Anthropogenic activities have led to increased levels of contaminants that pose significant threats to aquatic organisms, particularly fishes. One such contaminant is Selenium (Se), a metalloid which is released by various industrial activities including mining and fossil fuel combustion. Selenium is crucial for various physiological functions, however it can bioaccumulate and become toxic at elevated concentrations. Given that fishes are key predators in aquatic ecosystems and a major protein source for humans, Se accumulation raises considerable ecological and food safety concerns. Selenium induces toxicity at the cellular level by disrupting the balance between reactive oxygen species (ROS) production and antioxidant capacity leading to oxidative damage. Chronic exposure to elevated Se impairs a wide range of critical physiological functions including metabolism, growth and reproduction. Selenium is also a potent teratogen and induces various types of adverse developmental effects in fishes, mainly due to its maternal transfer to the eggs. Moreover, that can persist across generations. Furthermore, Se-induced oxidative stress in the brain is a major driver of its neurotoxicity, which leads to impairment of several ecologically important behaviours in fish including cognition and memory functions, social preference and interactions, and anxiety response. Our review provides an up-to-date and in-depth analysis of the various adverse physiological effects of Se in fishes, while identifying knowledge gaps that need to be addressed in future research for greater insights into the impact of Se in aquatic ecosystems.

The sonochemical reduction of MnO4(-) to MnO2 in aqueous solutions was investigated as a function of alcohol concentration under Ar. The rate of MnO4(-) reduction initially decreased with increasing alcohol concentration, and then increased when the alcohol concentration was increased further. The concentrations at which the reduction rates were minimum depended on the hydrophobic properties of the added alcohols under ultrasonic irradiation. At low concentrations, the alcohols acted as OH radical scavengers; at high concentrations, they acted as reductant precursors: Rab, formed by abstraction reactions of the alcohols with sonochemically formed OH radicals or H atoms, and Rpy, formed by alcohol pyrolysis under ultrasonic irradiation. The results suggest that the reactivity order of the sonochemically formed reducing species with MnO4(-) at pH 7-9 is the sum of H2O2 and H>Rpy>Rab. The peak wavelengths of MnO2 colloidal solutions formed at high 1-butanol concentrations shifted to shorter wavelengths, suggesting the formation of small particles at high 1-butanol concentrations. The rates of sonochemical reduction of MnO2 to Mn(2+) in the presence of 1-butanol were slower than that in the absence of 1-butanol, because the sonochemical formation of H2O2 and H, which act as reductants, was suppressed by 1-butanol in aqueous solutions.

A 90-day experiment was conducted to study the effect of sub-lethal concentration of cypermethrin exposure on mortality, Gonadosomatic index (GSI), ovarian cell development and hemato-biochemical parameters of Gangetic mystus (Mystus cavasius). According to previous findings of LC50 value (30µg/L) of cypermethrin the experiment was carried out using four sub-lethal concentrations as treatment (0.0, 4.0, 8.0, and 16.0µg/L). The whole water was exchanged in every 4th day and pesticide was mixed accordingly. For collection of gonad and blood, fish were sampled at 30, 60 and 90 days after starting of pesticide exposure. There were no significant changes on GSI. Several histopathological changes were observed in ovary. Wrinkle oocyte, cytoplasmic clumping, atretic follicle, degenerated granulose layer, degenerated oocyte wall, increased inter follicular space, adhesion, cyst, necrosis were found in the ovary of M. cavasius with increasing cypermethrin concentration and exposure time. Blood glucose level and WBCs were significantly increased whereas blood hemoglobin and RBCs were significantly decreased in exposure groups compared to control groups over the three sampling days. The current findings revealed that cypermethrin had negative impact on the hemato-biochemical parameters and female gonad of M. cavasius. So the use of cypermethrin in agriculture should be done with a great caution.

