Extinction of one of the world's largest freshwater fishes: Lessons for conserving the endangered Yangtze fauna

Taxonomic diversity, distribution, and ecology of the freshwater fishes of the Zambezian Lowveld Ecoregion in southern Africa: A systematic review

The Zambezian Lowveld Ecoregion (ZLE) is one of the 22 freshwater ecoregions covering southern Africa. This ecoregion covers ~520,418 km2 and extends from south of the Zambezi Delta in the north to the uMngeni River basin in the south. This study aimed to compile a comprehensive synthesis of the available information on the diversity and distribution of freshwater fishes in this ecoregion based on natural history collection holdings by reviewing the published scientific literature. In total, 105 native species belonging to 39 genera and 17 families were recorded from the ZLE. An important proportion, 20 (19%), of the species are considered to be narrow-range endemics. Nevertheless, the majority of them (81%) are considered to have broad geographic ranges, with some even extending into adjacent ecoregions. However, recent and ongoing studies indicate that the current taxonomy often underestimates the species diversity and therefore overestimates the distribution ranges of the latter group of freshwater fishes concerned. The present synthesis brings to the fore existing knowledge gaps in species diversity and distribution ranges of freshwater fishes of this ecoregion and highlights the need for the use of integrative approaches to address the prevailing taxonomic conflicts.

A chromosome-level genome assembly of critically endangered Ochetobius elongatus

Ochetobius elongatus, a critically endangered species found in the Yangtze River was the subject of our study in which we leveraged PacBio and Hi-C data to assemble a chromosome-scale genome. This assembly comprises 24 pseudo-chromosomes, yielding a genome size of 883.1 Mb with a scaffold N50 length of 35.1 Mb, indicative of a highly contiguous assembly. A BUSCO assessment ascertained the comprehensiveness of the genome at 98.3%. Annotation efforts identified 28,674 putative protein-coding genes, with 44.63% of the assembled genome annotated as repetitive sequences. Collinearity analysis between O. elongatus and two other species from the family Xenocyprididae revealed high collinearity, indicating good assembly quality of O. elongatus The completion of the O. elongatus genome assembly in this study signifies a critical advancement for its conservation, enabling deeper insights into its genetic diversity and facilitating the development of targeted preservation strategies.

The Influence of Dietary n-3 Highly Unsaturated Fatty Acids on Growth, Fatty Acid Profile, Lipid Metabolism, Inflammatory Response, and Intestinal Microflora in F2 Generation Female Yangtze Sturgeon (Acipenser dabryanus)

DHA and EPA, as indispensable n-3 highly unsaturated fatty acids (HUFAs), exert a fundamental influence on regulating fish growth, lipid metabolism, and overall well-being. However, there is a notable lack of data concerning their effects on the F2 female generation of Yangtze sturgeon. Over a ten-month period, this study assessed the impacts of various dietary concentrations of n-3 HUFAs (0.5%, 1.0%, 1.5%, 2.0%, and 2.4%) on growth, fatty acid composition, lipid metabolism, inflammatory response, and intestinal microbiota in the F2 female generation of Yangtze sturgeon. Seventy-five test fish, with an average body weight of 3.60 ± 0.83 kg, were housed in 15 ponds, with each dietary group being assigned to three ponds. The results indicated that the 1.0%~1.5% n-3 HUFA group was characterized by the highest values of weight gain rate; serum triglyceride levels peaked in the 0.5% n-3 HUFA group. The fatty acid profiles of the fish tissues closely mirrored those of the diets. Specifically, compared to the 1.5% and 2.0% n-3 HUFA groups, the diet containing 2.4% n-3 HUFA down-regulated the mRNA expression of transforming growth factor beta, and, compared to the 0.5% and 1.0% n-3 HUFA groups, the 2.0% n-3 HUFA diet up-regulated the mRNA expression of nuclear factor kappa B. Conversely, compared to the 0.5% n-3 HUFA group, 2.0% n-3 HUFA in the diet up-regulated the gene mRNA expression of fatty acid binding protein 1 and fatty acid synthase. Compared to the 0.5% n-3 HUFA group, 1.0% n-3 HUFA in the diet up-regulated the gene mRNA expression of lipoprotein lipase. The α-diversity indices (ACE, PD_whole tree, Richness, and Chao1) exhibited an upward trend with increasing dietary n-3 HUFA levels, and the 2.4% n-3 HUFA group reached the highest values. At the phylum level, Fusobacteriota, Proteobacteria, Firmicutes, and Bacteroidota were the primary dominant phyla. Cetobacterium was the dominant genus in all groups. Collectively, these findings underscore that moderate dietary supplementation of n-3 HUFA (1.3%) is optimal and does not impair growth. The deposition of fatty acids in muscle and ovarian tissues, as well as the mRNA expression of lipid-metabolism genes, are closely associated with the dietary n-3 HUFA content. High levels of n-3 HUFA did not suppress intestinal α-diversity. These discoveries provide novel insights into the regulation of growth, lipid metabolism, and health in the F2 female generation of Yangtze sturgeon and offer a nutritional strategy for the artificial conservation of this endangered species.

Inferring species extinction from sighting data

Understanding whether a species still persists, or the timing of its extinction is challenging, however, such knowledge is fundamental for effective species management.For the vast majority of species our understanding of their existence is based solely on sighting data that can range from museum specimens and clear photographs, through vocalisations, to markings and oral accounts.Here we review the methods that have been developed to infer the extinction of species from a sighting record, providing an understanding of their assumptions and applications. We have also produced an RShiny package which can be used to implement some of the methods presented in the article.While there are a number of potential areas that could be further developed, the methods reviewed provide a useful tool for inferring species extinction.

High spatial variability in a species-rich assemblage of diadromous fishes in Hong Kong, southern China

