Abstract. Dice snakes (Natrix tessellata, Laurenti, 1768) inhabit oligohaline and brackish waters along the Bulgarian Black Sea coast, where they often forage at sea. Under these conditions, this species should tolerate highly variable blood plasma sodium concentrations for extended time periods, but the effect of high sodium concentrations to blood cell morphology and physiology is largely unknown. In this experiment, we placed adult dice snakes in waters with different salinity for 84 h, representing the three typical saline concentrations in which dice snakes live and forage: 0.2 ‰ NaCl, fresh water, 16 ‰ NaCl, Black Sea water and 36 ‰ NaCl, World Ocean water. We collected blood through cardiocentesis after exposure to each treatment and used Giemsa's solution for staining the formed elements. We registered numerous nuclear abnormalities (nuclear buds, lobes and blebs) in the blood cells of all investigated specimens. In the snakes placed in the waters with high saline concentration (16 and 36 ‰ NaCl) we found numerous cells with ruptured membranes. These snakes also showed increased mitochondrial activity compared to those in fresh water (1.85 and 2.53 times higher, respectively). Our data suggest that even though dice snakes show a remarkable tolerance to waters with increased salinity, prolonged exposure to it induces significant physiological stress in N. tessellata, which poses a clear limit for their ability to stay in the sea for a long time.

Peninsula, Caucasian region, and Middle Asia). Thus, in Kyrgyzstan, Armenia, and Azerbaijan the glass lizard prefers habitats with dense vegetation (Yakovlieva, 1964;Alekperov, 1978;Arakelyan et al., 2011), while in Crimea it prefers habitats with rocky slopes with shrub communities and avoids dense vegetation (Shcherbak, 1966). In Middle Asia (Kazakhstan, Tajikistan, Uzbekistan) the glass lizard's populations are closely associated with rivers and oases (Paraskiv, 1956;Said-Aliev, 1979;Bogdanov, 1986). Wetlands were determined to be an important element in habitat preferences of the species in Uzbekistan (Siroitchkosky, 1958).P. apodus is defined as a typically daily active animal, avoiding the hot part of the day. Similar observations were also made by Muskhelishvili (1970). According to this author during July the animal was rarely noticed and in August it was not seen at all. The studies of Meek (1986) on the activity of the glass lizard in ex-Yugoslavia show that it was active in both sunny and cloudy and even in rainy weather. It is evident that the data of the habitat preferences and the activity of the European glass lizard are controversial, and the information is dated, mainly from the Caucasian and the Middle Asian and Crimean regions.