Chaetoceros muelleri and Isochrysis zhanjiangensis, known for their rapid reproduction, small size, and rich nutritional content, are commonly used as feed microalgae in aquaculture. This study aimed to sterilize these microalgal species and assess the effects of antibiotics on their algal cell density. Phycospheric bacteria were isolated and identified using the spread plate method and 16S rDNA sequencing, and antibiotic susceptibility tests were conducted using four antibiotics: ampicillin, streptomycin, kanamycin, and gentamicin sulfate. A sterile system was established for C. muelleri using ampicillin, streptomycin, and gentamicin, and for I. zhanjiangensis using kanamycin, ampicillin, and streptomycin. Based on the results, antibiotics with sterilization effects were selected and added to the algal cultures. Their effects on cell density were evaluated during a six-day co-culture. Ampicillin and streptomycin effectively inhibited bacteria associated with C. muelleri, initially increasing algal cell density but later causing a decline. For I. zhanjiangensis, kanamycin and ampicillin were effective, with kanamycin significantly promoting growth throughout the cycle, achieving a 36.92% higher cell density on day six (p < 0.05).

Sulfamethoxazole (SMZ), a widely used broad-spectrum antibiotic in aquaculture, lacks comprehensive research on its residual elimination kinetics in tilapia. This study investigated SMZ residue depletion, withdrawal periods, and dietary risks in 1-year-old GIFT tilapia (Genetically Improved Farmed Tilapia Oreochromis niloticus) weighing 500 ± 50 g, following oral gavage administration of a loading dose (200 mg/kg BW on day 1) and then 100 mg/kg BW daily for 6 more days, at 22 ± 2 °C. Tissue samples (plasma, muscle, skin, liver, kidney, gill, and remaining tissues) were collected from five fish per time point at intervals from 0.33 to 30 days post-administration, with SMZ residues quantified via HPLC-MS/MS. Results revealed peak SMZ concentrations at 0.33 days (8 h), ordered as liver > skin > plasma > kidney > remaining tissues > gill > muscle. Muscle residues fell below the maximum residue limit (MRL, 100 μg/kg) by day 3, while skin required 10 days. Kidney residues dropped below the limit of detection (LOD) earliest (16 days), followed by muscle, gill, and remaining tissues (25 days), whereas plasma, liver, and skin retained detectable levels until day 30. Elimination equations for SMZ across tissues exhibited first-order kinetics. Based on the specific conditions of this study, a minimum 11-day withdrawal period is recommended for edible tissues (muscle + skin) after SMZ administration. Hazard quotient (HQ) values for all tissues remained below the safety threshold (HQ = 1), indicating low dietary risk. These findings support SMZ use standardization in tilapia aquaculture to ensure food safety compliance.

Mactra veneriformis is highly susceptible to ocean acidification (OA) due to its low shell hardness during its rapid growth period. In this study, oxygen consumption rate, ammonia excretion rate, and transcriptome sequencing of mantle tissue analyses were conducted in white and purple shell-color populations of M. veneriformis under OA stress (pH = 7.6). The findings indicated a significant rise in oxygen consumption rates and ammonia excretion rates following acidification in both the two shell colors, while the clams with purple color showed comparatively lower basal metabolic levels. Transcriptomic analyses demonstrated the expression of key genes related to fatty acid synthesis were significantly inhibited, whereas genes involved in calcification, osmoregulation, and immune response were upregulated under OA exposure in the two shell-color groups. However, some genes such as CA and HSP showed a population-specific response between the two shell-color populations. KEGG enrichment analysis revealed that the MAPK signaling pathway and protein processing in the endoplasmic reticulum were significantly enriched in the two acidification groups. This study provides valuable insights into the response of M. veneriformis to OA stress and also helps to predict the future breeding of valuable strains of M. veneriformis.

