Abstract:Based on a comprehensive review of the current status of mariculture and its seed industry in China, this study establishes an evaluation index system encompassing two dimensions, “available varieties” and “industrial development,” to assess the seed industry development level of 36 major mariculture species in China. The study examines the phenomenon of “available varieties but underdeveloped industry” in the marine seed industry, analyzes the underlying causes, and proposes targeted recommendations. Key findings include: (1) The overall level of China’s marine seed industry requires improvement, with significant disparities in development among species; 72% of assessed species fall into medium-to-low tiers, indicating pronounced polarization. (2) The seed industry for multiple specific species still requires accelerated revitalization. The number of species exhibiting issues of "available varieties but underdeveloped industry," "weak variety resources despite strong industrial development," and "weak variety resources with underdeveloped industry" are 4, 3, and 11, respectively. Combined, these categories account for 50% of surveyed species. (3) The primary constraining factors of the development level of the marine seed industry include an incomplete intellectual property protection system, immature business models in the marine seed industry, and misalignment between seed industry research and development and market demand. To address these issues, this study recommends implementing the guiding principles on seed industry revitalization proposed by the Central Committee of the Communist Party of China and the State Council, strictly enforcing the requirements outlined at the National Symposium on Seed Enterprises, and balancing short-term and long-term objectives, public and commercial interests, government oversight and market mechanisms, and staple and specialty species cultivation, thereby promoting marine seed industry advancement and supporting high-quality development of mariculture.

Abstract:Aquatic germplasm resources refer to genetic materials of aquatic animals and plants that possess actual or potential value, including species, subspecies, varieties, strains, etc. These resources hold significant economic, ecological, and scientific importance in the fields of fisheries and agriculture, serving as the foundation for aquaculture, genetic improvement, and biodiversity conservation. Aquatic germplasm resources encompass aquatic organisms such as fish, shrimp, shellfish, algae, and echinoderms, along with their genetic material. They are not only a core element supporting the sustainable development and international competitiveness of the aquaculture industry but also a strategic resource for safeguarding national food security, ecological security, and biodiversity. Building strengthened protection of aquatic germplasm resources, how to enhance the scientific utilization and innovative development of superior aquatic germplasm is both an urgent national need for biological and genetic diversity security and a pressing demand for high-quality, healthy aquatic products driven by growing public expectations. This review conducts a safety assessment from three perspectives: fundamental research on the utilization of aquatic germplasm resources, relevant technologies and platforms, and artificially cultivated and genetically improved germplasm. Based on a review of research progress in these areas, it analyzes existing problems within these three aspects and proposes recommendations for improving the utilization of aquatic germplasm resources. Future recommendations for enhancing the safety of aquatic germplasm resource utilization in China mainly include: strengthening the construction of a shared genomic resource platform for aquatic species based on China's independent intellectual property; increasing efforts to decipher the genetic mechanisms underlying economically important traits; advancing the development of low-cost, high-throughput, and intelligent technologies for precise phenotyping and genotyping, along with platforms for precise functional gene manipulation; and enhancing science popularization regarding transgenic and gene editing technologies while strengthening the protection of intellectual property rights for new varieties.

Abstract:Coilia nasus, which can be migratory or sedentary, is a substantial economic fish in China and used to be an important anadromous fish in the Yellow River. It is a model organism for studying fish resource conservation. However, the C. nasus resources in Yellow River gradually depleted in the late 1980s because of river disconnection, water conservancy project construction, environmental pollution, and overfishing. With the promotion of the Yellow River water-sediment regulation and implementation of the Yellow River Fishing Ban period system, C. nasus has reappeared in the Yellow River basin. C. nasus has developed into a dominant population in Dongping Lake and Nansi Lake, but no large-scale population has been found in the channels and estuaries of the lower Yellow River. Some morphological differences can be observed between the samples from the Yellow River estuary and those from Dongping Lake and Nansi Lake. According to the relationship between age, body length, and weight, the samples from Dongping Lake and Nansi Lake collected in previous studies were basically 1+ age, and the proportion of 2⁺ age individuals was small, indicating miniaturization and younger age. Different geographical populations of C. nasus showed rich genetic diversity. Although the population of C. nasus in Dongping Lake appeared later than that in Nansi Lake, it was slightly more genetically diverse possibly owing to the unique geographical location of Dongping Lake. The convergence of the three major river systems, the Yellow River, the Wenhe River, and the Jinghang Canal, may be conducive to the reproduction and diffusion of C. nasus in Dongping Lake. Using mitochondrial control region (D-loop) gene sequencing, combined with the historical data and fishery monitoring results of Nansi Lake, we found that the C. nasus population in Nansi Lake was closely related to the population in the Yangtze River and Huaihe River. The C. nasus populations in Nansi Lake possibly originated from lakes in the Yangtze River by the urgent ecological water diversion in 2002. The population composition is not well understood because of the few samples in the main stream and estuary of the Yellow River. The habitat history of C. nasus in the Yellow River estuary is complicated. In history, the main migration channel of C. nasus in the Yellow River was the main stream of the Yellow River, the spawning ground was Dongping Lake, and the feeding and overwintering grounds were in the estuary of the Yellow River and offshore waters. Dongping Lake, as the only lake in the lower reaches of the Yellow River and once an important spawning ground for C. nasus, is still blocked by sluice at present. However, the water area of Pangkou Bay outside the sluice is in a significant position, and a certain amount of C. nasus resources already exist, which is the only habitat found at present. The resources of C. nasus in the Yellow River show a positive trend, but the resource amount and habitat conditions remain unclear. This review introduces the current research status of the biological characteristics, resource status, genetic diversity, and habitat history of C. nasus in the Yellow River and discusses the development trend and research direction. It may serve as a reference for the effective resource management, conservation efforts, and sustainable use of C. nasus in the Yellow River.

Abstract:With the ongoing rapid development of China’s mariculture industry, there has been a corresponding surge in the discharge of marine aquaculture tailwater, and the residual bait, feces, and other residues contained in this water have contributed to serious increases in the levels of nitrogen pollution. The wide geographical distribution of maricultural operations and the high concentrations of ions in tailwaters contribute to difficulties in the effective treatment and purification of this waste water, thereby seriously threatening the quality of the marine environment and the sustainable development of marine fishery resources. In this regard, microbial heterotrophic denitrification technology, which has the advantages of excellent nitrogen removal efficacy and less likelihood of contributing to secondary pollution, has increasingly become a focus of scholars engaged in research on mariculture tailwater purification. In this paper, we outline the mechanisms and advantages of heterotrophic denitrification and denitrification technology, summarize and assess the progress of research and the application of heterotrophic denitrification and denitrification technology in current mariculture tailwater treatment, and describe and evaluate the factors influencing the efficiency of denitrification, including the type of carbon source, hydraulic retention time, temperature, dissolved oxygen, and pH. On this basis of this information, we summarize the difficulties in applying denitrification technology to the commissioning of mariculture tailwater, assess potential solutions to the existing problems, and consider future research directions, with a view toward providing a theoretical basis and technical support for the application of denitrification technology in the efficient treatment of mariculture tailwater.

