Introduction: Elongation of very long-chain fatty acids protein 6 (ELOVL6) played crucial roles in regulating energy expenditure and fatty acid metabolism. Many studies have performed to investigate the physiological roles and regulatory mechanisms of elovl6 in fish and animals, while few studies were reported in crustaceans.Methods: Here we reported on the molecular cloning, tissue distribution and expression profiles in response to dietary fatty acids, ambient salinity and starvation stress in Scylla paramamosain by using rapid amplification of cDNA ends (RACE) and quantitative real-time PCR.Results: Three elovl6 isoforms (named elovl6a, elovl6b and elovl6c) were isolated from S. paramamosain in the present study. The complete sequence of elovl6a was 1345 bp, the full-length sequence of elovl6b was 1419 bp, and the obtained elovl6c sequence was 1375 bp in full length. The elovl6a, elovl6b and elovl6c encoded 287, 329 and 301 amino acids respectively, and exhibited the typical structural features of ELOVL protein family members. Phylogenetic analysis showed that the ELOVL6a from S. paramamosain clustered most closely to ELOVL6 from Portunus trituberculatus and Eriocheir sinensis, while the ELOVL6b and ELOVL6c from S. paramamosain gathered alone into a single branch. Quantitative real-time PCR exhibited that the relatively abundant expression of elovl6b was observed in intestine and stomach, and the elovl6a and elovl6c were highly expressed in hepatopancreas. In addition, studies found that replacing fish oil with soybean oil could significantly increase the transcriptional levels of three elovl6 in hepatopancreas of S. paramamosain, and the expression of elovl6a and elovl6c in hepatopancreas were more sensitive to dietary fatty acids than the elovl6b. Compared with the normal sea water group (27‰), the expression of sterol-regulatory element binding protein1c (srebp-1), elovl6a, elovl6b and elovl6c were upregulated in the low salinity groups, particularly in 7‰. On the contrary, the starvation stress suppressed the expression of srebp-1, elovl6a, elovl6b and elovl6c.Discussion: These results may contribute to understand the functions of elovl6 in fatty acid synthesis and regulatory mechanisms in crustaceans.

We investigate the early developmental characteristics of the skeletal system in Miichthys miiuy larvae using a double staining method on the whole larval skeletons (aged 130days). We systematically observed and described the morphological development of both the spinal vertebrae and appendicular skeleton. The results showed that there were 25 spinal vertebrae in M. miiuy, which consist of 12 abdominal vertebrae and 13 coccyges; The development of spinal vertebrae began at 12days after hatching (dah), with the emergence of ossified neural and haemal arches, where ossification appeared at 17 dah; and the appendicular skeletons were developed from the pectoral fins to ventral fins. The coracoid-scapula cartilage was clearly observable at three dah, and fin rays were clearly differentiated at 21dah. The cartilage of appendicular of the ventral fins was formed until 19dah, and the ventral fins were completely ossified at 21dah. Subsequently, the caudal, dorsal, and anal fins appeared at 10, 13, and 14dah, respectively. Additionally, the appendicular skeleton associated with the movement of M.miiuy was completely developed by 28dah. Finally we observed that there are four types of spinal vertebrae abnormalities during the early development of M.miiuy, which included spinal bulging, splitting of vertebrae, and supernumerary and bifurcated haemal and neural spines. Skeletal length and age (in days) were highly correlated (r²=0.942). There was a strong relationship between total length and skeletal length (r²=0.979) and between body length and skeletal length (r²=0.9779).

Phascolosoma esculenta ( P. esculenta ) is a benthic marine invertebrate endemic to China, and abrupt salinity shifts in aquaculture environment often cause mass mortality. In this study, individuals were divided into three salinity treatments: S20 (salinity 20), S10 (salinity 10), and S6 (salinity 6). Histological examination, enzyme activity assays, gut microbiota 16S rRNA sequencing, and transcriptome profiling were integrated to comprehensively evaluate intestinal responses to low‐salinity stress. After 30 days exposure, low‐salinity induced pronounced intestinal injury, including thinning of the mucosal layer (ML), villus collapse, and inflammatory cell infiltration. Malondialdehyde (MDA) concentrations showed a gradual elevation across groups (S20, S10, and S6), rising from 2.91 to 4.97 nmol/mg prot. Glutathione peroxidase (GSH‐PX) activity peaked at 37.23 U/mg prot in the S10, compared with 25.19 U/mg prot in the S20 and 29.36 U/mg prot in the S6, indicating activation of antioxidant defenses under moderate stress but exhaustion under severe stress. Acid phosphatase (ACP) activity declined from 4.32 U/mg prot (S20) to 3.99 U/mg prot (S10) and 3.02 U/mg prot (S6). Similarly, alkaline phosphatase (AKP) activity dropped from 1.33 U/mg prot (S20) to 1.03 U/mg prot (S6), reflecting suppressed immune function. Gut microbiota analysis showed significantly reduced Shannon diversity in the S6, with community composition shifting from Firmicutes/Bacteroidota toward Proteobacteria. Transcriptome analysis identified 125, 364, and 413 differentially expressed genes (DEGs) in the S10 vs. S20, S6 vs. S20, and S6 vs. S10 comparisons, respectively, which were primarily enriched in Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways such as TNF signaling, RIG‐I‐like receptor signaling, and osteoclast differentiation. In summary, P. esculenta can maintain homeostasis by activating the antioxidant system under moderate low‐salinity stress. However, under severe low‐salinity conditions, it exhibits activated inflammatory responses, reprogrammed amino acid metabolism, and excessive activation of apoptosis, ultimately leading to severe damage to the intestinal barrier. This study elucidates the salinity tolerance mechanisms of this species from an omics perspective, providing important insights for its healthy aquaculture and selective breeding for salinity tolerance.