China has experienced substantial coastal reclamation and damming of rivers. These changes have the potential to impact migrations of diadromous fishes between the sea and fresh waters, but the composition of these fishes and the impacts of barriers to their movement in China have received little attention. We inventoried the species composition and distribution of diadromous fishes, and the impacts of barriers on them, in the Hong Kong Special Administrative Region (HKSAR), southern China. Fish assemblages were surveyed using hand-nets, supplemented by cast-netting and single-pass snorkel surveys, in 24 small coastal streams across three regions. Surveys were undertaken on multiple occasions during the wet and dry seasons to account for the monsoonal tropical climate. Twenty-eight diadromous fishes were collected, mostly gobies, amounting to over half (53%) of the total richness of primary freshwater fishes; four additional species are known from literature records. Diadromous richness was 48% greater during the wet season, when all species were encountered. Richness varied substantially among streams, from a maximum of 17 (2 streams that were diversity hot spots) to none (3 streams). The most widespread diadromous fish was Glossogobius giuris (71% frequency of occurrence), followed by Mugil cephalus (58% occurrence) and Eleotris oxycephala (50% occurrence). The remaining 25 diadromous fishes occurred in fewer than half of the streams; 12 species were confined to a single stream and may be locally threatened. There were conspicuous spatial differences in diadromous assemblages across HKSAR, despite its limited extent (1114 km2), the proximity of the surveyed streams, and the broad geographic distribution of most species. Regional species assemblages were influenced by localized habitat characteristics, with a noticeable distinction between areas with and without large, fast-flowing, and highly oxygenated streams. The presence of in-stream barriers (weirs: 0.3-8.7 m high) did not affect spatial patterns in species assemblages, although, on average, diadromous richness was lower in weir-obstructed streams (4.0 vs. 6.9 species in unobstructed streams). In total, 18 species were confined to unobstructed streams or sections below weirs, whereas the remaining 10 species were recorded both above and below weirs. Only the mottled eel (Anguilla marmorata) and a goby (Stiphodon multisquamus) were able to ascend weirs over 2 m. Although at least 400 m of the lower course of each stream was sampled, diadromous fishes were confined to the first 300 m in 12 of the 13 weir-obstructed streams. Remarkably, the tally of 32 diadromous species in HKSAR exceeds the 19 known from mainland China, highlighting the need for further research on composition and conservation status of diadromous fishes.

Freshwater megafauna shape ecosystems and facilitate restoration

Freshwater megafauna, such as sturgeons, giant catfishes, river dolphins, hippopotami, crocodylians, large turtles, and giant salamanders, have experienced severe population declines and range contractions worldwide. Although there is an increasing number of studies investigating the causes of megafauna losses in fresh waters, little attention has been paid to synthesising the impacts of megafauna on the abiotic environment and other organisms in freshwater ecosystems, and hence the consequences of losing these species. This limited understanding may impede the development of policies and actions for their conservation and restoration. In this review, we synthesise how megafauna shape ecological processes in freshwater ecosystems and discuss their potential for enhancing ecosystem restoration. Through activities such as movement, burrowing, and dam and nest building, megafauna have a profound influence on the extent of water bodies, flow dynamics, and the physical structure of shorelines and substrata, increasing habitat heterogeneity. They enhance nutrient cycling within fresh waters, and cross-ecosystem flows of material, through foraging and reproduction activities. Freshwater megafauna are highly connected to other freshwater organisms via direct consumption of species at different trophic levels, indirect trophic cascades, and through their influence on habitat structure. The literature documenting the ecological impacts of freshwater megafauna is not evenly distributed among species, regions, and types of ecological impacts, with a lack of quantitative evidence for large fish, crocodylians, and turtles in the Global South and their impacts on nutrient flows and food-web structure. In addition, population decline, range contraction, and the loss of large individuals have reduced the extent and magnitude of megafaunal impacts in freshwater ecosystems, rendering a posteriori evaluation more difficult. We propose that reinstating freshwater megafauna populations holds the potential for restoring key ecological processes such as disturbances, trophic cascades, and species dispersal, which will, in turn, promote overall biodiversity and enhance nature's contributions to people. Challenges for restoration actions include the shifting baseline syndrome, potential human-megafauna competition for habitats and resources, damage to property, and risk to human life. The current lack of historical baselines for natural distributions and population sizes of freshwater megafauna, their life history, trophic interactions with other freshwater species, and interactions with humans necessitates further investigation. Addressing these knowledge gaps will improve our understanding of the ecological roles of freshwater megafauna and support their full potential for facilitating the development of effective conservation and restoration strategies to achieve the coexistence of humans and megafauna.

Investigation of Sexes and Fertility Potential of Female Russian Sturgeon (Acipenser gueldenstaedtii) and Male American Paddlefish (Polyodon spathula) Hybrids

In the present study, 10 allotriploid (3nALT) and 10 allopentaploid (5nALP) six-month-old hybrid fish and two 3nALT and four 5nALP 40-month-old hybrid fish, which resulted by crossing female Russian sturgeon Acipenser gueldenstaedtii (Brandt and Ratzeberg, 1833) and male American paddlefish Polyodon spathula (Walbaum, 1792), were investigated. It was revealed that six-month-old 3nALT and 5nALP hybrids initially had "undifferentiated" gonads, while in the 40-month-old hybrids, only testes were observed in one case of 3nALT and one case of 5nALP hybrids. The testis of 3nALT hybrids was partially developed with spermatogonia, while the testis of one 5nALP hybrid was in the second developmental stage with low spermatogonia density. We could not determine gonad differentiation in any of the cases when the hybrid individuals had the W sex chromosome. We concluded that the gonad differentiation of these interfamilial hybrids follows a similar pattern to interspecific hybrids of different ploidy parent species of the family Acipenseridae, which is consistent with the classical Haldane's rule. However, it cannot be excluded that the testis of this/these hybrid(s) may produce fertile sperm after sexual maturity, depending on additional genetic, hormonal and environmental factors, and further research is required for its evaluation.

Dams trigger exponential population declines of migratory fish

The impact of dams on global migratory fish stocks is a major challenge and remains seriously underestimated. China has initiated a dozen fish rescue programs for the dams on the Yangtze River, focusing on five flagship species-Chinese sturgeon, Chinese paddlefish, Yangtze sturgeon, Chinese sucker, and Coreius guichenoti. Despite 40 years of effort, these five fishes are on the verge of extinction. Here, we propose an analytical tool that includes a framework of fish migration taxonomy and six life cycle models, the concepts of invalid stock and the dam impact coefficient, and a simplified population model. We then clarify the migration patterns and life cycles of these fishes and show that the Yangtze dams have severely disrupted the life cycle integrity of these species, causing seven types of invalid stocks and their exponential population declines. Last, we discuss six scientific misjudgments underpinning the fish rescue programs and recommend reforms to China's fish rescue strategy.