Abstract:Antarctic krill (Euphausia superba) and its products have garnered increasing attention as valuable natural resources because of their rich nutritional profile, particularly that of their bioactive compounds such as proteins and phospholipids. These components offer significant health benefits, including antioxidant and anti-inflammatory properties, which have attracted consumers seeking nutritional supplements and functional foods. However, the safety and quality of Antarctic krill products are of critical concern, especially concerning the presence of elements such as arsenic (As) and fluoride (F) and the potential allergenic properties of krill proteins. This review provides a comprehensive analysis of the safety issues surrounding Antarctic krill and its products, addressing key concerns such as the presence of As, F, and allergenic proteins, and strategies to mitigate these risks. One of the primary food safety concerns of Antarctic krill is its potential to accumulate As through its diet of plankton and algae. Various forms of As exist in the natural environment; their toxicity depends on the chemical form. Inorganic arsenic, particularly arsenite (As(Ⅲ)) and arsenate (As(Ⅴ)), is highly toxic, whereas organic arsenic compounds, such as arsenobetaine (AsB), are considered nontoxic or of low toxicity. Studies have shown that Antarctic krill contain As primarily in the less harmful organic forms, with AsB accounting for a significant proportion of the total As content. The levels of inorganic As in Antarctic krill and its products are typically far below the regulatory limits set by food safety standards, such as the national standard of China (GB 2762-2022), which stipulates that the maximum allowable inorganic As content in aquatic products should not exceed 0.50 mg/kg. However, some krill oil samples exceeded the specific standard of 0.1 mg/kg for inorganic As in krill oil, highlighting the need for continued monitoring and control of As levels in these products. Another contaminant of concern in Antarctic krill is F; it naturally accumulates in marine organisms, particularly in the exoskeletons of crustaceans such as krill. Although F is an essential micronutrient at low concentrations, excessive F intake can lead to health issues such as skeletal fluorosis. Research has demonstrated that Antarctic krill and its products, especially krill meal and krill powder, contain elevated F levels because of the exoskeleton’s high F content. The presence of F limits the use of krill products in health supplements, but removing F remains a technical challenge. Current strategies for defluorination include physical and chemical treatments, such as enzymatic hydrolysis, calcium salt precipitation, and filtration. However, further research is required to improve the efficacy of these methods without compromising the nutritional quality of the products. The allergenic potential of Antarctic krill also poses a significant food safety issue. Krill contains proteins (e.g., tropomyosin) that are known allergens that can trigger immune responses in sensitive individuals, particularly those with shellfish allergies. Symptoms of krill protein allergies include skin reactions, respiratory issues, and gastrointestinal discomfort. Although there are no specific treatments for food allergies, several studies have explored methods to reduce the allergenicity of krill proteins. Techniques such as microwave treatment, ultrahigh-pressure processing, protease digestion, and electron beam irradiation have shown promising results in reducing the allergenic potential of tropomyosin and other proteins. For example, microwave and ultra-high-pressure treatments have been shown to reduce shrimp allergenicity, and protease digestion eliminates allergenic protein bands. Moreover, the Maillard reaction, commonly used in food processing, reduces the allergenicity of tropomyosin by altering its secondary structure. These findings offer potential solutions for rendering krill products safer for consumption by individuals with shellfish allergies. Given the growing demand for krill products in the food and nutraceutical markets, ensuring their safety and quality is essential. Future research should focus on understanding the mechanisms involved in As, F, and allergenic protein toxicity, developing more efficient methods for removing these harmful substances. Advances in processing technologies, such as improved defluorination techniques and novel methods for reducing protein allergenicity, are critical for enhancing the safety of krill products. In addition, regulatory frameworks must be strengthened to ensure that krill products meet food safety standards. This includes the use of advanced detection technologies for monitoring contaminants such as As and F and implementing strict labeling requirements for allergens. International cooperation is vital for sharing research findings and regulatory experiences, which can lead to the establishment of unified safety standards for krill products. Public education is another important aspect of ensuring consumer safety. Increasing awareness of the potential risks associated with krill consumption, including heavy metal contamination and allergenicity, can help consumers make informed decisions. Finally, ongoing policy development and establishing a comprehensive quality control system are crucial for protecting consumer health and ensuring the sustainable development of the Antarctic krill industry. In conclusion, Antarctic krill has immense potential as a sustainable and nutritious resource. However, ensuring the safety of its products is paramount. Through continued research, technological advancement, regulatory oversight, and public education, the krill industry can address safety concerns while meeting the growing consumer demand for healthy and functional foods.

Abstract:Chitosan is derived primarily from shrimp and crab shells. Expanding use of chitosan can promote the recycling of shrimp and crab shell waste while advancing green biotechnology. Chitosan and its derivatives exhibit a wide range of biological activities, including tissue repair, regeneration, and promoting coagulation and have antibacterial, anticancer, antioxidant, and absorption properties. Despite its excellent characteristics, chitosan has limitations, such as poor solubility and weak mechanical properties. The development of nanotechnology has provided a foundation for broadening the applications of chitosan. Chitosan-based nanocomposites are formed by introducing nanoscale inorganic or organic substances into chitosan, which serves as the matrix. Recently, chitosan-based nanocomposites have become the focus of research in various fields owing to their biocompatibility, degradability, and multifunctionality. In the medical field, chitosan nanocomposites can promote wound healing by enhancing epithelialization and collagen deposition in the dermis and are promising candidates for bone and cartilage regeneration. Furthermore, chitosan nanocomposites can deliver encapsulated drugs via various pathways; their nanoscale structure significantly improves the drugs’ bioavailability and targeting. Chitosan and its derivatives possess strong heavy metal adsorption capabilities in wastewater and pollutant treatment, owing to their multifunctional chemical groups, high hydrophilicity, high chemical reactivity, and flexible polymer structures. Chitosan nanocomposites can enhance these properties by improving their mechanical strength, stability, reusability, and adsorption capacity. In agriculture, chitosan nanocomposites are used as functional components in plant growth inducers, pesticide carriers, fertilizers, growth regulators, and stress inhibitors owing to their enhanced antimicrobial properties, targeting ability, and controlled release features. In the food industry, the antimicrobial, mechanical, and barrier properties of films and coatings can be improved by incorporating nanomaterials into chitosan, which enhances food quality and extends shelf life. In the fishery industry, chitosan nanocomposites serve as carriers, encapsulants, and immobilizers for bioactive ingredients, enabling the oral delivery of drugs, vitamins, nutrients, genes, and vaccines to the fish gastrointestinal tract. This paper systematically reviews the research progress and application potential of chitosan-based nanocomposites across the fields of medicine, agriculture, wastewater treatment, food, and fisheries to provide a comprehensive reference and foundation for expanding the applications of chitosan.

