4.2 Genetic structure and differentiation
Genetic distance is a crucial factor in determining the genetic relationship between species (Mather et al., 2017). Shaklee et al (1982) proposed a classification of fish genetic distance at the population level (0.05), species level (0.30), and genus level (0.90). In this study, it was found that the genetic distance between populations were small, indicating a close genetic relationship between these populations. According to the coalescent theory (Crandall & Templeton, 1993), more diverse populations have longer coalescence times and larger coalescent effective population sizes than less diverse populations (assuming the same mutation rate). Therefore, the ancestral haplotype was the most widely distributed. In the COI and Cyt b gene datasets, Hap_1 and Hap_2 were found to be the dominant haplotypes (Figure 2), and may be the origin of L. spadiceus .
In the phylogenetic analysis, the haplotypes of eight populations were randomly distributed. The haplotype network and phylogenetic tree also showed no clear pedigree structure corresponding to geographical location. This pattern suggests that the species went through a bottleneck event followed by a population expansion (Grant & Bowen, 1998). The dispersal of larvae with ocean currents is an important cause of the limited genetic differentiation of marine fishes that have a geographically large distribution range (Strathmann et al., 2002). In this study, L. spadiceus were caught in the spring and autumn, during this time, the China Coastal Current and the South China Sea Warm Current flowed northward into the East China Sea (Figure 1) (Yang et al., 2008; Wang et al., 2015). Previous studies have reported that extensive gene exchange occurs over a wide geographical range in marine fishes (Grant & Bowen, 1998; Niu et al., 2019; Yi et al., 2021).F st is a significant measure in evaluating genetic diversity among populations, as emphasized by Allendorf (1983). A higher F st value suggests a greater level of genetic differentiation. According to Wright (1951) classification,F st value of 0 ~ 0.05 suggests no differentiation, 0.05 ~ 0.15 suggests little differentiation, 0.15 ~ 0.25 suggests moderate differentiation, and F st value greater than 0.25 suggests significant genetic differentiation. TheF st between the BH and LZ population was the highest value among all populations (Table 2), indicating the greatest genetic differentiation is between these populations, other populations were lower.