Chromosome-level genome assembly and population genomic analysis reveal evolution and local adaptation in common hairfin anchovy (Setipinna tenuifilis)

Understanding the genetic structure and the factors associated with adaptive diversity has significant implications for the effective management of wild populations under threat from overfishing and climate change. The common hairfin anchovy (Setipinna tenuifilis) is an economically and ecologically important pelagic fish species, spanning a broad latitudinal gradient along marginal seas of the Northwest Pacific. In this study, we constructed the first reference genome of S. tenuifilis using PacBio long reads and high-resolution chromosome conformation capture (Hi-C) technology. The assembled genome was 798.38 Mb with a contig N50 of 1.43 Mb and a scaffold N50 of 32.42 Mb, which were anchored onto 24 pseudochromosomes. A total of 22,019 genes were functionally annotated, which accounted for 95.27% of the predicted protein-coding genes. Chromosomal collinearity analysis revealed chromosome fusion or fission events in Clupeiformes species. Three genetic groups of S. tenuifilis were revealed along the Chinese coast using restriction site-associated DNA sequencing (RADseq). We investigated the influence of four bioclimatic variables as potential drivers of adaptive divergence in S. tenuifilis, suggesting that these environmental variables, especially sea surface temperature, may play important roles as drivers of spatially varying selection for S. tenuifilis. We also identified candidate functional genes underlying adaptive mechanisms and ecological tradeoffs using redundancy analysis (RDA) and BayeScan analysis. In summary, this study sheds light on the evolution and spatial patterns of genetic variation of S. tenuifilis, providing a valuable genomic resource for further biological and genetic studies on this species and other closely related Clupeiformes.

Global introductions and environmental impacts of freshwater megafish

Freshwater megafish species, such as sturgeons, salmonids, carps, and catfishes, have a maximum reported weight ≥30 kg. Due to their charisma and economic value, they have been widely introduced outside of their native ranges. Here, we provide a comprehensive overview of the introduction of freshwater megafish and an assessment of their environmental impacts. Of the 134 extant freshwater megafish species, 46% have been introduced to new environments, and of these, 69% have established self-sustaining alien populations. These introductions affect 59% of the world's main basins, with the USA and western Europe being particular hotspots of megafish introductions. The common carp (Cyprinus carpio) is the most widely introduced species. Using the Environmental Impact Classification for Alien Taxa (EICAT and EICAT+) frameworks, we assessed the severity and type of negative and positive impacts posed by alien megafish on native species. Alien megafish caused negative impacts through nine different mechanisms, with predation being the most frequently reported mechanism, followed by herbivory and competition. Moreover, 58% of the alien megafish species with sufficient data to evaluate the severity of their impacts caused declining populations of native species, or worse, extirpations of native species populations. The positive environmental impacts of alien megafish were far less frequently documented. They include biotic interactions that benefit native species, and the provision of trophic resources or habitats. Widely introduced or extensively studied species are more likely to have documented severe impacts on native species. There is a clear trade-off between the economic benefits associated with megafish introductions and the severe adverse impacts they have on native biodiversity. Our study highlights the need for comprehensive risk assessments to evaluate the potential environmental impacts of megafish. More research and long-term monitoring schemes are required to inform management actions to protect biodiversity, particularly in the Global South.

Whether including exotic species alters conservation prioritization: a case study in the Min River in southeastern China

Conservation practices from the perspective of functional diversity (FD) and conservation prioritization need to account for the impacts of exotic species in freshwater ecosystems. This work first simulated the influence of exotic species on the values of FD in a schemed mechanistic model, and then a practical case study of conservation prioritization was performed in the Min River, the largest river in southeastern China, to discuss whether including exotic species alters prioritization. The mechanistic model revealed that exotic species significantly altered the expected FD if the number of exotic species occupied 2% of the community. Joint species distribution modelling indicated that the highest FD occurred in the west, northwest and north upstreams of the Min River. Values of FD in 64.69% of the basin decreased after the exotic species were removed from calculation. Conservation prioritization with the Zonation software proved that if first the habitats of exotic species were removed during prioritization, 62.75% of the highest prioritized areas were shifted, average species representation of the endemic species was improved and mean conservation efficiency was increased by 7.53%. Existence of exotic species will significantly alter the metrics of biodiversity and the solution for conservation prioritization, and negatively weighting exotic species in the scope of conservation prioritization is suggested to better protect endemic species. This work advocates a thorough estimate of the impacts of exotic species on FD and conservation prioritization, providing complementary evidence for conservation biology and valuable implications for local freshwater fish conservation.

Large mammals and trees in eastern monsoonal China: anthropogenic losses since the Late Pleistocene and restoration prospects in the Anthropocene

Massive human-induced declines of large-sized animals and trees (megabiota) from the Late Pleistocene to the Anthropocene have resulted in downsized ecosystems across the globe, in which components and functions have been greatly simplified. In response, active restoration projects of extant large-sized species or functional substitutes are needed at large scales to promote ecological processes that are important for ecosystem self-regulation and biodiversity maintenance. Despite the desired global scope of such projects, they have received little attention in East Asia. Here, we synthesise the biogeographical and ecological knowledge of megabiota in ancient and modern China, with relevant data mostly located in eastern monsoonal China (EMC), aiming to assess its potential for restoring functionally intact ecosystems modulated by megabiota. We found that during the Late Pleistocene, 12 mammalian megafaunal (carnivores ≥15 kg and herbivores ≥500 kg) species disappeared from EMC: one carnivore Crocuta ultima (East Asian spotted hyena) and 11 herbivores including six megaherbivores (≥1000 kg). The relative importance of climate change and humans in driving these losses remains debated, despite accumulating evidence in favour of the latter. Later massive depletion of megafauna and large-sized (45-500 kg) herbivores has been closely associated with agricultural expansion and societal development, especially during the late Holocene. While forests rich in large timber trees (33 taxa in written records) were common in the region 2000-3000 years ago, millennial-long logging has resulted in considerable range contractions and at least 39 threatened species. The wide distribution of C. ultima, which likely favoured open or semi-open habitats (like extant spotted hyenas), suggests the existence of mosaic open and closed vegetation in the Late Pleistocene across EMC, in line with a few pollen-based vegetation reconstructions and potentially, or at least partially, reflecting herbivory by herbivorous megafauna. The widespread loss of megaherbivores may have strongly compromised seed dispersal for both megafruit (fleshy fruits with widths ≥40 mm) and non-megafruit plant species in EMC, especially in terms of extra-long-distance (>10 km) dispersal, which is critical for plant species that rely on effective biotic agents to track rapid climate change. The former occurrence of large mammals and trees have translated into rich material and non-material heritages passed down across generations. Several reintroduction projects have been implemented or are under consideration, with the case of Elaphurus davidianus a notable success in recovering wild populations in the middle reaches of the Yangtze River, although trophic interactions with native carnivorous megafauna have not yet been restored. Lessons of dealing with human-wildlife conflicts are key to public support for maintaining landscapes shared with megafauna and large herbivores in the human-dominated Anthropocene. Meanwhile, potential human-wildlife conflicts, e.g. public health risks, need to be scientifically informed and effectively reduced. The Chinese government's strong commitment to improved policies of ecological protection and restoration (e.g. ecological redlines and national parks) provides a solid foundation for a scaling-up contribution to the global scope needed for solving the crisis of biotic downsizing and ecosystem degradation.