Abstract:Zooplankton are essential secondary producers that are pivotal in maintaining the stability of marine ecosystems. The Bohai Sea, which is known for its rich biological resources, serves as an important spawning and feeding ground for numerous economically important fish and shrimp species in northern China Sea. Studying seasonal variations in zooplankton communities and their relationships with environmental factors is vital for assessing the current status of the Bohai Sea ecosystem and promoting the sustainable development of marine ecosystems. In this study, we analyzed the community structure of zooplankton in the Bohai Sea using various indices, such as species turnover rate (A), dominance (Y), turnover rate of dominant species (R), Shannon-Wiener diversity index (Hʹ), Margalef richness index (d), and Pielou evenness index (Jʹ), based on survey data collected during two cruises in June (summer) and October (autumn) 2021. The results showed that 43 phytoplankton species were collected during summer, with an average abundance of (34,722.6±31,775.1) ind./m³. A total of 67 phytoplankton species were identified in autumn, with an average abundance of (464,376.6±422,191.0) ind./m³. The high-value zones for phytoplankton abundance in summer were mainly located in Laizhou Bay and adjacent waters, whereas the high-value zones shifted to the Bohai Bay entrance, and central and southern Bohai Sea in autumn. A total of 41 zooplankton species (taxa) were identified, including 14 copepod species, 13 pelagic larval taxa, 8 Hydrozoa species, 2 Amphipoda species, and one species each of Ctenophora, Cladocera, Chaetognatha, and Tunicata. Owing to the influence of high summer temperatures, the number of warm-water species significantly increased in autumn, yielding higher species richness during autumn (36 taxa) than during summer (30 taxa). Zooplankton abundance averaged higher in summer (5,460.0 ind./m³) than in autumn (4,116.9 ind./m³), with high-abundance zones occurring in warmer, lower-salinity marine areas. The spatial distribution of zooplankton abundance in summer was similar to that of phytoplankton, with primary concentrations in Laizhou Bay and Bohai Bay, whereas the distribution shifted to the central and southern Bohai Sea in autumn. The Hʹ, d, and Jʹ all exhibited higher values in autumn than in summer. The dominant zooplankton species in summer included the following: Acartia hongi, Oithona similis, Paracalanus parvus, Centropages abdominalis, Calanus sinicus, Acartia omorii, and Paracalanus crassirostris. Conversely, the dominant species in autumn comprised the following: A. hongi, P. parvus, P. crassirostris, O. similis, bivalvia larvae, Ditrichocorycaeus affinis, and Oikopleura dioica. Both P. parvus and A. hongi exhibited significant correlations with elevated temperatures and increased phytoplankton abundance. Redundancy analysis revealed that P. parvus and A. omorii were significantly positively correlated with phytoplankton abundance in summer (P＜0.05). Moreover, P. parvus and P. crassirostris were significantly positively correlated with temperature (P＜0.01) and negatively correlated with salinity and dissolved oxygen (P＜0.05). In autumn, D. affinis was positively correlated with temperature (P＜0.05), and O. dioica exhibited a highly significant negative correlation with salinity (P＜0.01). Cluster analysis classified zooplankton into three groups in both summer and autumn. During summer, Group A, located at select stations in the southern Bohai Sea, exhibited the lowest zooplankton abundance (371.0 ind./m³). Group B, located in the Bohai Bay and Laizhou Bay regions, had the highest zooplankton abundance (12,937.5 ind./m³). Group C encompassed the central and northern Bohai Sea regions. In autumn, Group D, primarily located in Liaodong Bay and the central–northern Bohai Sea, had the lowest zooplankton abundance (1,373.5 ind./m³). Group E, situated in the central Bohai Sea, had a relatively high zooplankton abundance (3,818.4 ind./m). Group F, mainly located in Bohai Bay and Laizhou Bay, had the highest zooplankton abundance (6,074.7 ind./m³). SIMPER analysis revealed that four zooplankton species, each contributing over 5%, accounted for cumulative contribution rates of 87.7% and 82.9% in Groups A and C, respectively. These species included A. hongi, O. similis, P. parvus, and C. abdominalis. In Group B, four species contributed 85.3% to the similarity: A. hongi, P. parvus, P. crassirostris, and O. similis. In Group D, five species accounted for 87.6% of the similarity: A. hongi, P. parvus, O. similis, bivalvia larvae, and P. crassirostris. In Group E, five species accounted for 83.2% of the similarity: A. hongi, P. parvus, bivalvia larvae, P. crassirostris, and D. affinis. In Group F, four species contributed 81.4% to the similarity: P. parvus, A. hongi, P. crassirostris, and O. similis. BIO-ENV analysis demonstrated that the best explanatory combination of factors for summer zooplankton community structure was surface temperature, surface salinity, bottom salinity, and phytoplankton abundance (ρ=0.303). Conversely, the best explanatory combination for autumn was surface salinity and surface dissolved oxygen (ρ=0.190). These findings are important for understanding the seasonal variation characteristics and dynamic changes in the marine ecological environment. They are crucial for assessing the current status of the Bohai Sea ecosystem and promoting the sustainable development of marine ecosystems. The findings also provide foundational data and a scientific basis for the seasonal variations in zooplankton in the Bohai Sea and the adaptive management of coastal ecosystems.

