South China presents an excellent opportunity to build a phylogeographic paradigm for complex geological history, including mountain lifting, climate change, and river capture/reversal events. The phylogeography of cyprinids, particularly Opsariichthys hainanensis, an endemic species restricted to South China, was examined to explore the relationship between the populations in Red River, Hainan Island and its adjacent mainland China. A total of 37 haplotypes were genotyped for the mitochondrial cytochrome b (Cyt b) gene in 115 specimens from 11 river systems. Relatively high levels of haplotype diversity (h = 0.946) and low levels of nucleotide diversity (π = 0.014) were detected in O. hainanensis. Four major phylogenetic haplotype groups revealed a relationship between phylogeny and geography. Our results found that (i) the ancestral populations of O. hainanensis were distributed south of the Wuzhishan and Yinggeling mountains, including the Changhua River on Hainan Island, and then spread to the surrounding areas, (ii) the admixtures within lineages occurred between the Red River in North Vietnam and the Changhua River in western Hainan Island and (iii) indicated that the exposure of straits and shelves under water retreat, provides opportunities for population dispersion during glaciations.

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 yellow pond turtle (Mauremys mutica) is widely cultured for food, medicine and for keeping as pets in China. The high risk of disease outbreaks and mortality caused by sudden sharp temperature drop events has significant negative effects on the aquaculture industry of M. mutica. However, the mechanism underlying the damage caused by cold stress is still unclear. To fill this gap, we performed transcriptome sequencing of M. mutica samples collected at three sampling time points (3h, 24h and 48h) during cold treatment, and at a recovery time point (96 h after the end of cold stress). The results showed that immunity and metabolism-related pathways (neuroactive ligand-receptor interactions, cytokine-cytokine receptor interactions and NOD-like receptor signaling pathways) were significantly enriched at both 24h and 48h. The expression of immunity and metabolism-related genes (BS2A1, PIWIL2 and Fads1) significantly decreased at all-time points compared to the control group. These results suggested that impaired immunity and depressed metabolism under cold stress may be the main cause of the massive cold-stress mortality of M. mutica. This study provides novel insights into the molecular mechanisms underlying mass disease and mortality caused by sudden sharp temperature drop events in M. mutica.

Alepes djedaba is a coastal fish species of high economic value. However, information on its early developmental biology remains incomplete, which limits its application in large-scale seed production. In this study, fertilized eggs were obtained through hormone-induced spawning. Under controlled conditions, microscopic observation and histological sectioning were used to systematically record and analyze embryonic development, larval growth characteristics, and the gonadal development potential of the artificially bred first-generation (F1) offspring. The results showed that the fertilized eggs of A. djedaba are spherical and pelagic with a single oil globule. The egg diameter is (672.02 ± 10.03) μm, and the oil globule diameter is (189.76 ± 8.82) μm. At a water temperature of 24.50–24.97 °C, embryonic development is divided into 6 periods (20 stages), taking 19 h 28 min to complete hatching with an average hatching rate of 22.9% ± 15.6%. The total length of newly hatched larvae is (1.43 ± 0.09) mm, with an average daily growth of 0.39 mm from 0 to 21 days post-hatching (DPH). Furthermore, the testes of the F1 generation cultured in captivity exhibit complete tissue structures and mature spermatozoa, while the ovaries show orderly development primarily dominated by stage II–III oocytes with some early stage IV oocytes. This confirms at the histocytological level that the artificially bred offspring possess normal gonadal development, providing strong evidence of its reproductive potential. This study clarifies the key stages of early development and the adaptability of A. djedaba in captive environments, providing critical theoretical support and basic data for subsequent large-scale seed production and larval rearing optimization.