EWHA's Research Power for Science & Engineering
February, 2018
EWHA's Research Power for Humanities, Arts & Social Sciences

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Phylogeography of Marine Species Impacted by the Yangtze River Outflow

 

               
by Prof. Joong-Ki Park  (jkpark@ewha.ac.kr)

Division of EcoScience

 

A recent study by Ewha research team, led by Prof. Park, shows how the largest river in Asia impacts the distribution of marine animals. The study, which was published in Molecular Ecology, analyzes data from existing publications and finds that the enormous current of freshwater outflow from the Yangtze River has often been claimed in scientific literature as the primary factor responsible for genetic population differences between marine animals on the north and south sides of the river’s mouth in the East China Sea. The Yangtze River propels a plume of fresh water many kilometers into the salty sea, as well as depositing a large bank of muddy sediment on the ocean floor, and so it is not surprising that earlier scientists should have assumed this fresh, muddy outflow was the main factor preventing species that are exclusively adapted to salt water or rocky substrates from crossing to the other side of the river’s mouth. However, the Ewha research team (Drs. Gang Ni, Elizabeth Kern and Prof. Park), determined that the story may be more complicated, and that in many cases large genetic differences between northern and southern populations are actually more likely due to other factors. This paper draws attention to how the paleogeographic history of the Yangtze River and sea level changes over past glacial periods suggest that populations have not been isolated by the river’s outflow long enough to create the deepest genetic divides reported in the literature. Rather, other factors like historical isolation in separate sea basins during the ice age are more likely to have caused a strong genetic divergence. On the other hand, the research team concluded that smaller genetic differences-which were also often seen in previously published studies-are probably indeed created and maintained by the barrier effect of the river’s freshwater outflow. In many of the studies analyzed, the authors noted that there is a shallow but meaningful level of genetic divergence between northern and southern populations in the East China Sea for animals like molluscs, fish, and crustaceans. This genetic divergence essentially suggests that the river’s continual outpouring of low-salinity water into the sea has deterred populations of some marine populations from mixing their genes with one another. Such isolation can be the first step towards speciation, and the fact that this shallow divergence was observed in many diverse species, from free-swimming fish to slow moving snails, offers an important insight into the generality of such phylogeographic barriers and their impact on species distribution, dispersal, and evolution.

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Figure 1. Map of East Asia showing seafloor topography (based on Schlitzer 2015, Ocean Data View.) and ecological factors (based on Johnson & Boyer 2015, East Asian Seas regional climatology (v. 2). National Centers for Environmental Information, NOAA). (a) The Yangtze River and the marginal seas of the northwestern Pacific. The dotted line separates two biogeographic regions. Four locations mentioned in the study are also marked. (b) Annual salinity (in practical salinity units, PSU) and (c) annual temperature (°C) at the surface (ten-degree grid).

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Figure 2. Representative cases of the impact of the Yangtze River outflow on coastal marine species. (a) The outflow as a biogeographic barrier for Nipponacmea (redrawn based on Yu et al. 2014, J Molluscan Studies, 80:420429.); (b) deep genetic divergence in Coelomactra antiquata potentially resulting from the historical separation of sea basins, rather than river outflow (based on Kong & Li 2009, Marine Biology, 156:15071515.); (c) shallow genetic divergence in Cellana toreuma resulting from Yangtze River outflow (based on Dong et al. 2012, PLoS ONE, 7:e36178.). Populations north of the Yangtze River are shown in red; southern ones are in blue.

 

 

* Related Article
Ni, G., Kern, E., Dong, Y. W., Li, Q., & Park, J. K., More than meets the eye: the barrier effect of the Yangtze River outflow. Molecular ecology. Volume 26, Issue 18, September 2017, Pages 4591–4602 




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