Abstract:Phytoplankton, which serve as primary producers within marine ecosystems, exhibit rapid responsiveness to ecological shifts in aquatic environments. Thus, they play a pivotal role in maintaining the health and stability of these systems. China is a leading aquaculture nation boasting approximately 70% of global aquaculture output, with shellfish accounting for 72% of total production. Phytoplankton serve as the primary food source for shellfish, which regulate their biomass through filter feeding mechanisms. Furthermore, the excretions from shellfish modify nutrient concentrations in the water, indirectly influencing the composition of phytoplankton communities and consequently impacting water quality and overall ecosystem health. By examining the intricate relationship between shellfish and phytoplankton and exploring the ramifications of shellfish farming activities on phytoplankton populations, we can anticipate and address the potential effects of marine environmental changes on aquaculture. This endeavor is crucial for assessing ecological carrying capacity and planning shellfish farming activities, thereby ensuring a harmonious balance between marine economic development and ecological preservation. Haizhou Bay, located between the southern part of the Shandong Peninsula and the northern part of Jiangsu Province, has shellfish and Porphyra as its main farming species, with shellfish being the predominant species. However, few studies focused on the relationship between shellfish culture and phytoplankton Thus, this study aimed to explore the effects of a mixed cultivation of oyster and mussel, and marine environmental factors on phytoplankton community structure. Surveys of phytoplankton and environmental factors in Haizhou Bay were investigated in March, July, September, October, and December 2023. The survey was divided into four areas: Area 1, Area 2, waterway, and offshore. Areas 1 and 2 served as aquaculture areas, whereas the waterway and offshore areas served as non-aquaculture areas. The shallow-water type Ⅲ plankton network was used to vertically dragged from the seabed to the sea surface to collect phytoplankton biological samples. Environmental factors of sea surface and bottom were investigated, and the average value was calculated for data analysis. Two-way analysis of variance was performed on environmental factors and phytoplankton communities for seasonal and regional changes. Canonical discriminant analysis (CDA) was used to analyze the similarity of phytoplankton community structure in different areas, and redundancy analysis (RDA) was conducted to study the relationship between predominant phytoplankton species and environmental factors. Significant seasonal differences in temperature, salinity, pH, dissolved oxygen, chemical oxygen demand, and nutrient concentration were observed in the survey area (P＜0.001). Water temperature and salinity were affected by terrestrial inputs. In July, September, and December, the nutrient salt concentrations in Areas 1 and 2 were higher than those in the other regions. A total of 69 species of phytoplankton in 33 genera and 3 phyla were identified in the survey area, with Bacillariophyta being the predominant group, accounting for 87% of the species. Fourteen dominant species appeared in the survey, including Chaetoceros lorenzianus, Chartoceros sp., Coscinodiscus grannii, and Skeletonema costatum, most of which belonged to Bacillariophyta, with significant seasonal and regional variations. Significant differences in phytoplankton abundance (1.40×10⁴ –739.11×10⁴ cell/m³ ) were found between seasons and regions (P＜0.05). After the red tide in September, the abundance of phytoplankton decreased compared with that in July and reached the highest value in October. Affected significantly by terrestrial inputs, the abundance in Area 1 was higher than that in the other areas in all seasons, and the surveyed area generally had higher abundance of nearshore phytoplankton than the offshore area. Significant regional and seasonal differences in species diversity index were found (P＜0.05). The survey conducted in September, following the occurrence of a red tide, showed that the phytoplankton diversity index was higher in the aquaculture areas than in the non-aquaculture areas. Although some oysters died, the proportion of remaining shellfish in farming was still significant, and shellfish activities possibly increased the stability of the phytoplankton community to a certain extent. CDA results showed that the similarity of phytoplankton community structure between the aquaculture and offshore areas was low, and shellfish activities can influence the composition of the phytoplankton community structure. The waterway area, due to its proximity to the bay and slower water exchange, had a high similarity in phytoplankton community structure to Area 2. RDA results showed that the abundance of dominant species of phytoplankton were affected by environmental factors such as temperature, pH, NO3-N concentration, and NO2-N concentration, and the abundance of dominant species positively correlated with nutrient concentration in July, September, and October. In Area 1, environmental factors such as water temperature and salinity and nutrient concentration were greatly affected by terrestrial inputs, and the changes in phytoplankton community in this area may be affected by geographical location and shellfish farming activities. This study preliminarily explored the relationship between phytoplankton and environmental factors in shellfish culture area, and its results may serve as a basis for shellfish culture planning and aquaculture capacity assessment in Haizhou Bay.

Abstract:Marine microorganisms, though microscopic, represent a vast reservoir of organic carbon and play a central role in element cycling and energy flow within marine ecosystems. As key contributors to the biogeochemical cycling of carbon, these microorganisms mediate essential processes, including carbon fixation, methane metabolism, and carbon degradation. Carbon fixation, where microorganisms convert CO₂ into organic carbon, is vital for understanding marine carbon sequestration potential. Six primary microbial carbon fixation pathways have been identified: Calvin-Benson-Bassham cycle (CBB), reductive acetyl-CoA (Wood-Ljungdahl) pathway (WL), reductive tricarboxylic acid cycle, 3-hydroxypropionate bicycle (3HB), 3-hydroxypropionate/4-hydroxybutyrate cycle (3HB/4HB), and dicarboxylate/4-hydroxybutyrate cycle (DC/4HB). Among these, the Calvin cycle dominates in phototrophic autotrophs, requiring high oxygen levels and contributing significantly to global CO₂ regulation via photosynthesis. By contrast, the 3-HB and WL pathways thrive in low-oxygen and anaerobic conditions, respectively, showing high carbon fixation efficiency, particularly in extremophilic communities. With advances in metagenomics, research on marine microorganisms in complex ecosystems such as the Beibu Gulf (Northern Bay of Guangxi) has deepened, offering valuable insights into microbial communities and their roles in carbon cycling. The Beibu Gulf is strategically important ecologically and economically, lying at the intersection of South China, Southwest China, and ASEAN economic zones. However, the carbon sink potential of its marine microbial communities remains largely unexplored, and the factors influencing this potential are not yet fully understood. Thus, this study aims to evaluate the carbon fixation potential of marine microorganisms in the Beibu Gulf and identify the environmental factors shaping microbial community structure. Using field data and metagenomic sequencing, we conducted seasonal sampling at 40 stations in the bay, coastal, and island regions during the wet season (October 2022) and dry season (January 2023). By analyzing water physicochemical parameters and the abundance of microbial carbon fixation pathways, we examined the spatiotemporal distribution of these pathways across regions and seasons and their correlation with environmental variables. The study reveals significant spatiotemporal variations in the abundance of microbial carbon fixation genes. During the wet season, carbon fixation primarily occurred via the CBB and 3HB pathways, with key genes such as meh and mcl showing notable regional variability in abundance, and K14469 was detected exclusively in the bay. During the dry season, prKB, meh, mct, and mcl exhibited significant regional variability. Notably, WL pathway genes cooS and acsE, which are crucial for carbon fixation under anaerobic conditions, were detected exclusively in island areas during the dry season. Principal coordinate analysis results demonstrated distinct regional differences in carbon sink gene composition across seasons, with the most prominent variance occurring between bay and island areas. Similarity analysis indicated significant differences in carbon fixation gene composition between regions during the wet (r=0.111, P=0.035) and dry seasons (r=0.416, P=0.001), with bay-island differences particularly pronounced. Seasonal analysis of the microbial carbon fixation pathways showed that the CBB pathway was dominant during the wet season, accounting for approximately 70% of the total carbon fixation pathways detected. By contrast, during the dry season, the relative abundance of the CBB and 3HB pathways was nearly equivalent, representing 49% and 50% of the total pathways, respectively. Furthermore, metagenomic data suggested that the overall carbon fixation potential of marine microorganisms in the Beibu Gulf was significantly higher during the wet season, with total pathway abundance ranging from 7.93 TPM to 55.39 TPM, averaging 23.68 TPM, which is 1.75 times greater than that during the dry season. In addition, spatial distribution analysis indicated that, during the wet season, the CBB pathway had a substantially higher relative abundance (4.96–48.43 TPM, averaging 16.58 TPM) than the 3HB pathway (2.97–14.51 TPM, averaging 7.10 TPM). The abundance of carbon fixation pathways significantly correlated with multiple environmental variables. The CBB pathway was highly correlated with factors such as nitrate (NO₃-N), nitrite (NO₂-N), salinity (SAL), total nitrogen (TN), dissolved inorganic phosphate (DIP), water temperature (WT), total organic carbon (TOC), and pH. For the 3HB pathway, WT and dissolved oxygen (DO) were significantly correlated. Overall, the total carbon fixation potential was predominantly influenced by SAL, DIP, NO₂-N, NO₃-N, TN, and TOC, indicating these variables as the primary drivers impacting microbial carbon sink potential in the Beibu Gulf. In summary, this study provides a comprehensive assessment of the carbon fixation potential of marine microorganisms in the Beibu Gulf and identifies key environmental factors influencing carbon sink capacity. The findings underscore the substantial role of environmental conditions in shaping the carbon fixation pathways across different seasons and regions, highlighting the complex and dynamic microbial carbon cycling processes in the Beibu Gulf. This study contributes valuable insights into marine microbial carbon sequestration and offers a scientific basis for future studies on the ecological impact of microbial communities in marine carbon cycling.

Abstract:Coilia nasus is among the “flagship species” of migratory fish within the Yangtze River. Although lakes such as Dongting Lake and Poyang Lake once served as spawning grounds for C. nasus, the migratory paths of these fish have now been obstructed and the spawning grounds have been lost, as a consequence of factors such as water pollution, overfishing, and the destruction of migratory channels and conservation areas. As a repercussion of these effects, since the beginning of the previous century, the Yangtze River population of C. nasus has been characterized a phenomenon of low age and miniaturization, and there has been a marked overall decline in fish stocks. However, despite these declines in fish status and stocks, in response to the implementation of a 10-year ban on fishing, the declining trend in C. nasus resources has been contained to a certain extent, and populations have gradually recovered in Poyang Lake. In this study, with a view toward confirming the potential ecological function of waterbodies as C. nasus habitats, and thereby provide basic data for an evaluation of the effects of the Yangtze River 10-year fishing ban, we applied otolith microchemistry technology to reconstruct the life history of 20 C. nasus individuals collected during the investigation of these fish in waters of the Poyang Lake region around Xiyuan Town, southern Duchang County, Jiangxi Province. Based on the findings of this study, we provide a theoretical basis for C. nasus resource conservation and habitat protection in Poyang Lake. In this context, some scholars have proposed that the developmental maturity of fish gonads can be used as a means of identifying spawning grounds. Accordingly, among the analyzed fish, zx01, zx04–08, zx11, zx13, zx17, and zx18 can be classified into a spawning group according to sexual maturity, whereas the zx02, zx03, zx09, zx10, zx14–16, zx19, and zx20 fish can be classed as non-spawning. The results of otolith microchemical analysis revealed a high variability in the Sr/Ca values of all otoliths, with low values corresponding to freshwater habitats (Sr/Ca×10³ ＜ 3), median values indicative of brackish habitats (3 ＜ Sr/Ca×10³ ＜ 7), and high values corresponding to marine habitats (Sr/Ca×10³ ＞ 7). The results also revealed that all individuals were typical anadromous C. nasus. Moreover, we not only identified individuals at reproductive stage Ⅲ, but also those with high gonadal development reaching stage Ⅳ. Combined with the analysis of environmental factors in the sampled water areas, we established that the water in the surveyed area was of sufficient quality to serve as a spawning ground for C. nasus. Consequently, we speculate that this water area might be an unreported spawning ground for migratory C. nasus in Poyang Lake, and could serves the ecological function of a C. nasus migratory channel. Furthermore, to briefly analyze the potential associations among different C. nasus populations, we also introduced the concept of the freshwater coefficient. In this regard, we speculate that there may be a close association between C. nasus in the Anqing section of the Yangtze River and C. nasus in local waters, although further investigations are needed to obtain basic data. Our findings in this study also indicated that the C. nasus population in the waters of Duchang County is a composite anadromous population with two migratory patterns. The zx01–08, zx10–15, and zx17–19 individuals were found to be characterized by one of these migratory patterns, whereas the zx09, zx16, and zx20 fish were found to have the second pattern. These findings thus tend to indicate that C. nasus has a strong ability to adapt to different habitat types. In this study, we reconstructed the migratory history of C. nasus in the waters of Xiyuan Town and revealed the migration patterns of this fish. Our findings provide evidence to indicate that this water area is of considerable significance to the replenishment and restoration of the Yangtze River C. nasus resources, and plays an indicative role in evaluating the effects of the Yangtze River fishing ban. These findings accordingly emphasize the importance of protecting this water area from human activities and habitat destruction.

