International Journal of Marine Science, 2025, Vol.15, No.2, 75-91 http://www.aquapublisher.com/index.php/ijms 77 2.3 Origin hypothesis: from tropical shallow sea to global spread Based on phylogenetic and paleogeographic evidence, scholars suggest that the genus Spanish mackerel originated from the ancient tropical shallow sea environment and spread to the world with the formation of marine channels and climatic events. Spanish mackerel is now particularly diverse in western Indo-Pacific, such as the Southeast Asia-Western Pacific Ocean region, which has gathered multiple species and is believed to be the center of origin of the genus. During the Cenozoic period, the closure of the Tetis Sea and the settlement of the Sunda ancient continent created vast tropical shallow seas such as the Indo-Taiwan Warm Pond, providing a suitable environment for the early evolution of Spanish mackerels. Subsequently, with the sea route between Africa and South American continents, some Indo-Pacific ancestor Spanish mackerels entered the Western Atlantic through the South Atlantic, achieving trans-ocean diffusion. For example, the Atlantic Ocean's king Spanish mackerel (S. cavalla) and the Spanish mackerel (S. maculatus) may have originated from the ancestral populations entering the Atlantic through the Cape of Good Hope in the late Miocene, and are closely related to the narrow-band Spanish mackerel in the Indo-Pacific (Herrera et al., 2015). The molecular phylogenetic tree shows that Atlantic species are phylogenetically embedded between Indo-Pacific species, suggesting that they originate from one or more westward spread events in the Indo-Pacific lineage. Meanwhile, some Spanish mackerel species may spread westward through the ancient Mediterranean-Tettis Sea channel, or cross the Eastern Pacific before and after the closing of the Panama Isthmus. Species existing in the Eastern Pacific (west coast of the Americas), such as S. sierra, are believed to have common origins with the Atlantic species, and their ancestors may have detoured through the southern end of the Central and American isthmus during the Pliocene. In addition, when sea levels in the Pleistocene ice age fell, the distribution of some nearshore species was limited by the shelf exposure, and the rise in the late ice age caused its re-expansion. 3 The Impact of Paleogeography and Climate Change on the Origin of Spanish mackerel 3.1 Shaping of distribution during the quaternary ice age and sea level changes The repeated ice-interglacial cycles during the Quaternary period had a profound impact on the geographical distribution of marine organisms. For species like Spanish mackerels that mainly live in shallow coastal coastal seas, a significant drop in sea levels during the glacial period has significantly changed their habitat area and connectivity. During the last glacier period (about 20 000 years ago), sea level fell by about 120 meters from today, with a large area of continental shelf exposed, and many of the now-separated sea areas were once connected (Schuerch, 2017). This environmental change may shrink the Spanish mackerel's habitat range and lead to population isolation. For example, the Sunda Shelf of Southeast Asia was connected to large land during the ice age, and the area of shallow waters such as the South China Sea and Java Sea reduced, which led to the differentiation of some nearshore populations. However, the rapid sea level rise after ice (the so-called “Flandrian transgression”) reconnects the previously isolated shallow sea areas, providing opportunities for Spanish mackerel population spread and genetic exchange (Figure 1) (González‐Wevar et al., 2023). The genetic diversity analysis of mitochondrial DNA supports this: For example, the proportion of samples sharing haplotypes in various places such as the Brazilian coastal Spanish mackerel (Brazilian Spanish mackerel S. brasiliensis) has a high proportion of samples and extremely weak genetic structure. It is inferred that it experienced population expansion about 10 000 years ago. This coincides with the time when seawater rises and re-expands habitat during the last period of ice elimination. Changes in the paleomarine environment not only affect horizontal distribution, but also vertical distribution. During the ice age, sea water temperature drops, and warm-water Spanish mackerels may migrate south to avoid cold. After the interglacial period, they will go north to occupy high-latitude waters again. This climate-driven distribution recombination has shaped the genetic pattern and adaptability of populations in various regional areas of modern Spanish mackerels (Oosting et al., 2022). Therefore, the fluctuations in the Quaternary climate and the rise and fall of sea level during the ice age are important background factors for understanding the history of the origin and diffusion of Spanish mackerel. 3.2 Changes in connectivity between current systems and habitats Marine circulation plays a media role in biological diffusion and gene exchange, and its historical changes have also affected the diffusion path and population connection of Spanish mackerel. During the Miocene-Pliocene,
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