Features of shape and size variability of blood ark (Anadara kagoshimensis) inhabiting biotopes in the northern part of the Black and Azov Seas were studied. Fourteen morphometric traits were measured for each blood ark shell. The principal component analysis extracted only one principal component (PC) whose eigenvalue is greater than 1.0. The PC 1 explained 84.5 % of the variation in morphometric traits and was interpreted to be an integral indicator of mollusk shell size. The variation of morphometric traits due to the size and asymmetry of the valves was subjected to correction using a Multiple General Linear Model with PC 1 as a continuous predictor and right or left valve as a discrete predictor. A cluster analysis was performed based on the residuals after size correction, as a result of which mollusk individuals were divided into four clusters. For mollusks from the Sea of Azov, a characteristic feature was an increased distance between pallial line and ventral shell margin. In turn, mollusks from the Black Sea exhibited an increase in the width of the posterior adductor scar. The observed morphological differences can be explained by different quantity and quality of food, since the soil composition in the biotopes studied is almost identical.

The bivalve species Cerastoderma glaucum (Poiret, 1789) was studied in this study. This species is allochthonous and belongs to the Mediterranean zoogeographic complex and was introduced in the Holocene. The C. glaucum is the dominant species among the bivalves in the Sea of Azov and has a wide range of distribution. The species is distributed in the coastal zone within 100-300 m from the shore, and it is also found in desalinated water bodies such as estuaries. The C. glaucum is fairly resistant to hypoxia. It is euryhaline with respect to salinity and eurybiontic with respect to soil. The species can settle on sandy, muddy or sandy-silty substrate. The aim of the study was to investigate the morphology of shells of this species in order to find out the reasons of morphological features change of Cerastoderma glaucum in different biotopes of the Sea of Azov. The study was conducted in early June 2021 on the northwestern coast of the Azov Sea. A total of 20 stations were investigated. Cerastoderma glaucum was found at all stations. The morphological variability of the bivalve Cerastoderma glaucum was investigated using the method of discriminant analysis. A notable morphological feature was the external alteration of the mollusc shell. A displacement of the apex to the anterior edge of the shell, lengthening of the posterior edge, and deformation of the shell shape, indicating the ecological characteristics of the study area and its inhabitants. Also, there is a difference in the ratio of shell height to shell length, indicating an increased level of siltation in the ground. Shell thickness varies in all survey areas, indicating different levels of salinity. The overall abundance of molluscs from the different biotopes indicates the factors determining the shape of cockle shells. In turn, morphological parameters indicate the general condition of the Sea of Azov. So, it can be assumed that siltation of the substrate on which benthic communities are located has increased as a consequence of massive deposition of phytoorganic residues. In addition, the hydrolytic regime has changed as a result of anthropogenic factors. As a consequence, salinity, oxygen levels are changing, etc.