Abstract:The Southern Ocean plays a pivotal role in global carbon cycling and climate regulation. As a fundamental component of Antarctic ecosystems, phytoplankton provide the main nutritional support for Antarctic krill (Euphausia superba Dana), a vital fishery resource in the Southern Ocean. Variations in phytoplankton community composition and abundance directly affect the growth, reproduction, and distribution of Antarctic krill, subsequently affecting the entire Antarctic food web and biological carbon pump processes. Understanding phytoplankton community dynamics is essential for assessing ecosystem responses to climate change and managing Southern Ocean fishery resources. In the context of global climate change, the physical environment and ecosystems of the Southern Ocean are undergoing rapid transformation. Since the beginning of the 21st century, environmental DNA (eDNA) metabarcoding has emerged as a powerful tool for studying phytoplankton diversity by enabling the rapid molecular-level assessment of distribution patterns across marine environments. This technique utilizes universal primers to amplify targeted DNA fragments for high-throughput sequencing, enabling the simultaneous analysis of multiple species in environmental samples, while effectively capturing microscopic and cryptic species. For eukaryotic phytoplankton, the 18S rDNA gene is most commonly used because of its universality, ease of amplification, and ability to provide phylogenetic information across taxonomic levels. Among the nine hypervariable regions (V1–V9), the V4 region is particularly valuable for marine phytoplankton studies. As the longest variable region, it shows superior species discrimination for diatoms and dinoflagellates, making it widely used in marine phytoplankton diversity research. However, the accuracy of diversity assessments depends heavily on bioinformatics methods for delineating taxonomic units, primarily operational taxonomic units (OTUs) and amplicon sequence variants (ASVs). Although both methods are widely used, their comparative performance in eukaryotic phytoplankton communities, particularly in Polar Regions, remains underexplored. This study compared two widely used bioinformatics pipelines for processing Antarctic phytoplankton eDNA metabarcoding data: One (VSEARCH-UPARSE) that employs a clustering method that generates OTUs at 97% similarity and the other (USEARCH-UNOISE3) based on a denoising (error-correcting) algorithm that generates ASVs. Specifically, we assessed differences in their taxonomic resolution, α and β diversity indices, and their implications for identifying ecologically significant taxa, providing insights into methodological choices for polar biodiversity studies. Seawater samples were collected from surface waters at seven stations in the Antarctic Peninsula, Cosmonaut Sea, and Prydz Bay during the austral summer (January–March 2022) aboard the icebreaker Xuelong2 as part of China’s 38th Antarctic Expedition. Samples were immediately filtered through 0.22 μm mixed cellulose ester membranes, which were then wrapped in aluminum foil and stored at –80°C until DNA extraction. Total eDNA was extracted using the EZNA Soil DNA Kit (Omega Bio-Tek). The V4 hypervariable region of the 18S rDNA was amplified using universal primers 573F (5ʹ-CGCGGTAATTCCAGCTCCA-3ʹ) and 951R (5ʹ-TTGGYRAATGCTTTCGC-3ʹ). The PCR products were purified, quantified, and paired-end sequenced on an Illumina MiSeq platform, with blank filters used as negative controls. The results showed that phytoplankton communities derived from both pipelines were dominated by three key phyla: Dinoflagellata (dinoflagellates), Bacillariophyta (diatoms), and Haptophyta (mainly Phaeocystis antarctica). In α diversity assessments, the Simpson and Shannon diversity indices showed no significant differences between OTU and ASV datasets (P＞0.05), with ecologically reasonable values ranging from 0.54–0.91 (Simpson) and 1.80–3.13 (Shannon), indicating that both pipelines can effectively characterize the overall structure of phytoplankton communities in the study areas. However, the OTU approach generated significantly higher richness indices than the ASVs (P＜0.05), whereas the ASVs exhibited greater Pielou’s evenness indices. This discrepancy likely stems from artifacts in OTU clustering, where sequence errors or intragenomic variants may inflate diversity estimates by generating low-abundance false-positive OTUs. At the species level, both pipelines identified common dominant taxa in Antarctic waters, including P. antarctica, Corethron inerme, Chaetoceros dichaeta, Gymnodinium sp., and Prorocentrum sp., which is consistent with known Antarctic assemblages. Among these species, P. antarctica has frequently been underestimated in traditional microscopic examinations owing to its small cell size (typically 5–10 μm). Crucially, ASVs provided more precise taxonomic information, identifying dominant species such as Fragilariopsis cylindrus and Fragilariopsis kerguelensis, which were underestimated or exhibited less accurate annotation in the OTU database. The ASV pipeline also detected the potentially invasive dinoflagellate Ptychodiscus noctiluca, which was absent in the OTU results. For β diversity analysis, weighted distance indices (Bray-Curtis and Weighted Unifrac) showed consistent patterns between pipelines (P＞0.1), whereas unweighted indices (Jaccard and Unweighted Unifrac) exhibited significant discrepancies (P＜0.05). Therefore, we suggest prioritizing abundance-weighted approaches to describe β diversities for more robust community comparisons. In conclusion, both OTU and ASV pipelines can effectively characterize the Antarctic phytoplankton community characteristics from eDNA metabarcoding based on high-throughput sequencing data, whereas the ASV pipeline appears to show greater potential for future applications in achieving finer taxonomic resolution or inter-study comparisons. A comparative analysis of different pipelines for minimum taxonomic unit divisions can provide a theoretical basis for the selection of methods for future phytoplankton diversity research. With the advances in data processing methods, the influence of different algorithms and genetic markers on metabarcoding-based phytoplankton community diversity analysis requires further exploration.

Abstract:Mytilus coruscus, an economically important cultured shellfish in China, undergoes settlement and metamorphosis during its transition from planktonic to benthic life. The settlement of marine invertebrates is regulated by microbial biofilms. Bacteria and diatoms, along with their secreted extracellular polymeric substances, adhere to the substrate's surface. Diatoms can form biofilms independently of bacteria and are preferred by settling marine invertebrates. Previous studies extensively investigated, the mechanisms by which bacteria regulate the settlement of invertebrates. However, the relationship between diatom settlement and marine invertebrates, such as mussels, has yet to be reported. Gouqi Island, in Zhoushan, Zhejiang Province features open near-shore waters, and its surrounding area constitutes an important marine region for the raft culture of M. coruscus. This study aimed to investigate the interactions between diatoms and marine invertebrates and evaluate the potential of diatom biofilms as settlement inducers for juvenile mussels in M. coruscus. Specifically, the relationship between the diatom community in biofilms with varying ages formed at Gouqi Island and the settlement of M. coruscus was examined. Natural biofilms were established for 7, 14, 21, and 28 days in the marine area surrounding Gouqi Island (122°46'E; 30°43'N). Ten juveniles were placed in sterilized glass dishes (64 mm × 19 mm) with 20 mL of autoclaved filtered seawater and natural biofilms to test the inducing effect of biofilms on M. coruscus settlement. The biological characteristics, including dry weight, bacterial density, diatom density, and chlorophyll a content, of biofilms were analyzed. The population composition was identified by 18S rRNA amplicon sequencing (573F CGCGGTAATTCCAGCTCCA, 951R TTGGYRAATGCTTTCGC) and scanning electron microscopy. On average, 90% of juvenile mussels settled on 21- and 28-day biofilms. Results indicated a positive correlation between and factors such as biofilm age, diatom density, chlorophyll a concentration, dry weight, and