Studies On The Genetic Diversity And Epibiota Of Pelagic Sargassum Horneri In The Yellow Sea And East China Sea

Since 2000,massive floating Sargassum horneri bloomed in the Yellow Sea（YS）and East China Sea（ECS）with increasing distribution and coverage in recent years.Large amount of floating S.horneri inundated the coastal waters forming“golden tides”,which caused serious losses in the aquaculture industry and threatened the safety of marine ecosystem and sustainable marine economy.The origin of the floating S.horneri and the blooming mechanism were the key questions for the scientific research.Since 2016,intrusion of S.horneri into the Subei Shoal region formed the unique bi-macroalgal blooms with the floating Ulva prolifera every spring.This phenomenon caused serious difficulties and confusions on the monitoring and prevention of the green tides.There are several key questions related to the source prevention of the green tides:what is the temporal and spatial distribution pattern of the floating S.horneri and U.prolifera,what’s the relationship between these two sympatric species.Field survey and sampling were conducted throughout the distributional range of the pelagic S.horneri in YS and ECS.The morphology and genetic diversity were compared between the floating and attached S.horneri in order to investigate the potential contribution from the sessile population in Changdao area.The species composition,distribution and abundance of sympatric and epiphytic macroalgae,and the epizoons as well,were also examined to study the relationship and functions of these epifauna with the pelagic S.horneri.This study has not only assisted the monitoring and prevention of the U.prolifera green tides,but also provided basic data for the further research on the blooming mechanism of S.horneri golden tides.The main research results are as follows:Studies on the genetic diversity and epibiota of pelagic Sargassum horneri in the Yellow Sea and East China1.Study on the morphology and genetic diversity of S.horneri in the Yellow Sea:Comparative study was conducted on the morphology and genetic differences between floating S.horneri from YS and attached populations from Changdao area during November of 2020 to June of 2021.The morphology of floating and attached S.horneri had distinct seasonal patterns.In November,there was no significant difference of the leaves and gas-filled vesicles between the floating and attached samples.From April to June,they had obvious morphological differentiation,such as leaves and vesicles of the attached S.horneri were growing while those of the floating samples tended to decrease.The receptacles of the floating S.horneri was obviously less and slimmer than that of the attached ones.Meanwhile,one nuclear（ITS2）and 4mitchondrial gene fragments（cox3,rnl-atp9,rpl5-rps3,cob-cox2）were selected to investigate the genetic diversity among these floating and attached S.horneri samples.The results showed that there were 2 polymorphic loci and 5 genotypes detected from ITS2 sequences,9 polymorphic loci and 5 haplotypes on the mitchondrial genes in these samples.The inter-population genetic distances among the geographic samples and between floating and attached samples were not significantly different with those intra-population genetic distances,which suggested these samples could be from the same genetic population.2.Distribution and diversity of the sympatric and epiphytic macroalgae of the floating S.horneri in the YS and ECS:Field survey and remote-sensing analysis were performed in the main blooming region in YS and ECS during April to June of2022.The study showed that both sympatric and epiphytic macroalgae commonly coexisted with the pelagic S.horneri and showed distinct distribution patterns.The sympatric green macroalgae,primarily U.prolifera,were initiated from the nearshore water of Jiangsu in late April followed by a rapid accumulation and expansion as the green tide developed.They were mostly restricted in the western coast of southern YS with biomass decreasing evidently toward offshore.There was no U.prolifera attached on S.horneri.The mixing of these two species in the green-tide blooming region was mostly driven by physical forces,such as wind trajectory and water circulation.Unlike the sympatric green macroalgae,the epiphytic brown algae were commonly distributed throughout the distributional range of the floating S.horneri.The percentage of S.horneri samples with epiphytes reached 80.00%,and the average abundance was 0.25±0.17 g _Epi/g _Sar.There were four epiphytes in Phaeophyta attached to the pelagic S.horneri,and Proselachista taeniiformis was dominant.The abundant epiphytes played an important ecological role in the floating micro-ecosystem formed by these S.horneri,which deserves further investigation.3.Species composition and tempo-spatial distribution of epizoans of the floating S.horneri in YS and ECS:The samples of floating S.horneri from YS and ECS collected in April to June of 2022 were screened and observed under microscope.A total of 7 epizoans were detected,including 4 species of Osteichthyes:Neosalanx anderssoni,Lateolabrax japonicus,Planiliza haematocheilus and Hyporhamphus sajori;3 species of Malacostraca:Sunamphitoe tea,Apohyale sp.and Idotea metallica.S.tea was dominant,and observed in ECS,Subei Shoal and Jiangsu coastal waters.S.tea was accounted for 75.00%of the total epizoans found,and the average abundance was 1.17 ind./plant.In RD and JS,the species composition of epizoans varied significantly from April to June of 2022.The average abundance of the dominant species S.tea increased from 1.00 ind./plant in April and May to 1.43 ind./plant in June.