One of the challenges in the aquaculture industry is providing nutritionally balanced and environmentally sustainable live food for fish larvae. Therefore, the rearing techniques of fish larvae with preferred starter food should be given importance for obtaining optimal hatchery production. Nile tilapia, Oreochromis niloticus larvae just after yolk absorption (body length 0.950 ± 0.004 mm; body weight 6.00 ± 0.02 mg) were reared in laboratory conditions for 16 days, feeding with 6 different diets to know their effect on survival and growth. The diets were live Monoraphidium littorale (T₁), live enriched copepods with M. littorale (T₂), powdered M. littorale (T₃), powdered enriched copepods (T₄), live M. littorale + live enriched copepods (T₅), and powdered M. littorale + powdered enriched copepods (T₆). The proximate composition, amino acid profile and fatty acid content of both M. littorale and copepods were analyzed. The biochemical analysis of the dried powder of enriched copepods and M. littorale revealed that both of them are excellent sources of protein, amino acids, and lipids, especially with monounsaturated and polyunsaturated fatty acids. The O. niloticus larvae fed the T₂ diet exhibited the most favourable outcomes, with significantly higher larval gain in weight and percent weight gain, in comparison to the larvae fed other diets (p < 0.001 for all comparisons). The LG% and SGR of the larvae were also significantly higher in T₂ in comparison to the T₁, T₃, T₄, and T₆ (p < 0.001 for all comparisons) except T₅. In addition, the highest percent survival rate of the larvae was observed in T₂ (95 %) followed by T₄ (93 %), T₆ (93 %), T₃ (82 %), T₅ (73 %) and then T₁ (43 %). Based on the present findings, it is recommended that live copepods enriched with M. littorale can be utilized as a starter food for the rearing of Nile tilapia, O. niloticus larvae in hatcheries because of its enriched nutritional profile.

Selenium (Se) plays a crucial role in fishes, but even a slight increase beyond physiological levels can make it highly toxic. In contaminated environments, fishes primarily accumulate Se as selenomethionine (SeMet). While pernicious effects of SeMet in adult fishes are well-documented, its embryotoxicity, beyond teratogenic outcomes, are underexplored. In current study, 2 h post fertilized (hpf) zebrafish embryos were subjected to waterborne Se at sub-lethal concentrations (0 [control], 5, 10, and 25 µg/L; as SeMet) until 5 days post-fertilization (dpf). Results revealed that SeMet exposure at 10 and 25 µg/L significantly increased larval mortality and deformity rates compared to the control group. Moreover, SeMet exposure (5 and 10 µg/L Se) impaired thigmotactic and reflexive behaviours at 5 dpf. Embryonic SeMet exposure also resulted in an escalation in reactive oxygen species levels and apoptosis with elevating concentration, along with dysregulation of proteins and genes related to nervous system development including dopaminergic, serotonergic, and cholinergic signaling pathways at 5 dpf. Interestingly, pretreatment with N-acetylcysteine (NAC), a potent antioxidant, resulted in amelioration of SeMet-induced oxidative stress, apoptosis, molecular and behavioural deficits, suggesting that oxidative stress is an initiating mediator in triggering neurobehavioural impairments in larval zebrafish. Overall, this study highlights that neurobehavioural responses are more sensitive to SeMet than its teratogenic effects in larval zebrafish, thus providing novel perspectives on the developmental toxicity of Se in fish.

The present study was conducted to understand the changes in gonads and hematological parameters in climbing perch (Anabas testudineus) exposed to synthetic estrogen and progestogen [mixture of ethinylestradiol (EE2) and desogestrel (DES)]. Climbing perch were exposed to four different concentrations of EE2/DES mixtures, viz. 0 ng of EE2 and DES/L (T0), 3 ng EE2 and 15 ng DES/L (T3), 30 ng EE2 and 150 ng DES/L (T30), and 300 ng EE2 and 1500 ng DES/L (T300) for 60 days. On days 45 and 60, samples were taken to assess changes in somatic indexes, gonad histology, and hematological parameters. The gonadosomatic index (GSI) increased in both females and males with increasing concentrations of estrogen mixtures except for T30 females, which was the lowest among all\four treatments. The hepatosomatic index (HSI) was observed to be increased in males as estrogen content increased. However, compared to fish at T0, HSI in female individuals did not vary in T30 fish, where the value was the highest among all the treatments. On day 45, histological observations showed no feminization or intersexuality but several germ-cell deformities in the ovary (adhesion, degenerated oocyte wall, degenerated granulose layer, increased interfollicular space, atretic follicle, and cytoplasmic clumping) and testes (increased interstitial area, focal loss of spermatocyte, dilation of the lumen, breakage of tubular epithelium, and elongated seminiferous tubule) were observed in fish exposed to EE2 and DES. Fish reared at T30 had lower RBC count, hemoglobin (Hb), glucose, and hematocrit levels. On day 60, fish reared at T30 had the highest Hb content compared to fish raised in other treatment conditions. WBC was progressively higher with increasing EE2/DES concentrations. Significant erythrocyte cytoplasmic abnormalities and erythrocyte nuclear abnormalities were observed in fish exposed to higher EE2/DES concentrations. The present study provides insights into the adverse impacts of synthetic estrogens sourced from human contraceptive pills on fish physiology.