bacterial density. The correlation between diatom density and juvenile mussel settlement was the strongest, with a coefficient of R = 0.96. Furthermore, the correlation between diatoms and juvenile mussel settlement was significantly higher than that observed for bacterial density, which had a correlation coefficient of R = 0.88. The dry weight of biofilms significantly increased with age, reaching 5.91 mg at 28 days. Bacterial density also increased over time, peaking at 2.68×10⁶ cell/cm² . The concentration of chlorophyll a exhibited a trend similar to that of diatom density, reaching its peak at 21 days. Although a decreasing tendency was observed at 28 days, the difference in concentration compared with that at 21 days was not statistically significant. The results of 18S rRNA sequencing showed significant differences (P ＜ 0.05) in eukaryotic microorganism diversity among different biofilm groups. The 18S rRNA sequencing analysis indicated that diatom phyla were predominant in biofilms of varying ages, exhibiting an average relative abundance of 0.96. The identified species within the diatom phyla encompassed 10 phyla, 4 orders, 17 families, 23 genera, and 32 species. The top 5 genera in terms of abundance in the sample were Licmophora, Odontella, Halamphora, Navicula, and Pauliella. The settlement of juvenile mussels positively correlated with the abundance of diatoms. Among the top 16 genera, the abundance of Thalassiosira, Licmophora, Minutocellus, and Achnanthes exhibited higher positive correlations with the settlement of juvenile mussels. Except for Licmophora, the abundance of Thalassiosira, Minutocellus, and Achnanthes showed higher relative abundance in the high biofilm day-age group than in the low biofilm day-age group. The relative abundances of Seminavis and Nitzschia negatively correlated with the settlement of juvenile mussels, and they were greater in the low biofilm day-age group than in the high biofilm day-age group. The relative abundance of diatoms with inducing activity for invertebrate settlement, such as Achnanthes, was higher in the high-age biofilms than in the low-age biofilms, whereas non-inducing diatoms such as Nitzschia had higher relative abundance in the low-age biofilms than in the high-age biofilms. The types and relative abundance of diatoms with and without inducing activity in biofilms were correlated with the induction capacity of these biofilms. This study indicated that diatoms may exhibit a higher inducing activity for invertebrates’ settlement, and the inducing activity may be related to their ecological habits, living environment, and physiological characteristics. The findings of this study provide a theoretical basis for examining the relationship between biofilm diatom community diversity and invertebrate settlement. This research offers a new perspective on addressing the challenge of seedling settlement during mussel aquaculture. Additionally, it has significant implications for applying diatoms and their biofilms to enhance shellfish aquaculture efficiency, particularly for M. coruscus. The insights from this study are crucial for understanding how to effectively utilize diatoms and their biofilms in cultivating M. coruscus.

Abstract:Epinephelus fasciatus, a small grouper, is valued for its delectable flesh and substantial market value. This study utilized wild red grouper parents to examine artificial breeding, embryonic and post-embryonic development, and juvenile fish cultivation under industrialized conditions in northern China. At present, no documented studies exist on the artificial breeding, embryonic and post-embryonic development, and feed conversion processes of E. fasciatus under local factory conditions in China. This study focused on fertilized eggs and juvenile fish of E. fasciatus artificially bred in factories and recorded their embryonic development and instances of rapid growth. The study also explored the relationship between juvenile fish development and feed conversion, providing a scientific reference for the industrial breeding and aquaculture of E. fasciatus and its high-value utilization. The parent experiments, including fish cultivation, fertilization, and hatching, were conducted at Laizhou Mingbo Aquatic Products Co., Ltd. For egg collection, female red groupers with stage Ⅳ gonad development were selected for artificial induction. The oxytocin used was a mixture of HCG and LHRH-A3, which was administered at dosages of 350 IU/kg and 20 μg/kg of fish weight, respectively. Two days before sperm collection, a halved mixture was injected into mature male fish to promote sperm production. To obtain fertilized eggs, mature eggs were first collected in a dry plastic basin by compressing the abdomen, while sperm with high microscopic vitality were simultaneously collected for artificial insemination. Dry insemination was performed at a sperm-to-egg volume ratio of 1:500. After stirring, the eggs were allowed to stand for 5 min. The eggs were then washed with an equal volume of seawater and allowed to stand for 5 min. The floating eggs were collected and incubated in an incubation bucket with seawater maintained at a water temperature of (23.4±0.8) ℃, dissolved oxygen levels greater than 6 mg/L, and a salinity range of 28–30. Upon completion of the embryonic hole closure period, the eggs were collected with gauze and immediately transferred to a breeding pool with an egg-laying volume of 2–3 mL/m³ . Starting with fertilized eggs, 10 floating eggs were regularly removed from the hatching bucket, and an optical microscope (Nikon E200) was used to observe the embryonic development process. Photos were captured, and the time and developmental characteristics of each developmental stage were recorded. The time point of each developmental stage was defined when two-thirds of the embryos reached this stage. The fertilized eggs completed embryonic development within 31 h and 12 min under water temperature and salinity conditions of (23.4±0.8) ℃ and 28–30, respectively. Starting with the initial hatching of fry, fish fry exhibiting good growth from the breeding pool were regularly selected. A microscope (Nikon E200), dissecting microscope (Olympus), and Image View software were used to capture photos, measure total length, and record the developmental stages and characteristics of the fry and juveniles. During this period, samples were collected daily from 0 to 10 days post-hatching (dph), every 2 days from 10 dph to 20 dph, and every 5 days from 20 dph to 40 dph. Fish fry (3–5) was randomly selected at each time point to measure their total length and record their growth status. One-way ANOVA was performed on growth data using SPSS 27.0, with significant differences between groups compared using the least significant difference (LSD) and Duncan tests. Origin Pro 2022 software was employed for figure creation. The post-embryonic developmental sequences were divided into the early larval (0–3 dph), late larval (4–29 dph), juvenile (30–54 dph), and young (>55 dph) stages. At 3 dph, the larvae were fed S-grade rotifers. At 9–20 dph, L-grade rotifers were provided. At 20–30 dph, brine shrimp larvae were provided. Starting at 31 dph, the fish larvae transitioned from animal-based feed to compound feed. Compared to the early developmental stage, the growth rate, phenotype, scales, and body color of the larvae rapidly increased after transitioning to compound feed. This indicated a high acceptance of compound feed during the transitioning stage. The feeding frequency was twice daily, occurring at 08:00 and 16:00. The dimensions of the cultivation tank were 6 m × 8 m× 2.5 m, with a water flow exchange rate of 8 m³ /h. Dissolved oxygen levels exceeded 6 mg/L, salinity ranged from 28 to 30, and water temperature varied between 25 ℃ and 28 ℃. At present, although red groupers can be artificially reproduced, cultivation technology remains imperfect, and large-scale breeding has not yet been realized. This study systematically investigated embryonic developmental processes and early morphological changes in red grouper larvae produced using artificial insemination under industrial conditions. The early developmental patterns of metamorphosis, growth, and early body color changes were systematically documented, providing a reference for the artificial cultivation and scientific feeding of early fry and juveniles to mitigate mortality risk.

