BACKGROUND: Diuronotus is one of the most recently described genera of Paucitubulatina, one of the three major clades in Gastrotricha. Its morphology suggests that Diuronotus is an early branch of Paucitubulatina, making it a key taxon for understanding the evolution of this morphologically understudied group. Here we test its phylogenetic position employing molecular data, and provide detailed descriptions of the muscular, nervous, and ciliary systems of Diuronotus aspetos, using immunohistochemistry and confocal laser scanning microscopy.

RESULTS: We confirm the proposed position of D. aspetos within Muselliferidae, and find this family to be the sister group to Xenotrichulidae. The muscular system, revealed by F-actin staining, shows a simple, but unique organization of the trunk musculature with a reduction to three pairs of longitudinal muscles and addition of up to five pairs of dorso-ventral muscles, versus the six longitudinal and two dorso-ventral pairs found in most Paucitubulatina. Using acetylated α-tubulin immunoreactivity, we describe the pharynx in detail, including new nervous structures, two pairs of sensory cilia, and a unique canal system. The central nervous system, as revealed by immunohistochemistry, shows the general pattern of Gastrotricha having a bilobed brain and a pair of ventro-longitudinal nerve cords. However, in addition are found an anterior nerve ring, several anterior longitudinal nerves, and four ventral commissures (pharyngeal, trunk, pre-anal, and terminal). Two pairs of protonephridia are documented, while other Paucitubulatina have one. Moreover, the precise arrangement of multiciliated cells is unraveled, yielding a pattern of possibly systematic importance.

CONCLUSION: Several neural structures of Diuronotus resemble those found in Xenotrichula (Xenotrichulidae) and may constitute new apomorphies of Paucitubulatina, or even Gastrotricha. In order to test these new evolutionary hypotheses, comparable morphological data from other understudied gastrotrich branches and a better resolution of the basal nodes of the gastrotrich phylogeny are warranted. Nonetheless, the present study offers new insights into the evolution of organ systems and systematic importance of so-far neglected characters in Gastrotricha.