Xenocypris davidi is a benthic fish species widely distributed in the water systems south of the Yellow River in China, playing a significant role in aquatic ecosystems. Despite its ecological and economic importance, genomic resources for X. davidi are limited, hindering a comprehensive understanding of its evolutionary adaptations and genetic improvements. This study presents the first chromosome-level genome assembly of X. davidi, utilizing PacBio long-reads, Illumina short reads, and Hi-C sequencing data. The genome assembly spans 1.05 Gb with a scaffold N50 length of 33.99 Mb, and 95.12% of the genome sequence was successfully anchored onto 24 pseudochromosomes. We identified 27,360 protein-coding genes, of which 26,672 were functionally annotated. This genome sequence provides a valuable resource for exploring the molecular basis of agronomic traits in X. davidi and will facilitate its genetic enhancement.

Background/Objectives: Liobagrus huaiheensis, an endemic fish in the Huaihe River basin, is a newly described species with limited molecular genetic research, hindering understanding of its evolutionary status, population structure, and genetic diversity. This study aimed to characterize its complete mitochondrial genome, clarify its phylogenetic position within Liobagrus, and assess its population genetic diversity. Methods: We obtained the complete mitogenome of L. huaiheensis (sourced from the Zhugan River) through sequencing, followed by detailed annotation of this genomic sequence. We analyzed its genomic structure, nucleotide composition, codon usage, and base asymmetry. Selection pressure on 13 protein-coding genes (PCGs) was evaluated using Ka/Ks ratios. Phylogenetic trees were generated by means of Bayesian inference (BI) and maximum likelihood (ML), using a dataset composed of 13 protein-coding genes (PCGs) from 37 species. Population genetic diversity was assessed using the cox1 gene. Results: The mitogenome is a 16,512 bp circular molecule encoding 37 genes and one control region, with a conserved structure typical of Liobagrus. It has high A + T content (55.74%) with A-preference and C-enrichment. All PCGs undergo purifying selection (Ka/Ks < 1). Phylogenetic analyses revealed L. huaiheensis is closest to L. obesus (100% support), with Liobagrus divided into three clades. The cox1 gene analysis showed low diversity (Hd = 0.656, π = 0.00171) and neutral evolution. Conclusions: This study fills the mitogenome data gap for L. huaiheensis, clarifies its evolutionary characteristics and phylogenetic position, and provides a basis for conservation genetics of Huaihe endemic fishes and molecular evolution research on Amblycipitidae.

Climate change has led to increasingly frequent and unpredictable droughts and high-temperature events, creating extreme conditions that profoundly impact the productivity of freshwater ecosystems. This study investigates the Nanwan Reservoir, a large and deep reservoir in Xinyang, China, to evaluate the effects of extreme drought events on water quality and phytoplankton production. Field investigations were conducted during both high-temperature and drought (HTD) conditions in 2019, and during non-high-temperature and drought (NHTD) conditions in 2020. Results showed that HTD conditions significantly disrupted the thermocline and oxycline structures, leading to prolonged stratification during the HTD period. Although phosphorus concentrations remained relatively stable across both periods, nitrogen levels were markedly lower during HTD events—likely due to uptake by phytoplankton—indicating a shift in nutrient limitation from phosphorus to nitrogen. Additionally, a complex relationship between environmental variables and phytoplankton biomass was observed under HTD conditions. These findings advance our understanding of primary production responses to extreme weather events in the Nanwan Reservoir and highlight the importance of incorporating such knowledge into water resource management and ecological conservation strategies.

Background/Objectives: The monotypic genus Araiocypris is currently classified in Xenocyprididae, yet its familial placement has long been controversial and lacks sufficient molecular phylogenetic support. Mitogenomic resources for the family Tanichthyidae remain limited, and the phylogenetic relationship between Araiocypris and tanichthyid species remains unexplored. This study was conducted to re-evaluate the taxonomic status of Araiocypris and characterize the mitogenomic evolution of Tanichthyidae. Methods: The complete mitochondrial genomes of Tanichthys huidongensis and Araiocypris batodes were sequenced, assembled and annotated. Comparative mitogenomic analyses were performed across seven Tanichthyidae species. A maximum-likelihood phylogenetic tree was reconstructed using concatenated sequences of 13 protein-coding genes and 2 ribosomal RNA genes to clarify the phylogenetic position of Araiocypris. Results: Based on mitochondrial phylogenomic evidence, phylogenetic reconstruction strongly supports the reassignment of A. batodes from Xenocyprididae to Tanichthyidae, as it nests within the Tanichthyidae clade and forms a sister group with the genus Tanichthys. Genetic divergence analysis confirms the validity of Araiocypris as a distinct genus. Tanichthyidae mitogenomes display highly conserved architecture with the typical vertebrate gene order. All protein-coding genes undergo strong purifying selection, with cox1 showing the slowest evolutionary rate. Conserved structural domains including CSB-F, CSB-E, CSB-D and CSB1–3 were identified in the control region. Conclusions: This study provides solid mitogenomic evidence for the familial reclassification of Araiocypris and systematically reveals the evolutionary patterns of Tanichthyidae mitogenomes, offering an important reference for further taxonomic and phylogenetic studies of Cypriniformes.

Climate change has led to increasingly frequent and unpredictable droughts and high-temperature events, creating extreme conditions that profoundly impact the productivity of freshwater ecosystems. In this study, we evaluated the effects of extreme temperature and drought events on Nanwan Reservoir, a large, deep body of water in Xinyang, China, by assessing water quality and phytoplankton biomass. Field investigations were conducted during both high-temperature and drought (HTD) conditions in 2019 and normal-temperature and non-drought (NTND) conditions in 2020. HTD conditions significantly disrupted the thermocline and oxycline structures, leading to prolonged stratification during this period. Although phosphorus concentrations remained relatively stable across both periods, nitrogen levels were markedly lower during HTD events, indicating a possible shift in nutrient limitation from phosphorus to nitrogen. Additionally, a complex relationship between environmental variables and phytoplankton biomass was observed under HTD conditions. These findings advance our understanding of primary production responses to extreme weather events in Nanwan Reservoir, highlighting the importance of incorporating this knowledge into water resource management and ecological conservation strategies.