Compensation mechanisms for fishermen quit fishing: A case of Jiangsu province, China

The 'Ten-year Ban on Fishing' policy was designed by the Chinese government to protect the biodiversity of the Yangtze River basin. Fishermen are the ultimate implementers of the fishing ban policy. Therefore, a scientific compensation mechanism for fishers to stop fishing is the basis for ensuring the continuous implementation of the policy. First, we conducted a survey with 309 fishermen in eight cities along the Yangtze River in Jiangsu province. We also analyzed living conditions of fishermen before and after quitting fishing based on descriptive statistical analysis. Based on the theory of sustainable livelihood, a binary logistic regression model was used to analyze the relationship between fishermen's willingness to quit fishing and five types of livelihood capital (natural, material, human, financial, and social capital). The results showed that fishermen face severe livelihood sustainability issues after ceasing to fish and that their willingness to quit is closely related to the five types of livelihood capital. Based on this, and according to different age groups, this study constructed a compensation mechanism for retired fishermen from two aspects: monetary and social security compensation. The research results can provide a theoretical framework for other provinces in the Yangtze River basin to formulate a compensation system for fishermen.

Independent rediploidization masks shared whole genome duplication in the sturgeon-paddlefish ancestor

Whole genome duplication (WGD) is a dramatic evolutionary event generating many new genes and which may play a role in survival through mass extinctions. Paddlefish and sturgeon are sister lineages that both show genomic evidence for ancient WGD. Until now this has been interpreted as two independent WGD events due to a preponderance of duplicate genes with independent histories. Here we show that although there is indeed a plurality of apparently independent gene duplications, these derive from a shared genome duplication event occurring well over 200 million years ago, likely close to the Permian-Triassic mass extinction period. This was followed by a prolonged process of reversion to stable diploid inheritance (rediploidization), that may have promoted survival during the Triassic-Jurassic mass extinction. We show that the sharing of this WGD is masked by the fact that paddlefish and sturgeon lineage divergence occurred before rediploidization had proceeded even half-way. Thus, for most genes the resolution to diploidy was lineage-specific. Because genes are only truly duplicated once diploid inheritance is established, the paddlefish and sturgeon genomes are thus a mosaic of shared and non-shared gene duplications resulting from a shared genome duplication event.

A review of recent and future marine extinctions

Between 20 and 24 marine extinctions, ranging from algal to mammal species, have occurred over the past 500 years. These relatively low numbers question whether the sixth mass extinction that is underway on land is also occurring in the ocean. There is, however, increasing evidence of worldwide losses of marine populations that may foretell a wave of oncoming marine extinctions. A review of current methods being used to determine the loss of biodiversity from the world's oceans reveals the need to develop and apply new assessment methodologies that incorporate standardized metrics that allow comparisons to be made among different regions and taxonomic groups, and between current extinctions and past mass extinction events. Such efforts will contribute to a better understanding of extinction risk facing marine flora and fauna, as well as the ways in which it can be mitigated.

Virome Profiling of an Eastern Roe Deer Reveals Spillover of Viruses from Domestic Animals to Wildlife

Eastern roe deer (Capreolus pygargus) is a small ruminant and is widespread across China. This creature plays an important role in our ecological system. Although a few studies have been conducted to investigate pathogens harbored by this species, our knowledge of the virus diversity is still very sparse. In this study, we conducted the whole virome profiling of a rescue-failed roe deer, which revealed a kobuvirus (KoV), a bocaparvovirus (BoV), and multiple circular single-stranded viruses. These viruses were mainly recovered from the rectum, but PCR detection showed systematic infection of the KoV. Particularly, the KoV and BoV exhibited closely genetic relationships with bovine and canine viruses, respectively, highly suggesting the spillover of viruses from domestic animals to wildlife. Although these viruses were unlikely to have been responsible for the death of the animal, they provide additional data to understand the virus spectrum harbored by roe deer. The transmission of viruses between domestic animals and wildlife highlights the need for extensive investigation of wildlife viruses.

The conservation paradox of critically endangered fish species: Trading alien sturgeons versus native sturgeon reintroduction in the Rhine-Meuse river delta

Sturgeons rank among the most endangered vertebrates in the world. Yet, the dwindling of wild sturgeon populations stands in stark contrast to their thriving status in aquaculture. Moreover, through the exotic pet trade, sturgeons are introduced outside their natural ranges where they may compete and hybridize with native species and transmit parasites and diseases. Here, we present an in-depth inventory of alien sturgeons in the delta of the rivers Rhine and Meuse, because several countries consider reintroduction of the native, critically endangered European sturgeon (Acipenser sturio). Our study is based on (a) an inventory of the industry of sturgeon cultivation; (b) reports on spread of alien sturgeons; (c) an analysis of pathways for introduction and spread; and (d) a risk assessment using the Harmonia⁺ protocol. In total, 11 alien Acipenseriformes (sturgeons and paddlefishes) were traded across an intricate network of >1000 distribution points in the Netherlands, Belgium, and Germany. Circa 2500 alien sturgeons were reported from 53 angling ponds and 64 other lakes and ponds, whereas circa 500 alien sturgeons were reported widespread across hydrologically connected waters. Species that posed the highest risk of introduction, establishment and spread are Siberian sturgeon (A. baerii), Russian sturgeon (A. gueldenstaedtii) and Sterlet (A. ruthenus). We recommend to implement stringent trade regulations and practical solutions to prevent spread of alien sturgeons. Measures must preferably be taken at the spatial scale of river basins.

The last giants of the Yangtze River: A multidisciplinary picture of what remains of the endemic Chinese sturgeon

The Chinese sturgeon, an important endemism of the Yangtze River, belongs to 'the most critically endangered group of species' worldwide, with overfishing and habitat destruction being the main drivers towards extinction. Newly obtained microchemical comparisons between animals and water from different river locations revealed a probable shifting of the spawning ground few kilometers downstream compared to the only previously known site, located under the Gezhouba Dam. This offers a glimmer of hope for an adaptive response to habitat perturbation caused by the recently built Three Gorges dam on the Yangtze River. On the other hand, genetic data provide an estimate of about 20 breeders participating in the only significant breeding event of the past 10 years. This warns of a near species extinction forecast if no in situ and ex situ conservation efforts occur promptly. Given these results we propose a list of priority conservation actions that urgently need to be promoted, supported, and put into practice.

