Identification And Diversity Analysis Of Ichthyoplankton In The Laibin Section Of The Xijiang River Based On DNA Barcoding

The Xijiang River is the mainstream of the Pearl River with rich fish resources and diversified water ecological environment.Fish resources have been damaged to some extent due to the construction of the terraced hydraulic hubs and environmental pollution as well as overfishing,leading to the disappearance of some fish spawning grounds and a decrease in ichthyoplankton.This study set up two sampling sites in the lower reaches of the Liujiang River and the Hongshuihe River in the Laibin section of the Xijiang River from April to August 2021.The ichthyoplankton was collected regularly and environmental factors were measured.Using DNA barcode method for species identification,we aimed to study the species composition,diversity status,and relationship between the ichthyoplankton and environmental factors in the Laibin section of the Xijiang River.The main results are as follows:（1）A total of 158 samples were selected from Liujiang River sampling sites for DNA extraction,and 118 valid sequences with an average length of 648 bp were finally sequenced.According to BLAST matching in the Gen Bank database,they belonged to 3 orders,6 families,and 18 species.Including 10 species of fish eggs and15 species of larval fish.The samples of eggs and larval fish obtained were dominated by species that produce drifting eggs.The dominant and common species are small fish such as Squalidus argentatus and Sinogastromyzon wui.（2）A total of 86 samples were selected from Hongshsuihe River sampling sites for DNA extraction,and 69 valid sequences with an average length of 648 bp were finally sequenced.According to BLAST matching in the Gen Bank database,they belonged to 3 orders,6 families,and 17 species.Including 14 species of fish eggs and 8 species of larval fish.The samples of eggs and larval fish obtained were dominated by species that produce drifting eggs.The dominant and common species are small fish such as Squalidus argentatus and Pseudohemiculter dispar.（3）The quantity of ichthyoplankton was mainly concentrated in the flood period,and the number of species was higher in the flood period than in the pre-flood and post-flood period in both the Liujiang and Hongshuihe River,with significant differences（P<0.05）.The species density during the flood period was higher than that in the pre-flood and post-flood period,but there was no significant difference（P>0.05）.The Margalef richness index(D_Ma)and biodiversity index（H’）during the flood period showed significant differences compared to the other two periods（P<0.05）.However,there was no significant difference in the uniformity index（J）of the Liujiang River among the three periods（P>0.05）.（4）The results of Principal Component Analysis（PCA）and Redundancy Analysis（RDA）showed that the environmental factors that had a significant impact on the community structure of ichthyoplanktons in the two river sections were transparency,water level,and discharge.The results of NMDS and ANOSIM analysis indicated that the species composition of the Liujiang River during the flood period showed significant differences compared to the species composition in the pre-flood and post-flood period（P<0.05）.However,in the Hongshuihe River,the impact of rising water on species composition changes was not significant（P>0.05）.（5）Mitochondrial whole genome sequencing analysis of a fish species in the Xijiang River,Pseudohaemiculter hainanensis,which was not recorded in the Gen Bank,revealed that the full-length mitochondrial genome of P.hainanensis was16,647 bp,showing higher A+T content than G+C content.An NJ phylogenetic tree was constructed by combining 10 species from the Culterinae.The phylogenetic tree showed that P.hainanensis formed a separate branch and was in a subclade with Toxabramis houdemeri.