The seasonal dynamic of lipids and their fatty acid constituents in the lipid sac and muscles of pelagic postlarval Leptoclinus maculatus, an ecologically important fish species in the Arctic food nets, in Kongsfjord, Svalbard waters was studied. The determination of the qualitative and quantitative content of the total lipids (TLs), total phospholipids (PLs), triacylglycerols (TAGs), cholesterol (Chol), cholesterol esters (Chol esters) and wax esters was analyzed by TLC, the phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidylcholine (PC), lysophosphatidylcholine (LPC) and sphingomyelin (SM) were determined by HPLC, and fatty acids of total lipids using GC. The lipid sac is a system of cavities filled with lipids, and it is not directly connected to organs of the digestive system. The wall’s inner layer is a multinuclear symplast that has a trophic function. The results provide additional knowledge on the role of lipids in the biochemical and physiological adaptation of fish to specific environments and clarify the relationship between fatty acids and the food specialization of postlarvae. Analysis of the fatty acid (FA) profile of TLs in the muscles and lipid sac of daubed shanny pelagic postlarvae showed it to be tissue- and organ-specific, and tightly associated with seasonal variations of environmental factors (temperature conditions and trophic resources).

The influence of two light regimes, 16:8 h light/dark (LD 16:8) and 24:0 h light/dark (LD 24:0), in comparison to a usual hatchery light regime (HL), on the fatty acids content and weight gain in hatchery-reared underyearlings (at 0+ age) and yearlings (at 1+ age) of Atlantic salmon in the summer–autumn period was studied. The total lipids were analyzed by Folch method, the lipid classes using HPTLC, and the fatty acids of total lipids using GC. The increase in EPA and DHA observed in October in underyearlings and yearlings salmon (especially under LD 24:0) suggests they were physiologically preparing for overwintering. The changes in fatty acids and their ratios in juvenile Atlantic salmon can be used as biochemical indicators of the degree to which hatchery-reared fish are ready to smoltify. These associated with an increase in marine-type specific DHA and EPA, an increase in the 16:0/18:1(n-9) ratio, in correlation with a reduction in MUFAs (mainly 18:1(n-9)). These biochemical modifications, accompanied by fish weight gain, were more pronounced in October in yearlings exposed to continuous light (LD 24:0). The mortality rate was lower in experimental groups of underyearliings with additional lighting. Exposure to prolonged and continuous light did not affect yearlings mortality rate.

Photoperiod is associated to phenotypic plasticity of somatic growth in several teleost species, however, the molecular mechanisms underlying this phenomenon are currently unknown. The effect of a continuous lighting (LD 24:0), compared with the usual hatchery lighting (HL) regime, on the growth rate and gene expression of myogenic regulatory factors (MRFs: MyoD1 paralogs, Myf5, and MyoG) myosin heavy chain (MyHC), and MSTN paralogs in the white muscles of hatchery-reared Atlantic salmon yearlings was evaluated over a 6-month period (May to October). The levels of gene expression were determined using real-time PCR. Continuous lighting was shown to have a positive effect on weight gain. MyHC, MyoD1c, MyoD1b, and MSTN1a/b mRNA expression was influenced by the light regime applied. In all the studied groups, a significant positive correlation was observed between the expression levels of MRFs and MSTN paralogs throughout the experiment. The study demonstrated seasonal patterns regarding the simultaneous expression of several MRFs. MyoD1a, MyoG, and MyHC mRNA expression levels were elevated in the mid-October, but MyoD1b/c, and Myf5 mRNA levels decreased by the end of this month. In general, the findings showed that constant lighting affected the regulatory mechanisms of muscle growth processes in salmon.

The general ideas regarding ecological biochemical adaptations in aquatic organisms are based on the results obtained by using a set of field and experimental methods over decades of research on members of marine and freshwater fish fauna and on their food items (mollusks, crustaceans) carried out by Science School in Ecological Biochemistry. Examples of biochemical adaptations in aquatic organisms are provided. Certain conclusions are drawn regarding the common and specific pattern of the metabolic adaptive response in the studied organisms growing and developing under normal conditions and upon changes in the environment. The results improve our understanding of the mechanisms behind ecological biochemical adaptations and their role in maintaining homeostasis in aquatic organisms.

New data on the effect of additional lighting on the lipid profile of Atlantic salmon 0+ fingerlings during growth and development under artificial cultivation in fish farming conditions were obtained. The influence of different lighting modes, such as standard farming lighting conditions (control), experimental modes of 16 h of light and 8 h of dark (16 : 8), and constant lighting (24 : 0), were studied. Specific modifications of the lipid profile—mainly in the content of structural lipids (phospholipids) as well as indexes of the ratios of structural and reserve lipids: the cholesterol/phospholipids (Chol/PL) and triacylglycerols/phospholipids (TAG/PL)—were found in the fingerlings whose development took place under the conditions of experimental photoperiod regimes. These changes were more pronounced in young salmons cultivated under round-the-clock lighting (24 : 0).

The general ideas regarding ecological biochemical adaptations in aquatic organisms based on the results obtained by using a set of field and experimental methods over decades of research on members of marine and freshwater fish fauna and on their food items (molluscs, crustaceans) carried out by the Ecological Biochemistry Think Tank team. Examples of biochemical adaptations in aquatic organisms are provided. Certain conclusions are drawn regarding the common and specific pattern of metabolic adaptive response in the studied organisms growing and developing under normal conditions and upon changes in the environment. The results improve our understanding of the mechanisms behind ecological biochemical adaptations and their role in maintaining homeostasis in aquatic organisms.