Swimming performance of a pelagic species in the Yangtze River under different exposure modes of the total dissolved gas supersaturation

During flood discharges of upstream dams in the Yangtze River, the pelagic fish have a stress risk from total dissolved gas (TDG) supersaturation in the river water. This study took the silver carp as the object and systematically evaluated the effects of TDG supersaturation levels and exposure time on their critical swimming speed (Ucrit) at different temperatures. The external symptoms of gas bubble disease were found when TDG levels exceeded 130%. Both exposure time and TDG level did not significantly impact the Ucrit of fish under 6 days of non-lethal exposure (110%, 120%, 130% TDG) with lower or higher water temperature. Significant differences in Ucrit were found among different exposure times at 11.0 ± 1.0°C under 10 hours of lethal exposure (135%, 140%, 150% TDG) and the Ucrit reduced by 59.88%, 83.32%, and 92.40%, respectively. TDG level had a significant impact on the Ucrit at 21.0 ± 1.0°C when exposure time exceeded 8 hours. Ucrit at 21.0 ± 1.0°C water were significantly greater than those at 11.0 ± 1.0°C water where conditions had the same TDG supersaturation and exposure time. Differences in Ucrit between temperatures ranged from 3.24 to 6.12 BL/s under non-lethal exposure and from 6.38 to 13.88 BL/s under lethal exposure. The results of this study can provide a reference for fish conservation during flood discharge.

Managing Water Level for Large Migratory Fish at the Poyang Lake Outlet: Implications Based on Habitat Suitability and Connectivity

River–lake interaction is important for maintaining biodiversity, yet it is vulnerable to hydrological alteration. The connectivity of the channel connecting Poyang Lake and the Yangtze River not only ensures the regular migration of fish but also makes Poyang Lake a feeding and fattening ground for them. Unfortunately, human activities have dramatically changed the hydrodynamic conditions of Poyang Lake, which is experiencing severe drought due to the obvious decline in the water level in autumn and winter, especially since 2003. However, the possible impacts of the changes in the water level on the habitats of migratory fish remain unclear due to the limitation of traditional techniques in spatiotemporal analysis. Here, we combined a hydrodynamic model and habitat suitability model to simulate variations in the suitable habitat area and their connectivity under different water-level conditions. The conditions for the migration pathway of the target fish were obtained by a hydroacoustic survey using the Simrad EY60 echosounder. The results showed that the change in water level will significantly affect the spatiotemporal change in the suitable habitats and their connectivity. In particular, we found the existence of two thresholds that play a dominant role in illuminating the connectivity of effective suitable habitats (HC). Firstly, the maximum value of the weighted usable area (WUA) and HC can be achieved when the water level is more than 16 m. Secondly, when the water level is between 10 and 16 m, the changes in the HC are sensitive and rapid, and the area flooded at this stage is called the sensitive area. HC is a crucial element in fish migration and habitat conditions. Under the condition of continuous drought in the middle reaches of the Yangtze River, our research contributes to clarifying the influence of water level on key habitats for fish and optimizes the practice of river–lake ecological management.

Updated species checklist of fishes from Lake Dongting in Hunan Province, South China: Species diversity and conservation

A lack of an updated checklist of freshwater fish species from Lake Dongting is a great hindrance to further biodiversity analysis. A seasonal survey of fishes in the lake was conducted from October 2017 to January 2019. Based on the data obtained during the field survey and coupled with known literature and the latest taxonomic development of relevant taxa, the species checklist of fishes from Lake Dongting was updated. A total of 130 species from 12 orders, 30 families and 76 genera has been documented, containing 126 native species and four alien species. Its fish fauna is dominated by the Xenocyprididae that has the highest number of included species (30), followed by the Gobionidae (25) and Acheilognathidae (11). This checklist comprises 20 species undergoing nomenclatural changes and 11 new records, eight of which are native and three exotic. It excludes 20 species, which have been reported in error in historical works, due to synonyms, erroneous records, taxonomic changes and unconfirmed records. Unsampled in this survey were 34 species that are ecologically specialised: migratory, rheophilic, predatory, shellfish-dependent or pelagic-egg-spawning. While some of these species eluded capture likely due to the paucity of population, others may have been extirpated in Lake Dongting perhaps owing to human perturbations, such as river damming across affluents or the Chang-Jiang mainstem, sand dredging, overfishing or water pollution. The updated checklist lays a sound foundation for biodiversity conservation of fishes in Lake Dongting.

Temporal Changes of Fish Diversity and Driver Factors in a National Nature Reserve, China

Simple Summary

Freshwater-fish diversity declined rapidly due to multiple anthropogenic disturbances. The loss of fish diversity often manifested itself in taxonomic homogenization over time. Knowledge of multi-faceted diversity (i.e., species, functional, and phylogenetic diversity) perspectives is important for biodiversity assessment and conservation planning. The results showed that the diversity of fish has declined from 2008 to 2021, with five species lost over time. We found an overall homogenization trend in the fish fauna of the study area, with a 4% increase in the taxonomic similarity among the rivers. Additionally, we found that the community structure of fish was significantly different among the rivers, and environmental filtering was the main contributor to the phylogenetic diversity of fish in 2008 and 2021. This study provides new insight into the patterns and drivers of fish-biodiversity change in the broader Yangtze River basin and informs management efforts.

Abstract

Freshwater-fish diversity declined rapidly due to multiple anthropogenic disturbances. The loss of fish diversity often manifested itself in taxonomic homogenization over time. Knowledge of multi-faceted diversity (i.e., species, functional, and phylogenetic diversity) perspectives is important for biodiversity assessment and conservation planning. Here, we analyzed the change of the species diversity and phylogenetic diversity of fish in 2008 and 2021 as well as explored the driver factors of the biodiversity patterns in the Lushan National Nature Reserve. The results showed that the species diversity and phylogenetic diversity of fish have declined from 2008 to 2021, with five species lost over time. We found an overall homogenization trend in the fish fauna of the study area, with a 4% increase in taxonomic similarity among the rivers. Additionally, we found that community structure of fish was significantly different among the rivers, and environmental filtering was the main contributor to the phylogenetic diversity of fish in 2008 and 2021. This study provides new insight into the patterns and drivers of fish-biodiversity change in the broader Yangtze River basin and informs management efforts.

