In the Asia-Pacific region, the COI DNA test revealed the divergence of the bivalve mollusc Mactra chinensis into three species; can these species be distinguished using shell coloration and sperm structure?

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.

Analysis of genetic diversity in two different shell colors of the giant triton snail (Charonia tritonis) based on mitochondrial COI sequences

“…Different shell colors resulting from phenotypic plasticity due to environmental factors are indistinguishable by genetic information. It was found that three genetic species of Mactra chinensis could not be determined reliably based on analysis of the shell coloration due to the plasticity of these morphological characters (Reunov et al, 2021); in Corbicula population, morphotypes defined by internal shell color were indistinguishable in genetic background (Li et al, 2022), and there are also examples of different species with the same shell color (Williams et al, 2016), different species of marine snails Clanculus pharaonius and Clanculus margaritarius were examined that contained different pigments but produced similar colors. This fact shows that there may be some adaptive value for these colors and patterns despite differences in their habitats.…”

Investigating the Link between Microalgal Nutrition and the Environment in Hen Clam (Mactra chinensis) Larvae Growth and Survival

“…Hen clam (Mactra chinensis) is a clam species widely found in the soft sandy bottom of the upper subtidal and intertidal zones of the coastal areas of Russia, Korea, Taiwan and Japan [1]. According to the results of a cytochrome coxidase I (COI) DNA investigation, hen clams living in the Asia-Pacific region can be categorized into three types of species [2]. In Korea, they are abundant in the low and mid-intertidal zones of the west and south coasts, within a depth range of approximately 10 m; around Jeju Island, they are only known to be found off the coast of Gwakjiri, a village in the town of Aewol (Eup).…”