The environmental problems brought about by factory-based aquaculture have become increasingly prominent. Reducing nitrogen and phosphorus concentrations in tailwater has become the key to tailwater management. In order to assess the potential of microalgae in removing nitrogen and phosphorus ions from aquaculture wastewater, four microalgae species, i.e., Chlorella sp., Dicrateria zhanjiangensis, Nitzschia closterium minutissima, and Platymonas subcordiformis, were used in this study, and their growth and nitrogen and phosphorus removal rates in four nutrient concentrations of simulated aquaculture wastewater were systematically evaluated. After 15 days of cultivation, the cell counts of all four types of microalgae increased. Three species, i.e., Chlorella sp., N. closterium minutissima, and P. subcordiformis, grew best in high PO₄³⁻ and low NH₄⁺ medium, whereas D. zhanjiangensis possessed best growth in low PO₄³⁻ and high NH₄⁺ medium. The removal rate of PO₄³⁻, NH₄⁺, NO₃⁻, and NO₂⁻ by four microalgae species exceeded 82.64%, 89.06%, 59.27%, and 42.15%, respectively, even though the four microalgae had different performance in the removal of nitrogen and phosphorus. All microalgae in the low-phosphorus groups removed PO₄³⁻ at significantly lower rates than those in the high–phosphorus groups, while high NH₄⁺ removal rates were observed in all four microalgae groups. Moreover, in phosphorus-limited conditions, four microalgae exhibit lower removal rates of NO₃⁻ when nitrogen content was high. The chlorophyll a contents of microalgae in four culture media strictly corresponded to their final cell densities. P. subcordiformis exhibited the highest intracellular polysaccharide accumulation in high PO₄³⁻ and low NH₄⁺ type medium, whereas D. zhanjiangensis demonstrated the strongest protein synthesis capacity in high PO₄³⁻ and low NH₄⁺ medium. The activities of acid phosphatase in all microalgae were higher under phosphorus–deficient conditions than phosphorus-sufficient conditions. Our results might provide useful references for microalgae selection in the treatment of different aquaculture wastewater conditions.

Mitochondrial genomes are playing an increasingly important role in molluscan taxonomy, germplasm, and evolution studies. The first complete mitochondrial genome of the commercial big brown mactra clam, Mactra grandis, was characterized using Illumina next-generation sequencing in this study. The 17,289 bp circular genome has a typical gene organization of 13 protein-coding genes (PCGs), 2 rRNAs, and 22 tRNAs, with an obvious (A + T)-bias of 64.54%. All PCGs exhibited a homogeneous bias in nucleotide composition with a (A + T)-bias, a positive GC skew, and a negative AT skew. Results of phylogenetic analysis showed that Mactra grandis was most closely related to Mactra cygnus. The functional gene arrangement of the two species was identical but different from other Mactra species. The congeneric relationships among Mactra species were demonstrated by genetic distance analysis. Additionally, the selective pressure analysis suggested that cox1 was highly efficient for discriminating closely related species in genus Mactra, while nad2 was the most appropriate marker for population genetic analysis.

This study elucidates the role of the steroidogenic acute regulatory protein (StAR) in sex steroid hormone dynamics and the gonadal development of the commercially important marine bivalve ark shell Scapharca broughtonii. The sequence of the StAR gene was obtained and verified from the transcriptome of ark shell, then the tissue localization and expression pattern during the gonad development of the StAR gene were detected by in situ hybridization and quantitative real-time PCR, respectively. Additionally, the concentrations of three critical sex steroid hormones (progesterone, testosterone, and estradiol) were measured throughout gonadal development using enzyme-linked immunosorbent assay (ELISA). The results showed that the length of the coding region of StAR was 1446 bp, encoding 481 amino acids. The results of qRT-PCR showed that the expression of the StAR gene varied with gonadal development, increased from the early active stage to the development stage, and decreased from the mature stage to the spent stage. Notably, the expression level in ovaries was higher than that in testes, suggesting the potential involvement of StAR in sex differentiation and gonadal development. Additionally, the results indicated that progesterone, testosterone, and estradiol accounted for 80%, 10%, and 10% of the total hormone content in the gonads, respectively. Correlation analysis revealed a highly significant strong positive correlation between progesterone/estradiol levels and StAR gene expression, demonstrating that StAR serves as a key regulator in sex steroid hormone biosynthesis. These findings provide crucial molecular evidence for StAR-mediated steroidogenesis in bivalve reproduction, offering fundamental insights into invertebrate endocrinology.

With the rapid development of oyster farming and the emergence of new varieties, the identification and evaluation of genetic resources have become fundamental to the effective utilization of oyster germplasm resources. To evaluate the impact of selective breeding on the germplasm of Crassostrea ariakensis, this study conducted genetic structure analysis on five successive generations including four generations selected (F1 to F4) focusing on both fast growth and high glycogen content and one base population (F0) based on partial mitochondrial cox1 and rrnL gene sequences. Results showed that the 649 bp cox1 and 488 bp rrnL sequences exhibited (A+T)-biased composition and showed positive AT skew. A total of 12 haplotypes were found for the cox1 sequences, while only three haplotypes were detected for the rrnL sequences. However, among the F1 to F4 populations, only three cox1 haplotypes and two rrnL haplotypes were observed. Based on cox1 sequences, the values of the haplotype diversity index, average number of nucleotide differences, and nucleotide diversity index all decreased progressively from F1 to F4, indicating a reduction in genetic variation due to selective breeding. All populations exhibited a low level of nucleotide diversity (<0.05). In addition, the intra-population genetic distance declined from F0 to F4, with significant genetic differentiation between F0 and F1, as well as between F0 and F4 (p < 0.05). In contrast, only the F0 and F4 population had multiple rrnL haplotypes, and no statistically significant genetic differentiations were observed based on rrnL fragments (p > 0.05). The AMOVA results showed that intra-population genetic variation exceeded inter-population variation, regardless of whether it was assessed using the cox1 gene or the rrnL gene, indicating that substantial genetic diversity persists within populations despite multiple generations of selection, and complete genetic differentiation across generations has not been achieved. These findings indicate that the selectively bred lines retain considerable genetic potential and can serve as a valuable resource for future breeding programs.

The coloration of shellfish significantly influences both environmental adaptability and economic value. In the Jinjiang oyster (Crassostrea ariakensis), soft-body color varies between individuals, with an orange-yellow phenotype distinct from the milky white coloration of the wild type. To elucidate the compositional differences and molecular mechanisms underlying orange-yellow (designated as CaR) versus milky white (CaW) soft-body color in C. ariakensis, we conducted comparative ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS) non-targeted and transcriptomic analyses. A total of 280 differential accumulation metabolites (DAMs) and 691 differentially expressed genes (DEGs) were detected between the CaR and CaW groups. The metabolite set enrichment analysis (MSEA) revealed that DAMs were significantly enriched in pigment metabolism pathways, including tyrosine metabolism, porphyrin metabolism, and lipid metabolism. Furthermore, genes associated with melanin synthesis and carotenoids conversions or transports were upregulated in the CaR vs. CaW group. These genes included Cyp4z1, Cyp4f22, Cyp17a1, Cyp1a5, Cyp2d28a, Lrp4, Aldh, and Tyr-3, potentially driving the accumulation of pheomelanin and carotenoids. This study demonstrates the vital roles of melanin and carotenoid metabolism in Jinjiang oyster body color formation, providing key insights into the molecular mechanisms of color determination in shellfish.