Abstract:The transcription factor gene figla (factor in the germ line, alpha), a member of the basic helix-loop-helix (bHLH) family, has been extensively documented for its pivotal role in mammalian ovarian development and primordial follicle formation. However, studies of figla in teleosts are less detailed than those in mammals. Turbot (Scophthalmus maximus), an important aquaculture species in Europe and China, exhibits sexual dimorphism in growth and body size and possesses a female heterogametic sex determination system (ZW female and ZZ male). Therefore, it is crucial to understand the mechanisms underlying sex determination and differentiation. This study exclusively used female individuals as experimental subjects to elucidate the complete cDNA sequence of figla in turbot and its role during early ovarian development. The full-length cDNA sequence of figla was cloned using RT-PCR and rapid amplification of cDNA ends techniques. Tissue expression distribution and spatiotemporal expression patterns during early ovarian development were investigated using semi-quantitative RT-PCR, real-time fluorescence quantitative PCR, and in situ hybridization. The full length of the figla gene cDNA had 1,006 base pairs (bp), with an open reading frame ranging from 150 bp to 758 bp, encoding 202 amino acids with a conserved bHLH domain. In the turbot gonads, figla exhibited specific expression, showing higher levels in the ovaries than in the testes. The expression pattern of figla mRNA was detected throughout various stages of ovarian development, commencing at 25 dph and progressively increasing until reaching its peak at 90 dph. In situ hybridization experiments revealed the predominant localization of figla mRNA within the cytoplasmic region of oocytes. In conclusion, the distinct expression pattern of figla in male and female turbot, its localization within primary oocytes, and its expression patterns during early ovarian development suggest a crucial role for this gene in the normal development of oocytes and ovaries. Further research is required to investigate the regulatory mechanisms underlying oocyte development and gonadal differentiation. The study of figla function not only enhances our understanding of sex determination and differentiation mechanisms in turbot but also provides novel insights and methodologies for sex control and breeding applications in this species.

Abstract:The razor clam, Sinonovacula constricta, has become a major economic mollusk in coastal East Asia because of its short culture cycle, rapid growth, and high yield, which have profoundly impacted the growth and development of mollusks in the aquaculture industry. However, compared with other mollusks, razor clam reproduction presents several challenges such as asynchronous spawning, poorly regulated reproduction, and a high rate of embryonic deformities, which greatly affect artificial breeding and the development of new varieties. Therefore, it is important to study the mechanisms underlying gonadal development. Gonadal development is the basis of animal breeding and reproductive control, and its mechanisms vary across different species. Therefore, a deeper understanding of gonadal development can provide a theoretical basis for genetic breeding and gender control. Gonadal development is affected by external factors (e.g., temperature and food) and endogenous gender-related genes. The Dmrt (double-sex and mab-3 related transcription factor) gene family is a class of transcription factors involved in sex determination and gonadal development in animals, with all encoded proteins containing highly conserved zinc-finger DM domains. Dmrt5 is an important member of this family, and its encoded protein is somewhat different from that of other family members because it includes conserved DM and DMA domains. It plays important roles in sex determination, gonadal development, and organ function maintenance in animals. In this study, two Dmrt5 genes were identified through whole-genome and gonadal transcriptome analyses of razor clams. To investigate the roles of Dmrt5 genes (ScDmrt5) in gonadal development of S. constricta, the full-length cDNA sequences of the two ScDmrt5 genes were cloned using the rapid amplification of cDNA ends (RACE) technique. The proteins encoded by the ScDmrt5 genes were analyzed for sequence comparison, conserved structural domains, and phylogenetic relationships using various bioinformatics tools. The expression patterns of ScDmrt5 genes/proteins in different tissues and gonadal development stages were investigated by qRT-PCR and immunofluorescence techniques. The open reading frame (ORF) sequence of ScDmrt5-1 was 1,170 bp, encoding 389 amino acids, and that of ScDmrt5-2 was 1,125 bp, encoding 374 amino acids. Both possessed conserved DM and DMA structural domains. The phylogenetic analysis showed that ScDmrt5-1 and ScDmrt5-2 first converged into a small clade with molluscan Dmrt5 proteins. Their motif locations, types, and quantity were similar, and their amino acid identities with other molluscan Dmrt5 proteins exceeded 50%, suggesting that the two Dmrt5 proteins of S. constricta were highly similar to Dmrt5 in other mollusks. ScDmrt5-1 and ScDmrt5-2 were localized on the same chromosome (chr12), but their positions were far apart without tandem duplication. Based on sequence comparisons, conserved domains, phylogenetic analysis, and chromosome localization, the two Dmrt5 genes were designated ScDmrt5-1 and ScDmrt5-2. The qRT-PCR analysis revealed that ScDmrt5-1 and ScDmrt5-2 were expressed in seven tissues (gill, mantle, testis, ovary, siphon, hepatopancreas, and foot), with significantly higher expression levels in the gills than in other tissues (P<0.05). In the mature stage of the gonads, ScDmrt5-1 and ScDmrt5-2 expression levels were significantly higher in the testes than in the ovaries (P<0.05). Based on the morphological characteristics of gonad cells of razor clams using paraffin sections and HE staining, the gonadal development process of the testis and ovary was categorized into four stages: proliferative, growing, mature, and spawning. During these stages, the expression level of ScDmrt5-1 gradually increased with testis development, peaking at the mature stage (P<0.05). In contrast, its expression exhibited an opposite trend in the ovary. ScDmrt5-2 expression also gradually increased with testis development, reaching higher levels in the mature and spawning stages, where it was significantly higher than in the ovary at the same stage (P<0.05). However, ScDmrt5-2 expression in the ovary remained relatively low and did not differ significantly between developmental stages. The immunofluorescence analysis revealed that ScDmrt5-1 and ScDmrt5-2 proteins were mainly located in the germ cells of female and male gonads, including oocytes and mature oocytes in the ovaries, and spermatogonia, spermatocytes, and mature sperm in the testis. In conclusion, ScDmrt5-1 and ScDmrt5-2 may be primarily involved in testis development and functional maintenance with potential roles in respiration and neuroperception. This study elucidates the expression characteristics of Dmrt5 in razor clam gonadal development. It also preliminarily explores the role of Dmrt5 in the gonadal development of razor clams, which could provide theoretical guidance for reproductive regulation, artificial breeding, and the development of new S. constricta varieties.