Ecological valence of North Caspian mollusks to oxygen

North Caspian molluscs are characterized by high frequency of occurrence and wide variability of quantitative indicators. In studies conducted in the period from 2013 to 2017 in the water area of the western part of the Northern Caspian the frequency of molluscs was 64%. The population was in the range of 10-8290 species/m2; biomass – 0.003-1162.066 g/m2. Molluscs lived at depths of 2.5-29.0 m, with an absolute oxygen content of 1.29-8.52 ml/l, and a relative oxygen content of 21-134%. Most abundant was Abra ovata (29%). Mytilaster lineatus occured with frequency 24%. According to the results of the research, there were determined the limits of tolerance to the oxygen content for all molluscs living in the Northern Caspian, as well as the optimum zone for the most frequent representatives of malacofauna. The minimum oxygen concentration registered in the bottom water of the western part of the Northern Caspian during the study period (1.29 ml/l) is not critical for species Mytilaster lineatus, Hypanis angusticostata and Didacna barbotdemarnyi. Oxygen concentration of less than 3 ml/l is resistant to species Cerastoderma lamarcki and Abra ovata, less than 4 ml/l – to Hypanis vitrea. Other species including those encountered in rare cases are found at concentrations above 4.06 ml/l. The oxyphilous species are Hypanis semipellucida and Didacna pyramidata. Mytilaster lineatus, Didacna barbotdemarnyi and Cerastoderma lamarcki have a wide environmental valence to oxygen. The absolute oxygen content within the range 5.78-9.08 ml/l is considered the most favorable for the of molluscs development, where all species of molluscs can be found. Most species demonstrated a quantitative increase with oxygen concentrations rising from 4 to 6 ml/l.