The evolution of the protonephridial terminal organ across Rotifera with particular emphasis on Dicranophorus forcipatus, Encentrum mucronatum and Erignatha clastopis (Rotifera: Dicranophoridae)

Abstract Riemann, O. and Ahlrichs, W.H. 2009. The evolution of the protonephridial terminal organ across Rotifera with particular emphasis on Dicranophorus forcipatus , Encentrum mucronatum and Erignatha clastopis (Rotifera: Dicranophoridae). — Acta Zoologica (Stockholm) 91 : 199–211 We report on the ultrastructure of the protonephridial terminal organ in three species of dicranophorid rotifers ( Dicranophorus forcipatus , Encentrum mucronatum and Erignatha clastopis ). Differences between the three species relate to shape and size, the morphology of the filter region and the number of microvilli and cilia inside the terminal organ. A comparison across Rotifera indicates that the terminal organs in D . forcipatus display a number of plesiomorphic characters, but are modified in E . mucronatum and Er . clastopis . This is in accordance with the results of phylogenetic analyses suggesting a basal position of D . forcipatus compared with the more derived species E . mucronatum and Er . clastopis . Moreover, we survey available data on the terminal organ in Rotifera and discuss its evolutionary transformations. The protonephridial terminal organ in the common ancestor of Rotifera consisted of a cytoplasmic cylinder with cilia united into a vibratile flame and a single circle of circumciliary microvilli. Depending on the topology on which characters are optimized, the site of ultrafiltration was formed by longitudinal cytoplasmic columns spanned by a fine filter diaphragm or by pores in the wall of the terminal organ. In several taxa of Rotifera, the terminal organ – probably independently – lost its circumciliary microvilli.