| Ophiuroids are important echinoderms,commonly occurring in many marine benthic ecosystems.Ophiura sarsii is a common circumpolar species in the North Pacific,Arctic waters,and Atlantic Ocean.Their subspecies,O.sarsii vadicola,is a dominant benthic species in the Sea of Japan and the Yellow Sea Cold Water Mass(YSCWM).O.sarsii vadicola lives at the southernmost boundary of O.sarsii distribution,33°N.Although the significant morphological divergence on radial shield and comb papillae were well studied between typical species and subspecies,it is still unclear about their genetic relationships and differences on the population structure between them.Compared with the well understandings of O.sarsii populations from the Sea of Japan,Chukchi Sea,and Atlantic waters,little is known about the current population structure of O.sarsii vadicola.In this study,the distribution,size,and age structure of the O.sarsii vadicola population from the Yellow Sea were described to understand their population structure.Combined with the existing studies of O.sarsii,the genetic diversity and structure for O.sarsii and O.sarsii vadicola were revealed by comparisons among populations using mitochondrial cytochrome c oxidase subunit I gene(CO?),nuclear gene Internal transcribed Spacer 2(ITS2),and single nucleotide polymorphism(SNPs).Based on the above content,this study revealed the population structure and evolution history of O.sarsii vadicola.The results are as follows:(1)Population structure of O.sarsii vadicola in the Yellow SeaBased on the 18 surveys from 1958 to 2019,the major distribution range of O.sarsii vadicola in the Yellow Sea was 34°–39°N,122°–124°E.The distribution of minority was shown in spots outside the range of the YSCWM.Individuals could reach the northernmost at 39°N,the southernmost at 32°N,the westernmost at 120°E,and the easternmost at 125°E.The distribution of O.sarsii vadicola was related to the seasonal changes of temperature in the Yellow Sea Bottom Water.There was a reduced phenomenon for distribution of O.sarsii vadicola individuals in the South Yellow Sea in fall.According to the survey on the spring and summer in 2018 and 2019,the maximum density of O.sarsii vadicola population occurred in summer 2018,reached 331 ind./m2.In 2018,the average density for individuals in the northern Yellow Sea was respectively 83 ind./m2 in spring,73 ind./m2 in summer,higher than individuals from the southern Yellow Sea.The average density for individuals collected in spring 2019 was reached to 47 ind./m2.The regression analysis was in a great correlation between wet weight,dried weight,and ash-fresh dried weight.The average biomass and ash-free dried weight in 2018 spring and summer were respectively 10.84 g/m2 and 12.36 g/m2,0.034 g/m2 and 0.032 g/m2.The secondary production of populations from the south and north Yellow Sea were 0.67 g/m2 a and 1.26 g/m2 a?Based on the samples from 13 stations in the northern and southern Yellow Sea in spring and summer 2018,the distribution of body disc was analyzed for this population.The disc size of O.sarsii vadicola population ranged 1.00–17.55 mm.In the spring,the average disc size of individuals was in 7.00 mm.In the summer,the average disc size was 9.77 mm for individuals from the southern Yellow Sea,whereas it was only 7.55 mm for those from the northern Yellow Sea.Besides,the age correction was conducted for 186 individuals from the Yellow Sea in the spring and summer 2018 and 2019.A maximum of 9-year extra age was added to the final age of individuals.Based on the corrected age dataset,Size-at-Age(SAD)analysis indicated that the age of individuals with 3–17 mm disc was 2–16 years.Based on the Special von Bertalanffy and Gompertz model,the growth curve for the Yellow Sea population of O.sarsii vadicola fitted as Dt=27.02(1-e-0.058t)and Dt=22.35e-e-0.124(t-6.226),in which the maximum disc size may be 22.35 mm or 27.02 mm.According to the good-fit Gompertz growth model,the age structure was predicted for the population with size structure analysis in spring and summer 2018.The average age for northern individuals was 5 year and the maximum age was 15 year.However,for the southern individuals,the average age increased from spring(5 year)to summer(7 year)and the maximum age was 17 year.(2)The genetic divergence between O.sarsii vadicola and O.sarsii based on DNA barcodesThe population genetic analysis was conducted for 8 geographic population of O.sarsii and O.sarsii vadicola by using mitochondrial gene CO? and nuclear gene ITS2.The results showed that the haplotype of populations from different localities had high diversity,which could separate into 3 mitochondrial lineages: O.sarsii vadicola populations from Yellow Sea,North Pacific–Arctic O.sarsii populations,and Arctic–Atlantic O.sarsii populations.The mitochondrial genetic structure and divergence clearly showed the genetic divergence between subspecies and typical species reached 13.5–15.2%,higher than the internal divergence within O.sarsii(0–7.4%),even though there were only two different sites in the ITS2 fragment from subspecies and typical species.Based on the Iterative Calibration method,the divergence between O.sarsii and subspecies could happen at 3.6 Ma(late Pliocene)in the North Pacific Ocean.O.sarsii may have dispersed from the Pacific to the Atlantic Ocean since early Pleistocene(1.5–1.2 Ma).The expansion of the subspecies was likely coeval with transgressions events in the Yellow Sea during the late Pleistocene period(111.8–60.1 ka).(3)The genetic differentiation between O.sarsii vadicola and O.sarsii populations based on the 2b–RAD sequencing dataThe genetic structure was also studied for the O.sarsii vadicola populations from the Yellow Sea and Sea of Japan and O.sarsii populations from the Sea of Japan,Bering Sea,and Barents Sea.Combined with the analysis of phylogenetic and genetic differentiation based on 2b–RAD sequencing data,3 subgroups were determined as O.sarsii vadicola subgroups,Pacific–Arctic O.sarsii subgroups,and Atlantic O.sarsii subgroups.Within 2 O.sarsii vadicola population,there were no divergence among individuals.Moderated differentiation was founded between populations from the Sea of Japan and Bering Sea.Besides,Barents Sea population highly differentiated with North Pacific–Arctic subgroups.According to the preliminary results of mitochondrial gene and nuclear genes,O.sarsii should be a complex species and the Sea of Japan could be the key zone to generate divergence within species.The internal evolution within this complex species is still happening.The subspecies O.sarsii vadicola is in a highly differentiation level underlying the historical changes,while the divergence process gradually occurs between the North Pacific-Arctic and Atlantic subgroups. |
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