Abstract
According to COI DNA barcoding testing, the marine bivalve mollusc Mactra chinensis, which is native to the Asia-Pacific region, diverged into three species. These species were preliminary characterized as M. chinensis COI clade I, M. chinensis COI clade II and M. chinensis COI clade III. To find out whether it is possible to morphologically distinguish samples representing genetic clades, we examined the color of the shells and the structure of the spermatozoa. It was found that the number of detected coloration types exceeds the number of detected species. In addition, it was shown that individuals belonging to the same genetic clade can have shells of different colors. Consequently, it is impossible to choose one type of shell coloration as a species-specific trait. For sperm, the sperm morphological patterns found in each of the three species are consistent with the M. chinensis sperm model described in previous reports. However, the single sperm variant is also not applicable to discriminate between species derived from M. chinensis, since heterogeneous variants of spermatozoa differing in the length of the acrosomal rod were found. We hypothesized that genetic divergence of species could cause a shift towards predominance of one of the sperm variants, and that species-specific sperm morphs could be quantitatively dominant in molluscs belonging to different clades. However, the dominant sperm morphs were the same in COI clade I and COI clade III. Thus, dominant sperm morphs are useless as species-specific traits. However, shell color and sperm parameters are specific to different geographic regions, and it seems that unique environmental factors can determine shell color and sperm morphology. As a result, both shells and spermatozoa can be used to distinguish the geographical forms of M. chinensis, regardless of the belonging of the forms to a particular genetic clade. Here we propose the introduction of geographic identifiers, in which the shell color and parameters of sperm sets are used as morphological criteria to determine the geographical origin of mollusc specimens belonging to the M. chinensis species complex.
Highlights
Bivalve molluscs of the family Mactridae Lamarck, 1809, the so-called “surf clams”, inhabit the AsiaPacific region along the coasts of Russia, Japan, Korea and Chinese Academy of Sciences (China) [1,2,3,4,5,6,7,8,9,10]
Since there are no new names for these new species yet, we preliminarily characterized them as M. chinensis COI clade I, M. chinensis COI clade II, and M. chinensis COI clade III
As for spermatozoa, in order to test whether spermatozoa can be used to distinguish between species derived from M. chinensis, we examined the structure of male gametes in specimens belonging to the species M. chinensis COI clade III, and combined this with data obtained earlier for species M chinensis COI clade I and M. chinensis COI clade II
Summary
Bivalve molluscs of the family Mactridae Lamarck, 1809, the so-called “surf clams”, inhabit the AsiaPacific region along the coasts of Russia, Japan, Korea and China [1,2,3,4,5,6,7,8,9,10]. The identification of M. chinensis was carried out only on the basis of morphological characters. The first test of the nucleotide sequence of the mitochondrial COI gene, carried out at the Canadian DNA Barcoding Center (Canada), showed that there are three clades of M. chinensis, such as clade COI I, clade COI II and clade COI III, which constitute separate species inhabiting the Russian, South Korean and Chinese parts of the Asia-Pacific region [15]. A second test, carried out at the Institute of Oceanology of the Chinese Academy of Sciences (China), confirmed this conclusion (present report). Since there are no new names for these new species yet, we preliminarily characterized them as M. chinensis COI clade I, M. chinensis COI clade II, and M. chinensis COI clade III
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