International Journal of Marine Science, 2025, Vol.15, No.3, 130-143 http://www.aquapublisher.com/index.php/ijms 132 hibernate. This seasonal migration may lead to reproductive isolation of groups in different waters, thereby promoting intraspecific population differentiation (Oliveira et al., 2021). Figure 1 The appearance of Indo-Pacific king mackerel (Adopted from Gao et al., 2024) 2.2 Current status and challenges of genetic diversity The genetic diversity of the Spanish mackerel genus is essential for its adaptation to the environment and long-term survival (Garner et al., 2020). However, due to pressures such as overfishing and habitat changes, the genetic diversity of many populations is under threat. Some regional studies have shown that Spanish mackerel populations often show low mitochondrial genetic diversity and limited genetic differentiation, suggesting that they may have experienced population bottlenecks or homogenization caused by continuous gene flow in history. For example, a study of narrow-band Spanish mackerel along the coast of Tanzania found that there was no significant genetic differentiation between sampling points, and the whole population constituted a single genetic pool. The high gene mobility may be due to the migration of adults and juveniles between different spawning and foraging grounds. For example, mitochondrial analysis of Brazilian Spanish mackerel (S. brasiliensis) in the southwest Atlantic showed that the genetic distance between its different geographical populations was limited and the population structure was relatively simple. These results indicate that many Spanish mackerel populations are connected at the regional scale, and the highly migratory life history leads to frequent gene exchanges between populations. However, it is necessary to be vigilant that traditional molecular markers (such as mitochondrial genes) may underestimate the actual level of genetic differentiation. Recent higher-resolution data reveal that some species of Spanish mackerel, which were thought to be widespread, actually contain hidden genetic branches and population structures. For example, a multi-gene analysis of the Indo-West Pacific Spanish mackerel complex found that the Indo-Pacific Spanish mackerel (S. guttatus), which was originally considered a single species, actually includes two highly genetically differentiated lineages: one is restricted to the coast of the Bay of Bengal and was reconfirmed as a strictly defined Spanish mackerel (sensu stricto), and the other is widespread in the Indo-Pacific region and was restored to the name of Leopard Spanish mackerel (S. leopardus) (Gao et al., 2024). The mitochondrial genetic distance between these two lineages exceeds 10%, which is enough to constitute different species. In addition, about 2% of genetic differences were detected within the Leopard Spanish mackerel, which was subdivided into two operational taxonomic units (MOTUs) in the Pacific and Indian Oceans. For another example, in the narrow-band Spanish mackerel (S. commerson), genetic data revealed two lineages that may be geographically isolated in the Indian Ocean and the Western Pacific. The existence of these cryptic lineages indicates that some widespread species of this genus are actually composed of several genetically differentiated populations or closely related species. Since they are difficult to distinguish by traditional morphology, they have been "hidden" under nominal species and were only discovered with the help of molecular methods. The existence of cryptic species not only means that we do not have enough knowledge about the actual species diversity of Spanish mackerel, but also poses a challenge to fishery management and aquaculture selection. If different genetic lineages differ in adaptive traits, ignoring their differences may lead to errors in management measures or breeding decisions.
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