Abstract Tidal freshwater reaches may provide early‐life habitat for diadromous fishes, but long‐term evidence of their use remains limited. Size‐resolved records are needed as a reference for monitoring under the Yangtze fishing ban introduced in 2021. We analysed monitoring data from Jingjiang, 240 km upstream of the river mouth (2002–2017; 519 effective surveys; 10,560 measured individuals), combining size‐based catch indices, environmental‐window analyses and otolith Sr/Ca profiles from 115 Japanese grenadier anchovy Coilia nasus . Individuals <120 mm standard length were classified operationally as juveniles during August–October, when scale readings supported an age‐0 interpretation, whereas individuals ≥180 mm were treated as a descriptive large‐fish category throughout the year. Juveniles occurred in every survey year; annulus‐free individuals also occurred in winter and spring. The Juvenile Recruitment Index (JRI) showed negative associations with tidal range across the April–July spawning and larval‐transport period, although none of the 72 environmental‐window correlations remained significant after Benjamini–Hochberg correction at a 5% false‐discovery rate. Monthly models also showed a negative lagged association. None of the 40 autumn recruits sampled across four capture years had an accepted elevated Sr/Ca zone; such zones were more frequent in later developmental groups. Total catch obscured contrasting size structures: 2016 combined the series‐high JRI with few large fish, whereas 2010 combined the series‐high large‐fish catch rate with a mid‐range JRI. These findings document recurrent early‐life use of a tidal freshwater reach and provide a size‐resolved pre‐ban reference, without establishing a disproportionate contribution to adult production.
Abstract Estuarine wetlands are critical zones of land–sea interaction that play vital roles in maintaining biodiversity and ecosystem functioning. As the only remaining natural estuarine wetland in the Yangtze River Estuary, Jiuduansha's fish and macrobenthic diversity and community assembly mechanisms remain poorly understood due to limitations of traditional survey methods in protected areas. We conducted eDNA metabarcoding surveys at 11 sampling sites (6 intertidal zone and 5 riverine sites) in the Jiuduansha Wetland Nature Reserve during May and July of 2023–2024, combined with measurement of environmental variables, to systematically analyse the mechanisms of species composition, diversity patterns and community assembly of the Jiuduansha Wetland. A total of 64 fish species and 35 macrobenthos were detected. The eDNA approach identified approximately 40% more species than historical traditional surveys. Spatially, fish diversity was higher in riverine sites (average 18.6 ± 2.7 species/site), while macrobenthos were more concentrated in the intertidal zone (average 14.2 ± 2.5 species/site). Seasonal patterns differed: fish species richness was higher in May, whereas macrobenthos peaked in July. Generalized linear models (GLMs) indicated that total nitrogen (TN) and chemical oxygen demand (COD) were key environmental factors that have negative influence on both communities' richness. Beta diversity decomposition showed that species turnover was the dominant component of community variation within the study scale (70.7%–70.9% for fish, 60.0%–69.3% for macrobenthos). Neutral community model (NCM) fitting revealed increased migration rates, with Nm values increased from 4 in 2023 to 17 in 2024 for fish and from 9 to 18 for macrobenthos, based on samples collected in May and July of each year. This indicates enhanced dispersal between communities. Both βNTI analysis (|βNTI| < 2) and the RCbray index (with >90% contribution of ecological drift) demonstrated that stochastic processes, particularly ecological drift, dominated community assembly.
Variation in fish body morphology represents an important aspect of phenotypic diversity and plays a fundamental role in understanding fish evolution and ecology. The complex oceanographic conditions of the Northwest Pacific Ocean create diverse selective pressures, making it an ideal system for studying how environmental variability shapes fish body morphology. While previous macroecological studies have linked fish morphology to environmental factors, the driving factors of regional morphological differences were seldom explored. In this study, morphological variations among different body regions (entire body, head, torso and tail) of 22 fish species from the Northwest Pacific Ocean were analysed to explore the phylogenetic and ecological drivers of these variations with geometric morphometrics (GMs) in a phylogenetic framework. The results showed significant morphological differences among orders and species. Entire body, head, torso and tail exhibited low to moderate phylogenetic signals. Ecological factors, including latitudinal range, temperature preference, fecundity and trophic level, significantly shaped morphological variations, thus emphasizing the complex interplay between ecological and evolutionary pressures. We confirmed that different body regions are subject to distinct selective pressures, reflecting the combined influence of genetic and environmental factors. Overall, this study highlights the combined influence of phylogenetic history and ecological pressures in shaping fish morphology. These findings improve understanding of fish evolutionary biology and provide a foundation for conservation and species management in the Northwest Pacific.
Acrossocheilus fasciatus is an economically important freshwater fish species in China. In recent years, Citrobacter freundii infection has caused significant harm to the aquaculture industry. In this study, we performed histological observation and RNA-seq analysis of the intestines and spleens of control (CK) and C. freundii infected A. fasciatus (EG). The results showed that infected fish exhibited abdominal haemorrhage, intestinal villous shedding, hydropic degeneration of hepatocytes and splenic sinus dilation with congestion. RNA-seq analysis identified 3054 DEGs in the intestine and 7706 DEGs in the spleen, with 1247 overlapping DEGs identified by combined analysis. A substantial portion of these genes clustered within biological activities linked to immunity and metabolic regulation, particularly engaging immune-related routes such as endoplasmic reticulum-based protein maturation, alongside the complement system and coagulation signalling cascades. This study systematically reveals the synergy and division of labor between the intestine and spleen during C. freundii infection, providing fundamental data for understanding the immune response mechanisms of A. fasciatus and a theoretical basis for optimising disease prevention and control strategies.
The Cape galaxias, formerly treated as the single African representative of the family Galaxiidae under the name Galaxias zebratus (Castelnau, 1861), has long masked substantial hidden diversity. We consolidated all available molecular data and generated additional sequences to fill sampling gaps, providing the most comprehensive assessment of historically isolated lineages within this complex to date. Analysis of mitochondrial cyt b and COI sequences identified 15 genetically distinct lineages, most with restricted geographic ranges. Previous phylogenetic studies demonstrated that the African galaxiids are not closely related to Galaxias truttaceus Valenciennes, 1846, the type species of Galaxias, suggesting they may be misclassified. Detailed morphological and osteological examinations identified consistent diagnostic characters separating the Cape galaxiids from Galaxias and other galaxiid genera. Molecular and morphological evidence supports reinstatement of the genus Agalaxis Scott, 1936, for the African galaxiids. We further revalidate Agalaxis punctifer (Castelnau, 1861) and A. dubius (Gilchrist & Thompson, 1917), describe A. peninsulae sp. nov. and designate a neotype for and redescribe A. zebratus s.s. By resolving the application of available names, we establish a taxonomic framework for resolving the remaining undescribed diversity and recognise the Cape galaxiids as a diverse endemic radiation within the Cape Fold Ecoregion.
