In the Sea of Japan and the Adriatic Sea, a COX1 DNA test revealed genetic similarity of Manila clams, although shell parameters, spermatogenesis patterns, and sperm characteristics are area specific

The two Manila clam forms living in the Sea of Japan and in the Adriatic Sea were compared genetically and morphologically to identify traits that could be used to authenticate these geographic forms. COX1 DNA barcoding confirmed that both forms of these venerid bivalve belong to the species Ruditapes philippinarum (Adams and Reeve, 1850). It has also been shown that, based on COX1 analysis, it is not possible to separate these forms into different clusters based on geographic origin. However, some morphological features make it possible to differentiate these forms. It is noted that the contour of the shells of the Adriatic and Pacific molluscs is rounded and oblong, respectively. Underwater analysis of Pacific clams showed that shell color may be specific to specimens found at certain collection sites and differ between specimens taken from different collection sites. In the spermatogenesis of Adriatic and Pacific clams, a difference was found in the cellular mechanism of interaction between germplasm granules and mitochondria, manifested by distant contact and direct contact. Spermiogenesis has three similar lines of acrosome development, but differs in the number of ways in which the nucleus is formed. Due to the variability of the ways of sperm formation, heteromorphic morphs of spermatozoa are formed, the total number of which is six for the species R. philippinarum. In the Adriatic Manila clam, five morphs from this set are expressed. The Pacific Manila clam has only three sperm morphs. Only two sperm morphs are similar in both forms. In addition, each form is distinguished by the presence of unique sperm morphs and each form has its own type of dominant spermatozoon. Thus, the contour and color of the shells, as well as cytologic markers that have been found in the mechanisms of differentiation of meiotic cells and the characteristics of spermatozoa, can be used to distinguish the Adriatic and Pacific forms of the bivalve mollusc R. philippinarum.