The dorsal plate is a critical feature in the reassessment of the rotiferan genus Ptygura (Monogononta; Gnesiotrocha; Flosculariidae)

All species of Flosculariidae (Rotifera; Monogononta; Gnesiotrocha; Flosculariaceae) possess a flexible integument; they are termed illoricate. But species of four genera (Beauchampia, Floscularia, Limnias, and Ptygura) possess a distinctive hardened region on the dorsal side of their anterior end, which unfortunately has received little attention beyond the species level. In our work we study the ultrastructure of these hardened regions. In Beauchampia, Limnias, and some species of Ptygura the hardened zone takes the form of a dorsal plate (hereafter DP) with or without additional protruding processes. The DP has the shape of a ~50 µm diameter disc enfolding the anterior dorsal region of the animal, with a perimetral inward fold, smooth surface, and pores. The DP of Limnias ceratophylli shows three main ultrastructural differences from the rest of the body: (1) type of glycocalyx, (2) absence of small mounds on its surface, and (3) presence of broad cisternae at the base of the pores. In Floscularia and some Ptygura the hardened structures are reduced to a pair of dorsal projections protecting the dorsal antenna: i.e., lacking a full DP. These structures deserve attention because their adaptive importance has not been studied. Here we augment those observations with other characters: i.e., presence/absence of a birefringent body in late-stage embryos and morphology of their trophi and corona. Our goal was to provide information to help develop better taxonomy, as well as advance our understanding of the evolution of sessile taxa in general and of the potentially polyphyletic Ptygura genus in particular.