DISTRIBUTION AND POPULATION SYSTEMS OF WATER FROGS (THE Pelophylax esculentus COMPLEX) IN NORTHWESTERN RUSSIA

Peculiarities of distribution and population systems of water frogs of the Pelophylax esculentus complex in the northwest of Russia were studied using cytogenetic (genome size) and external morphological characters (439 specimens from 101 localities). We registered three species: P. lessonae, P. ridibundus, and P. esculentus of a hybrid origin. All frogs, including hybrids, were diploid. The first species was prevailing (82% localities); the second species was revealed in 18% localities and hybrid frogs in 13% localities. All species reach the northernmost limits of their distribution. Pelophylax lessonae is widely spread through Pskovskaya and Novgorodskaya oblast's, as well as in southern Leningradskaya oblast', preferring forest habitats. Pelophylax esculentus is found in Pskovskaya oblast' (forest and agricultural biotopes) in co-existence with P. lessonae only. Pelophylax ridibundus, which was introduced since the 18(th) century, is distributed in St. Petersburg City and Leningradskaya oblast' along southern coast of the Gulf of Finland and Narva River on the Estonian border. All populations of the species were usually observed in open water bodies and rivers that flow down on cultivated, urban or barren lands and were allopatric in relation to P. lessonae and P. esculentus. Water frogs of northwestern Russia form three types of population systems only: so called pure single-species (L or R) systems and a mixed (L - E) system; no type with unisexual hybrids was revealed. Thus, their diversity is lower than in other regions of central and eastern Europe.