[Distribution of abra ovata [bivalvia] in the southern part of the caspian sea and the effect of the physico-chemical parameters of water and substral on it]

The Abra ovata belongs to family of Semelidae. This Bivalvia is one of the native species of the black sea and Azof lake and introduced to the Caspian sea be Russian scientists. In the Caspian sea, Abra has most frequency between 20-50m depth, in spring and summer frequency of Abra ovata is also more than other seasons and density in easthern areas is also more; and reaches to 3000 N/m[2]. Frequency and settelinq down of Abra ovala depend on Physico-chemical factors of environment, such as; grain size of sediment, rate of site, organic material and sands. Rate of silt have more important than others. In an area with more silt, Abra can remove and exchange better where the sediment have 75 percent silt, 15 percent sand and 10 percent organic matter, Abra has the best condition. The tolerance of Abra ovata relative to salinity is high but the best salinity is between 10-12%