Cadmium (Cd) exposure can affect metal handling, metabolism and tissue condition in bivalves on different timescales. We examined these responses in the blood clam Anadara kagoshimensis (Tokunaga, 1906) exposed to a nominal Cd concentration of 100 μg/L. Respiration, tissue Cd, redox biomarkers and histology were assessed over 480 h, alongside hepatopancreas RNA profiles at 0 h, 24 h and 21 d. Oxygen consumption reached its lowest exposed mean at 96 h. At 480 h, Cd concentrations were 44.23 ± 8.99 μg/g dry weight in the hepatopancreas and 31.75 ± 1.81 μg/g in the gill. SOD and CAT activities followed tissue-specific trajectories, GSH content declined, and MDA showed non-monotonic changes. Histological observations included epithelial detachment and disruption of digestive tubule structure. Descriptive expression comparisons identified metal handling, redox, cell-death and immune-related candidates, with more twofold expression differences at 21 d than at 24 h. qRT-PCR examined ten selected targets using three tank-derived RNA samples per time. Together, the physiological measurements, tissue observations and exploratory expression profiles describe a tissue- and time-dependent response to Cd exposure and identify candidates for further functional study.

This study evaluated the effects of salinity stress on survival, gill morphology, and immune-, osmoregulatory-, and antioxidant-related enzyme activities in Mercenaria mercenaria. Nine salinities (0–50, practical salinity units, PSU) were tested, with 30 as the control. Survival was monitored for 168 h, and median lethal salinities under hypo- and hypersaline conditions were estimated. Gill histology was examined at 24, 72, and 168 h under salinities 15, 30, and 50, while ACP, AKP, SOD, CAT, and Na+/K+-ATPase activities were measured at five time points. M. mercenaria showed substantial tolerance to salinity stress. After 168 h, cumulative mortality was 20% at salinity 45 and 22% at salinity 20, with median lethal salinities of 52.232 and 15.097, respectively. Both low and high salinity caused gill damage, including filament shrinkage at 24 h and wider inter-filament spaces after 72 h. Enzyme activities generally increased initially, then declined and stabilized, although response timing differed among enzymes. These results indicate strong early phase physiological plasticity (72 h), which could increase mortality risk, may reflect either impaired osmoregulation, antioxidant capacity and immune defense, or the establishment of physiological acclimation. The findings support pond culture management and breeding of salinity-resilient strains.

To elucidate the toxic effects and adaptive mechanisms of copper stress on Ruditapes philippinarum , and to screen for sensitive biomarkers, this study established a 14-day stress experiment using the 48h median lethal concentration (LC₅₀) of copper as a reference. Two treatment groups were established: 0.0568 mg/L (low concentration, 1/100 of 48h LC50) and 0.568 mg/L (high concentration, 1/10 of 48h LC50). Enzyme activity results showed that LDH, PK, and HK in the clams peaked on Day 2 and then slowly declined, indicating enhanced anaerobic respiration in the early stage of stress; SOD and CAT peaked on Day 7, reflecting the organism’s response to oxidative stress during the middle stage; ACP first dropped to a minimum and then peaked in the middle-to-late stage, marking the activation of immune function in the later stage; TG and TCHO levels both showed a trend of initial decrease followed by an increase, indicating energy consumption for stress response in the early stage and adaptive regulation to maintain membrane function in the later stage; AKP showed no significant regularity. There were significant differences in core response indicators at different stages of copper stress: PK, HK, and LDH responded significantly in the initial stage; SOD and CAT responded strongly in the middle stage, serving as key indicators for the organism to resist oxidative damage; ACP, TCHO, and TG responded significantly in the later stage. Among them, LDH, SOD, ACP, and TG showed stable and prominent responses throughout the stress cycle and can be used as core biomarkers for assessing the toxic effects of copper pollution on Ruditapes philippinarum . Transcriptomic analysis indicated that the number of differentially expressed genes (DEGs) between the high and low concentration groups increased significantly with stress duration, reaching 5,337 (5,088 upregulated, 249 downregulated) on Day 14. GO and KEGG enrichment analyses revealed that the low-concentration group initiated rapid stress responses in material transport and fatty acid metabolism pathways in the early stage, activated antioxidant defense pathways in the middle stage, and coordinated multiple pathways to maintain homeostasis in the later stage. The high-concentration group resisted acute toxicity through lipid metabolism and mitophagy in the early stage, and strengthened detoxification, repair, and metabolic-immune coordinated regulation to adapt to long-term stress in the later stage. This study systematically elucidates the dynamic response patterns of Ruditapes philippinarum to copper stress., providing important physiological and molecular data support for heavy metal pollution monitoring and adaptive mechanism research in marine bivalves.