General patterns of salinity influence on the energy balance of aquatic animals in hypersaline environment

The balance-energy approach, based on the principles of thermodynamics, makes it possible only to outline the area of the possible, to cut off what cannot be. It is widely used in hydrobiology/ecology to describe ecosystems, but very rarely to understand certain empirically found patterns. In the article, based on the generalization and analysis of our own and literature data, several empirical generalizations were made on the effect of salinity in the hypersaline interval from 35 to 300 g/L on animals, which were analyzed using the balance-energy approach. According to the type of osmoadaptive mechanisms, animals are divided into active osmoregulators of salt concentration in body fluids and osmoconformers, in which osmoadaptation is carried out inside the cells (accumulation of osmolyte substances), as in unicellular organisms. Animals-osmoconformers can not only synthesize different types of osmolytes themselves but also use osmolytes of consumed primary producers or dissolved organic matter. With an increase in salinity above the optimum for both types, there is an increase in energy expenditure for these mechanisms operation. The upper limit of halotolerance in osmoconformers can be primarily determined by the presence of available osmolytes in the environment, rather than by the physiological characteristics of the species. With an increase in salinity, in proportion to the increased viscosity of the solution, there is an increase in the cost of movement for all animals. So, spending on movement will tend to the maximum possible, sharply limiting the possibilities of locomotor activity. This reduces the possibility of obtaining the necessary diet. Thus, the empirical generalization: as salinity increases above optimal level, total metabolic costs will increase, and the resulting diet will decrease. Analysis of this generalization based on the balance-energy approach allows us to draw some conclusions: 1) the possible definitive size will decrease with increasing salinity, which is observed in nature and experiments; 2) the higher the concentration of food, the less pronounced this effect will be; 3) a decrease in body weight allows the species to exist in wider ranges of salinity and food concentration. This, first of all, can explain the fact that with an increase in salinity and under other unfavorable/stressful conditions, large-sized animal species "drop out" of the community in the first place, which is observed in nature. The balance-energy approach can be used to understand the found empirical patterns. At the same time, it only outlines the areas of the possible and defines trends, not to mention specific realizations of those possibilities and their mechanisms. It cannot give an understanding of the uniqueness of specific complex processes and systems.