasys in light of the newinformation on Redudasys fornerise makes the alliance between these two taxa very likely. As a result, we createAnandrodasys gen. nov. to contain members of the previously described D. agadasys and erect Redudasyidae fam. nov. toreflect this novel relationship between Anandrodasys and Redudasys. From an ecological perspective, the derived position ofRedudasys, which is deeply nested within the Macrodasyida clade, unequivocally demonstrates that invasion of freshwaterby gastrotrichs has taken place at least twice, in contrast with the single event hypothesis recently put forward.