Quantifying the Colonization of Environmental Microbes in the Fish Gut: A Case Study of Wild Fish Populations in the Yangtze River

In aquatic animals, gut microbial communities shift with host development and living environments. Understanding the mechanism by which the environment impacts the gut microbial communities of aquatic animals is crucial for assessing and managing aquatic ecosystem health. Here, we proposed a simplified framework for the colonization and dynamics of gut microbial communities. Then, to quantify the colonization of environmental microbes in the wild fish gut, the current study used 16S rRNA gene amplicon sequencing to obtain the structure of the water environmental microbial community and the gut microbial community in 10 wild fish populations (Leiocassis crassilabris, Leiocassis longirostris, Pelteobagrus vachelli, Silurus asotus, Siniperca chuatsi, Coilia brachygnathus, Aristichthys nobilis, Hypophthalmichthys molitrix, Coreius heterodon, and Xenocypris argentea) from the Wuhan section of the Yangtze River, and the relationship of these microbial communities was analyzed. The results identified that in most individuals, approximately 80% of gut microbes [at the operational taxonomic unit (OTU) level] were shared with the water environmental microbial community (except for individuals of Siniperca chuatsi and Coilia brachygnathus, approximately 74%). In approximately 80% of individuals, more than 95% of microbial species (OTUs) in the gut were transient. For fish species, more than 99% of microbial species (OTUs) that were introduced into the gut were transient. Nearly 79% of OTUs and 89% of species of water environmental microbes could be introduced into the fish gut. Driven by the introduction of transient microbes, fishes with similar feeding habits had similar gut microbial communities. The results indicated that for adult wild fishes, most gut microbiota were transient from the environmental microbiota that were related to fish feeding habits. We therefore encourage future research to focus on environmental microbiota monitoring and management to promote the better conservation of aquatic animals. It was important to note that, because of various influence factors, interspecific differences and individual variations on gut microbial community characteristics, the quantification of gut microbes in the current work was approximate rather than accurate. We hope that more comparable research could be conducted to outline the quantitative characteristics of the relationship between gut microbial community and aquatic environment microbial community as soon as possible.

Societal extinction of species

The ongoing global biodiversity crisis not only involves biological extinctions, but also the loss of experience and the gradual fading of cultural knowledge and collective memory of species. We refer to this phenomenon as 'societal extinction of species' and apply it to both extinct and extant taxa. We describe the underlying concepts as well as the mechanisms and factors that affect this process, discuss its main implications, and identify mitigation measures. Societal extinction is cognitively intractable, but it is tied to biological extinction and thus has important consequences for conservation policy and management. It affects societal perceptions of the severity of anthropogenic impacts and of true extinction rates, erodes societal support for conservation efforts, and causes the loss of cultural heritage.

Riverside underwater noise pollution threaten porpoises and fish along the middle and lower reaches of the Yangtze River, China

The Yangtze River exhibits a high biodiversity and plays an important role in global biodiversity conservation. As the world's busiest inland river in regard to shipping, little attention has been paid to underwater noise pollution. In 2017, the underwater noise level in 25 riverside locations along the middle and lower reaches of the Yangtze River mainly at night time were investigated by using passive acoustic monitoring method. Approximately 88% and 40% of the sampled sites exhibit noise levels exceeding the underwater acoustic thresholds of causing responsiveness and temporary threshold shift, respectively, in cetacean. Noise pollution may impose a high impact on fish with physostomous swim bladders and Weberian ossicles, such as silver carp, bighead carp, goldfish and common carp, whereas it may affect fish with physoclistous swim bladders and without Weberian ossicles, such as lake sturgeon and paddlefish, to a lesser extent. Noise levels reductions of approximately 10 and 20 dB were observed in the middle and lower reaches, respectively, of the Yangtze River over the 2012 level. The green development mode of the ongoing construction of green shipping in the Yangtze River Economic Belt, including the development of green shipping lanes, ports, ships and transportation organizations, may account for the alleviated underwater noise pollution. Follow-up noise mitigation endeavors, such as the extension of ship speed restrictions and the study and implementation of the optimal navigation speed in ecologically important areas, are required to further reduce the noise level in the Yangtze River to protect local porpoises and fish.

Using environmental DNA methods to improve detectability in an endangered sturgeon (Acipenser sinensis) monitoring program

Background

To determine the presence and abundance of an aquatic species in large waterbodies, especially when populations are at low densities, is highly challenging for conservation biologists. Environmental DNA (eDNA) has the potential to offer a noninvasive and cost-effective method to complement traditional population monitoring, however, eDNA has not been extensively applied to study large migratory species. Chinese sturgeon (Acipenser sinensis), is the largest anadromous migratory fish in the Yangtze River, China, and in recent years its population has dramatically declined and spawning has failed, bringing this species to the brink of extinction. In this study, we aim to test the detectability of eDNA methods to determine the presence and relative abundance of reproductive stock of the species and whether eDNA can be used as a tool to reflect behavioral patterns. Chinese sturgeon eDNA was collected from four sites along the spawning ground across an eight month period, to investigate the temporal and spatial distribution using droplet digital PCR (ddPCR).

Results

We designed a pair of specific primers for Chinese sturgeon and demonstrated the high sensitivity of ddPCR to detect and quantify the Chinese sturgeon eDNA concentration with the limit of detection 0.17 copies/μl, with Chinese sturgeon eDNA been intermittently detected at all sampling sites. There was a consistent temporal pattern among four of the sampling sites that could reflect the movement characteristics of the Chinese sturgeon in the spawning ground, but without a spatial pattern. The eDNA concentration declined by approximately 2–3 × between December 2018 and December 2019.

Conclusions

The results prove the efficacy of eDNA for monitoring reproductive stock of the Chinese sturgeon and the e decreased eDNA concentration reflect that Chinese sturgeon may survive with an extremely small number of reproductive stock in the Yangtze River. Accordingly, we suggest future conservation measures should focus on both habitat restoration and matured fish restocking to ensure successful spawning. Overall, this study provides encouraging support for the application of eDNA methods to monitor endangered aquatic species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01948-w.

Foundation and Prospects of Wild Population Reconstruction of Acipenser dabryanus

Acipenser dabryanus is an endemic fish inhabiting the upper reaches of the Yangtze River in China. It is classified as a first-class nationally protected animal in China and is listed in the International Union for Conservation of Nature (IUCN) Red List of Critically Endangered Species (CR). Recently, there has been a decrease in natural reproduction of A. dabryanus, and the wild population is almost extinct. This paper summarizes the changes observed in the natural population of A.dabryanus and the factors leading to its endangerment. Based on the process of artificial propagation and achievement of relevant protection goals, this paper presents the concept and technical framework for reconstruction of the wild population of A. dabryanus. In addition, by comprehensively reviewing the research findings and existing problems in the resource protection and monitoring of A. dabryanus in recent years, we assessed the possibility of wild population reconstruction and resource restoration for A. dabryanus. Reconstruction and restoration measures for the wild population of A. dabryanus are proposed, with the aim of providing a scientific basis for the reconstruction of the natural population and the improvement and restoration of critical habitat of this species. Furthermore, it is hoped that this paper will serve as a reference for the protection and restoration of other endangered fishes.

