The ichthyofauna of continental islands is characterized by immigration through a land bridge due to fluctuating sea levels. Hainan Island is adjacent to the southern margin of mainland China and provides opportunities for understanding the origin and diversification of freshwater fishes. The aim of our study was to evaluate the level of genetic variation and phylogeographic structure of Opsariichthys hainanensis on Hainan Island and mainland China, using mtDNA cyt b gene (1140 bp) and D-loop (926 bp), nuclear RAG1 gene (1506 bp), and 12 microsatellite loci. Mitochondrial phylogenetic analysis identified five major lineages according to the geographical distribution from different populations. We suggested that two dispersal events occurred: the population in the Changhua River migrated to the Red River (Lineage B), and the populations in the South Hainan region moved northwards to the North Hainan region. However, populations in Northwest Hainan Island dispersed to the populations around the Gulf of Tonkin (Lineage A1) and populations in Northeast Hainan Island dispersed to the populations in mainland China (Lineage A2). Our results indicated that the populations of O. hainanensis suffered a bottleneck event followed by a recent population expansion supported by the ABC analysis. We suggest that O. hainanensis populations were found mostly in the lowlands and a lack of suitable freshwater habitat in southern mainland China and Hainan during the Last Interglacial period, and then expansion occurred during the Last Glacial Maximum.

The barbel chub Squaliobarbus curriculus, is an economically important freshwater fish in China. The fishery production of the wild populations has declined dramatically, making the development of aquaculture urgently needed. However, the lack of high-quality genome has impeded its artificial breeding and genetic breeding. Herein, we present a chromosome-level genome assembly for S. curriculus by combining HiFi sequencing, Hi-C sequencing, Iso-seq and short-reads RNA-seq data. This assembly was 910.27 Mb in size, with a contig N50 length of 34.70 Mb. 99.50% of the assembled sequences were placed onto 24 chromosomes supported by Hi-C contact map. Using Iso-seq and short-reads RNA-seq data, we identified 28,329 protein-coding genes based on three prediction methods. Of these genes, 27,207 genes (96.04%) were functionally annotated to at least one of the six commonly used databases. Additionally, we annotated 2,041 miRNAs, 16,426 tRNAs, 5,488 rRNAs and 1,536 snRNAs in the S. curriculus genome. Overall, the chromosome-level genome of S. curriculus will provide valuable genomic resources for genetic breeding, population genomics, sex-related marker identifications, and other future studies.

We sequenced the mitochondrial genome of Pseudorasbora interrupta using Illumina technology and additional Sanger sequencing. The assembled 16601 bp mitogenome had a GC content of 40.98% and consisted of 13 protein-coding genes, 22 tRNA genes, two rRNA genes, and one non-coding control region (CR), with a gene order identical to the fishes. In addition, we downloaded the mitogenome of the closely related species Topmouth Gudgeon P. parva. The mitogenomes of P. interrupta and P. parva showed a sequence identity of 99.2% with the previously published P. parva mitogenome. The phylogenetic reconstruction based on 14 Gobioninae mitogenomes supported P. interrupta and its sister species P. parva as a monophyletic group. However, Pseudorasbora was proved to be a polyphyletic group which means amendments will be needed for the taxonomy of this genus.

Macropharyngodon meleagris is a coral reef-dwelling, benthic predatory fish renowned for its striking coloration and distinctive body patterns. It exhibits pronounced sexual dimorphism and is characterized by the presence of prominent canine-like teeth. Here, we employed PacBio HiFi sequencing combined with Hi-C assembly technology to generate a high-quality, chromosome-level genome assembly of M. meleagris. The final assembly spans 666 Mb across 24 chromosomes, with high contiguity (a contig N50 of 20.57 Mb and a scaffold N50 of 29.79 Mb). Approximately 27.63% of the genome is composed of repetitive elements. A total of 21,940 protein-coding genes were predicted, with 98.76% successfully assigned functional annotations. The assembled genome exhibits high completeness (98.7% BUSCO completeness) and accuracy (98.05% for WGS short reads, 99.86% for HiFi long reads and 92.64% for RNAseq reads).

The gold barb (Barbodes semifasciolatus), a member of the Cyprinidae family, exhibits remarkable adaptability to highly acidic environments, making it an ideal model for studying extreme environmental adaptation. However, its genome has not been previously characterized. To address this, we assembled a high-quality chromosome-scale genome for B. semifasciolatus using High-Fidelity (HiFi) sequencing and Hi-C technology. The resulting haplotype-resolved assemblies, spanning 776 Mb and 779 Mb across 25 chromosomes, achieved genome coverages of 99.5% and 99.7%, respectively, and included four gap-free chromosomes. Genome quality assessment using BUSCO indicated a high completeness score of 98.2% for haplotype1 and 98.3% for haplotype2, further validated by strong synteny with the zebrafish (Danio rerio), confirming the assembly’s integrity and continuity. Through integration of full-length transcriptome data, RNA sequencing, and homology-based annotation, we identified 26,057 protein-coding genes with 2,087 pseudogenes in haplotype 2, and 25,622 protein-coding genes with 2,101 pseudogenes in haplotype 1. This high-resolution genome assembly is a crucial resource for advancing research in the Cyprinidae, particularly for understanding adaptive evolution in extreme environments.