Parasitic nematodes of Pool Frog (Pelophylax lessonae) in the Volga Basin

Objetive. Present a modern review of the nematodes fauna of the pool frog Pelophylax lessonae (Camerano, 1882) from Volga basin populations on the basis of our own research and literature sources analysis. Materials and methods. Present work consolidates data from different helminthological works over the past 80 years, supported by our own research results. During the period from 1936 to 2016 different authors examined 1460 specimens of pool frog, using the method of full helminthological autopsy, from 13 regions of the Volga basin. Results. In total 9 nematodes species were recorded. Nematode Icosiella neglecta found for the first time in the studied host from the territory of Russia and Volga basin. Three species appeared to be more widespread: Oswaldocruzia filiformis, Cosmocerca ornata and Icosiella neglecta. For each helminth species the following information included: systematic position, areas of detection, localization, biology, list of definitive hosts, the level of host-specificity. Conclusions. Nematodes of pool frog, excluding I. neglecta, belong to the group of soil-transmitted helminthes (geohelminth) and parasitize in adult stages. Some species (O. filiformis, C. ornata, I. neglecta) are widespread in the host range. The latter two are able to reach high invasion indices and also to be the background parasites of the pool frog. It happens due to these nematode species life cycle specificity and the long-term connection of the amphibian with water.


INTRODUCTION
The pool frog Pelophylax lessonae (Camerano, 1882) (Amphibia: Anura) is common in Europe from southern France in the west to Tatarstan and Bashkortostan (Russia) in the east (1,2,3,4). It is restricted to the forest zone, in which it inhabits plain broad leaved and mixed forests till the south taiga. In the forest the frog prefers shallow standing water bodies with dense vegetation: lakes, ponds, former river-beds, marshes; it occasionally occurs in the shallows of rivers and streams, in flood meadows and islands (1,5,6). Through riverine thickets and forested floodplains, this frog penetrates the forest-steppe and steppe zones; adults are able to migrate to neighboring water bodies. It is common in the anthropogenic landscapes where it inhabits temporary and drying reservoirs like: road tracks, roadside pits, ditches and quarries with water (7,8,9,10,11). The helminths fauna of the pool frog on the territory of the habitat has been studied unevenly and insufficiently. Reviews on parasites of frogs and toads for European countries clearly confirm this (12,13,14,15,16). This paper continues a series of publications devoted to the modern characterization of the helminth fauna of certain amphibian species in the Volga basin (17,18,19,20,21). Data on the fauna of trematodes (Trematoda) of this species of amphibians from the Middle Volga region are presented in the works of Chikhlyaev et al (22,23). The aim of the study is to present a modern review of the nematodes (Nematoda) fauna of the pool frog from Volga basin populations on the basis of our own research and literature sources analysis.

MATERIALS AND METHODS
Site study. The Volga basin covers an area of 1360 thousand km 2 , which is almost 13% of the area of Europe (Figure 1). In Russia, 41 regions belong fully or partially to this basin. The channel of the Volga and its tributaries are located in lowlands, and only in some places the river flows through elevations (for example, the Samarskaya Luka near the Zhiguli Mountains). The vast basin is characterized by a significant variety of landscapes. Forest ecosystems predominate in the northern part of the basin. At the same time, there are the mixed and coniferous forests in the northwestern part, while the forests of the taiga type with larch and spruce prevail in the northeastern part. In the middle part of the Volga basin, the forest ecosystems gradually pass into the zone of the forest-steppe landscapes, then to the south, the steppe and semi-desert ecosystems are more expressed. Beyond the southern margin of the dry steppes the flat and anhydrous Caspian semidesert with its saline soil cover, drainage rivers, low-lying and sparse vegetation begins.  (Table  1). Own researches are conducted in 8 regions of Russia. In total, more than 568 specimens of P. lessonae have been studied. Studies were performed by the method of full helminthological autopsy. The collection, fixation and processing of the material were carried out according to the standard methods (24).
For species determination of helminths, were used reports of Ryzhikov et al (24). To refer nematodes species to according systematic taxones, we used systems developed by Hodda (25).

RESULTS
Annotated list of pool frog nematodes species with an indication of their systematic position, areas of detection, localization, biology and geographic distribution is given below. The information on degree of specificity of parasites to hosts is also provided. For each species of nematodes, a list of their definitive hosts within Russia is indicated, corrected according to literary sources analysis (18,19,(22)(23)(24) Description (n=2): The shape of the buccal capsule is close to cylindrical. The size of the buccal capsule, which is 10-11 μm long and 9 μm wide. The shape of the tail is slender; body length, which is 9-10 mm. Biology: It is a soil-transmitted parasite (geohelminth). Infection of amphibians results from percutaneous penetration of invasive larvae which migrate with a blood flow in the host's lungs on the land (26). Participation of paratenic hosts -land mollusks, oligochaetes is probable (24). Is a widely specific parasite of anurans.

DISCUSSION
In the pool frog on the territory of Volga basin there are 9 species of nematodes from 7 genera, 4 families, 3 orders and 1 class (