Technical fishway passage structures provide high passage efficiency and effective passage for adult Pacific salmonids at eight large dams

Fishways have been widely used for upstream passage around human-built structures, but 'success' has varied dramatically. Evaluation of fishway success has typically been conducted at local scales using metrics such as fish passage efficiency and passage time, but evaluations are increasingly used in broader assessments of whether passage facilities meet population-specific conservation and management objectives. Over 15 years, we monitored passage effectiveness at eight dams on the Columbia and Snake rivers for 26,886 radio-tagged spring-summer and fall Chinook Salmon O. tshwaytscha, Sockeye Salmon O. nerka, and summer steelhead O. mykiss during their migrations to spawning sites. Almost all fish that entered dam tailraces eventually approached and entered fishways. Tailrace-to-forebay passage efficiency estimates at individual dams were consistently high, averaging 0.966 (SD = 0.035) across 245 run×year×dam combinations. These estimates are among the highest recorded for any migratory species, which we attribute to the scale of evaluation, salmonid life history traits (e.g., philopatry), and a sustained adaptive management approach to fishway design, maintenance, and improvement. Full-dam fish passage times were considerably more variable, with run×year×dam medians ranging from 5-65 h. Evaluation at larger scales provided evidence that fishways were biologically effective, e.g., we observed rapid migration rates (medians = 28-40 km/d) through river reaches with multiple dams and estimated fisheries-adjusted upstream migration survival of 67-69%. However, there were substantive uncertainties regarding effectiveness. Uncertainty about natal origins confounded estimation of population-specific survival and interpretation of apparent dam passage 'failure', while lack of post-migration reproductive data precluded analyses of delayed or cumulative effects of passing the impounded system on fish fitness. Although the technical fishways are effective for salmonids in the Columbia-Snake River system, other co-migrating species have lower passage rates, highlighting the need for species-specific design and evaluation wherever passage facilities impact fish management and conservation goals.

Identification of hotspots of threatened inland fish species and regions for restoration based on longitudinal river connectivity

Dams have reduced longitudinal river connectivity (LRC) worldwide, impairing habitat and migration opportunities for many freshwater fish species. However, national assessments linking LRC and threatened inland fish species distributions are lacking. Here, we show the trends in the LRC in China over the past 60 years, and quantitatively analyse their implications for threatened inland fish species. The average LRC in China decreased from 93.5% in 1960 to 25.1% in 2018, and the significant deterioration in LRC occurred over the last 20 years. Water resource regions with a high number of threatened fish species are concentrated in southern China, and the degree of threat to inland fish species increases with a decreasing average LRC. A total of 125 inland fish species are threatened by habitat degradation and loss, which is mainly caused by the construction of dams. Intrinsic factors and pollution are two other key threats to fish species in addition to the reduction of LRC. The results of this study can help us better understand the trends in LRC in China and their implications for inland fishes. Meanwhile, this study provides guidance for river management to help preserve biological diversity, including enhancing management of natural reserve areas, establishing ecological compensation mechanisms, preventing biological invasion, and monitoring and evaluation of restoration efforts.

The American Paddlefish Genome Provides Novel Insights into Chromosomal Evolution and Bone Mineralization in Early Vertebrates

Sturgeons and paddlefishes (Acipenseriformes) occupy the basal position of ray-finned fishes, although they have cartilaginous skeletons as in Chondrichthyes. This evolutionary status and their morphological specializations make them a research focus, but their complex genomes (polyploidy and the presence of microchromosomes) bring obstacles and challenges to molecular studies. Here, we generated the first high-quality genome assembly of the American paddlefish (Polyodon spathula) at a chromosome level. Comparative genomic analyses revealed a recent species-specific whole-genome duplication event, and extensive chromosomal changes, including head-to-head fusions of pairs of intact, large ancestral chromosomes within the paddlefish. We also provide an overview of the paddlefish SCPP (secretory calcium-binding phosphoprotein) repertoire that is responsible for tissue mineralization, demonstrating that the earliest flourishing of SCPP members occurred at least before the split between Acipenseriformes and teleosts. In summary, this genome assembly provides a genetic resource for understanding chromosomal evolution in polyploid nonteleost fishes and bone mineralization in early vertebrates.

Opportunities to improve China's biodiversity protection laws

Since 1989, China has established a system of powerful laws and regulations aimed to preserve its rich natural flora and fauna. However, this legislative framework still has shortcomings, in terms of sentencing standards across related crimes and the extent of scientific basis for sentences. Here, we review Chinese biodiversity protection laws and some example cases with the goal of suggesting ways to increase law compliance and thus better protect biodiversity. In particular, our suggestions involve regular updates of threat assessments based on scientific evidence including herbaceous plants, fungi and algae; considering ecological differences among the species groups and ensuing ecological damage and financial profit gained; and a differentiation of punishment between organized and individual crimes, with a preference for custodial sentences for the former and monetary fines for the latter, to comply better with international standards and to minimize the incentive to engage in such conduct.

Combined effects of life-history traits and human impact on extinction risk of freshwater megafauna

Megafauna species are intrinsically vulnerable to human impact. Freshwater megafauna (i.e., freshwater animals ≥30 kg, including fishes, mammals, reptiles, and amphibians) are subject to intensive and increasing threats. Thirty-four species are listed as critically endangered on the International Union for Conservation of Nature (IUCN). Red List of Threatened Species, the assessments for which are an important basis for conservation actions but remain incomplete for 49 (24%) freshwater megafauna species. Consequently, the window of opportunity for protecting these species could be missed. Identifying the factors that predispose freshwater megafauna to extinction can help predict their extinction risk and facilitate more effective and proactive conservation actions. Thus, we collated 8 life-history traits for 206 freshwater megafauna species. We used generalized linear mixed models to examine the relationships between extinction risk based on the IUCN Red List categories and the combined effect of multiple traits, as well as the effect of human impact on these relationships for 157 classified species. The most parsimonious model included human impact and traits related to species' recovery potential including life span, age at maturity, and fecundity. Applying the most parsimonious model to 49 unclassified species predicted that 17 of them are threatened. Accounting for model predictions together with IUCN Red List assessments, 50% of all freshwater megafauna species are considered threatened. The Amazon and Yangtze basins emerged as global diversity hotspots of threatened freshwater megafauna, in addition to existing hotspots, including the Ganges-Brahmaputra and Mekong basins and the Caspian Sea region. Assessment and monitoring of those species predicted to be threatened are needed, especially in the Amazon and Yangtze basins. Investigation of life-history traits and trends in population and distribution, regulation of overexploitation, maintaining river connectivity, implementing protected areas focusing on freshwater ecosystems, and integrated basin management are required to protect threatened freshwater megafauna in diversity hotspots.

