The incidence of the anomaly P syndrome in water frogs (Anura, Ranidae, Pelophylax) from the Middle Volga River (Russia)

 The anomaly P is a widespread morphological anomaly, which occurs in some groups of amphibians, caused by the trematode parasite Strigea robusta (Digenea: Strigeidae). This anomaly has been previously recorded in water frogs of the genus Pelophylax and toads of the genera Bufo and Bufotes. The anomaly P includes symmetrical polydactyly cases as a mild attenuated form of the complex syndrome, which in severe cases includes strong deformations of hindlimbs and forelimbs. Strigea robusta has a complex 3-host life cycle using planorbid mollusks as the first intermediate hosts, amphibian larvae as the second intermediate hosts, and anatid birds as the definitive hosts. Herein, we described new records of the anomaly P syndrome in water frogs of the genus Pelophylax from the northeastern parts of their ranges. Symmetrical polydactyly (as a mild form of the anomaly P syndrome) was found in 30 individuals of three species of water frogs from seven localities: in 25 individuals of P. lessonae from four waterbodies, in four individuals of P. ridibundus from three waterbodies, and one individual of P. esculentus. In Gusevo pond, three individuals of P. lessonae with severe cases of the syndrome were found. This is the first record of the anomaly P in reliably identified hybridogenetic edible frogs (P. esculentus) that have been identified in nature. Additionally, we provided new data about the occurrence of the anomaly P and the prevalence of the trematode S. robusta in mollusks taken from two water bodies where anomalous water frogs were found.