Background: Snipe eels (family Nemichthyidae) are a group of pelagic fishes with unique specializations; yet, species within this study are not well-studied due to a lack of molecular data. As typical mesopelagic-to-bathypelagic fishes, snipe eels exhibit extreme body elongation, reduced skeletal ossification, and highly specialized beak-like jaws that facilitate survival in deep-sea midwater environments. Methods: The complete mitochondrial genome of the deep-sea eel Nemichthys curvirostris (Anguilliformes: Nemichthyidae) was sequenced and annotated, representing the first mitogenomic resource for this species. The phylogenetic position of N. curvirostris was also explored. Results: The circular genome of N. curvirostris was determined to be 16,911 bp in length and contained 37 genes, including 13 protein-coding genes, 22 tRNAs, 2 rRNAs, and a single control region, with an overall A + T bias of 56.67%. The maximum-likelihood phylogeny inferred from concatenated mitochondrial protein-coding genes recovered a well-supported monophyletic Nemichthys clade, with N. curvirostris positioned as the sister taxon to N. scolopaceus. The genera Avocettina and Labichthys were recovered as sister taxa, and Nemichthys clustered within a broader clade alongside them. The COX1 haplotype phylogeny showed that the two public database sequences (HQ563894.1 and MN123435.1) appeared as long, isolated branches outside the main N. curvirostris lineage, with COX1 genetic distances from typical N. curvirostris haplotypes reaching 12–13%, far exceeding the expected range of intraspecific variation. Conclusions: This mitogenome provides a valuable molecular resource for phylogenetic, evolutionary, and population genetic studies of deep-sea Anguilliformes.

The Na⁺/K⁺/2Cl⁻ cotransporter (NKCC) gene encodes a critical membrane transporter involved in cellular ion homeostasis and plays a pivotal role in osmoregulation and salinity adaptation in aquatic organisms. This study identified and validated SNP markers in the NKCC gene associated with low-salinity tolerance in Scylla paramamosain. Four SNPs (g.196C>A, g.8374T>A, g.8385T>A and g.91143T>A) were screened and genotyped in low-salinity tolerant and intolerant groups. Association analysis revealed that mutant genotypes at all four sites were significantly enriched in the tolerant group (p <0.05), with the values of odds ratios (OR) greater than 1. The tolerant group exhibited significantly higher genetic diversity than the intolerant group. Haplotype analysis showed the wild CTTT haplotype dominated in the intolerant group, while mutant-containing haplotypes were significantly elevated in the tolerant group. A positive correlation was observed between the mutant and NKCC expression. Functional validation by qRT-PCR demonstrated that mutant allele carriers exhibited significantly higher NKCC mRNA expression levels than the wild-type carriers. Moreover, the expression level of homozygous mutations is significantly higher than that of heterozygous mutations. These validated SNPs could provide effective molecular markers for marker-assisted selection breeding of low-salinity tolerant S. paramamosain strains, offering important theoretical and practical implications for sustainable aquaculture development.