Spatial variation in biodiversity loss across China under multiple environmental stressors

Biodiversity is essential for the maintenance of ecosystem health and delivery of the Sustainable Development Goals. However, the drivers of biodiversity loss and the spatial variation in their impacts are poorly understood. Here, we explore the spatial-temporal distributions of threatened and declining ("biodiversity-loss") species and find that these species are affected by multiple stressors, with climate and human activities being the fundamental shaping forces. There has been large spatial variation in the distribution of threatened species over China's provinces, with the biodiversity of Gansu, Guangdong, Hainan, and Shaanxi provinces severely reduced. With increasing urbanization and industrialization, the expansion of construction and worsening pollution has led to habitat retreat or degradation, and high proportions of amphibians, mammals, and reptiles are threatened. Because distributions of species and stressors vary widely across different climate zones and geographical areas, specific policies and measures are needed for preventing biodiversity loss in different regions.

Continental diatom biodiversity discovery and description in China: 1848 through 2019

In this paper we inventory the continental diatom taxa described from inland waters in China, from the first species descriptions dating back to 1848 through 2019. China's geography and hydrography are complex, including the world's highest mountains, many large rivers, salty lakes, and large karst regions. From this area, a total of 1128 taxa have been described from China over this time period. We examine the number of taxa described in ca. 20-year intervals and note the periods of time of no to few descriptions, versus time intervals with many taxon descriptions. Early on, taxon descriptions of freshwater diatoms from China were done by mostly by Europeans working alone, and the time frame of 1948 to 1967 had few descriptions, as a devasting famine and the cultural revolution impacted scientific work and productivity. B.V. Skvortzov produced a large number of taxon descriptions, during his time in residence in Harbin, later while in Sao Paulo, Brazil, and even posthumously. More recently, a wide range of labs and collaborations across China, and with a diverse array of international partners, is ushering in a new, robust era of research on the biodiversity of continental diatoms. A few areas of research and work for the future are discussed.

Establishment and characterization of a cell line derived from fin of the endangered Yangtze sturgeon (Acipenser dabryanus)

Yangtze sturgeon (Acipenser dabryanus) is an endangered endemic freshwater fish of China. Cell-line is a potential means used for long-term preservation of germplasm resources and an ideal in vitro model in place of living organisms for biological studies. Here, culture condition and characterization of fin-derived cell in Yangtze sturgeon were carried out. Tissue explant techniques have been efficiently used in the Yangtze sturgeon caudal fin (YSCF) culture. The YSCF cell line showed a fibroblast-like morphology and stable growth in minimum essential medium eagle's (MEME) supplemented with 10-20% fetal bovine serum at 25°C. Cells were cryopreserved with preservative DMSO in liquid nitrogen and grew normally after recovery. No bacterial, fungal, or mycoplasma contamination was detected in the YSCF cells. Karyotype analysis of the YSCF cells showed that the chromosome numbers of the YSCF ranged from 242 to 273, and the modal chromosome number was identified as 264 at passage 9. The YSCF cells were confirmed from A. dabryanus by assay of 16S rRNA and COI. Furthermore, GFP reporter gene was successfully transferred into YSCF cells and expressed. The established YSCF cell lines will contribute to the preservation of germplasm resources and provide a useful vitro tool for further biological studies in sturgeon species.

First observations of intersex development in paddlefish Polyodon spathula

This study reports the first cases of intersex (abnormal development of both male and female reproductive tissues in a gonad) paddlefish Polyodon spathula, an Acipenseriform species of large rivers in the central U.S. and in aquaculture worldwide. Despite a large, multi-decadal data set in Oklahoma, Montana, and North Dakota, intersex development was not observed until 2019, when two individuals were harvested from the Grand Lake/Neosho River stock in Oklahoma. This suggests that intersex development in mid-water, zooplanktivorous paddlefish is rarer than in bottom-dwelling sturgeons for which intersex development is regularly observed. Although contaminants are implicated in causing intersex development in other Acipenseriformes, more investigation is needed.

Assessment of Yangtze sturgeon as recipient for the production of American paddlefish gametes through spermatogonia transplantation

The Chinese paddlefish (Psephurus gladius), one of the world's largest freshwater fish, was last seen alive in 2003; they are presumed now to be extinct. In fish, germ cell transplantation is currently known as one of the most powerful assisted reproductive technologies for the conservation of endangered species. In the event that a Chinese paddlefish is unexpectedly caught in the near future, we aimed to develop an experimental strategy to produce paddlefish gametes in the gonads of surrogate sturgeon. Spermatogonia were collected from the testes of 2.5-year-old immature male American paddlefish (Polyodon spathula), the species most closely related to the Chinese paddlefish, by Percoll gradient centrifugation, and transplanted into the peritoneal cavity of Yangtze sturgeon (Acipenser dabryanus) larvae at 7-8 days post-hatch. At two months post-transplantation, donor-derived spermatogonia had efficiently colonized in the recipient gonads and proliferated. A PCR analysis developed to detect xenogenic donor-derived mtDNA sequences in recipient gonads revealed that American paddlefish germ cells survived for at least seven months after transplantation in the gonads of Yangtze sturgeon recipients. These results show that the somatic microenvironment of Yangtze sturgeon gonads was able to support the colonization, proliferation, and survival of xenogeneic germ cells from a different taxonomic family. This study provides key information that could lead to future restoration of Chinese paddlefish using germ cell transplantation.

Hybridization of Russian Sturgeon (Acipenser gueldenstaedtii, Brandt and Ratzeberg, 1833) and American Paddlefish (Polyodon spathula, Walbaum 1792) and Evaluation of Their Progeny

Two species from the families Acipenseridae and Polyodontidae, Russian sturgeon (Acipenser gueldenstaedtii, Brandt and Ratzeberg, 1833; functional tetraploid) and American paddlefish (Polyodon spathula, Walbaum 1792, functional diploid) were hybridized. The hybridization was repeated using eggs from three sturgeon and sperm from four paddlefish individuals. Survival in all hybrid family groups ranged from 62% to 74% 30 days after hatching. This was the first successful hybridization between these two species and between members of the family Acipenseridae and Polyodontidae. Flow cytometry and chromosome analysis revealed two ploidy levels in hybrids. The chromosome numbers of the hybrids ranged between 156-184 and 300-310, in "functional" triploids and "functional" pentaploids, respectively. The hybrid origin and the ploidy levels were also confirmed by microsatellite analyses. In hybrids, the size and the number of dorsal and ventral scutes correlated with the ploidy levels as well as with the calculated ratio of the maternal and paternal chromosome sets. An extra haploid cell lineage was found in three hybrid individuals irrespective of the ploidy level, suggesting polyspermy. Although the growth performance showed high variance in hybrids (mean: 1.2 kg, SD: 0.55), many individuals reached a size of approximately 1 kg by the age of one year under intensive rearing conditions.