General Patterns of Salinity Influence on the Energy Balance of Aquatic Animals in Hypersaline Environment

Abstract

Abstract— The energy balance approach based on the principles of thermodynamics makes it possible only to outline the area of the possible and 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, on the basis of the generalization and analysis of our own and published data, several empirical generalizations have been made concerning the effect of salinity in the hypersaline range from 35 to 300 g/L on animals, which were analyzed using the energy balance approach. According to the type of osmoadaptive mechanisms, animals are divided into active osmoregulators of salt concentration in body fluids and osmoconformers with osmoadaptation performed inside the cells (the accumulation of osmolytic substances) as in unicellular organisms. Osmoconformer animals are able not only to synthesize different types of osmolytes per se but also to 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 the operation of these mechanisms. The upper limit of halotolerance in osmoconformers can be primarily determined by the presence of available osmolytes in the environment rather than by physiological characteristics of a species. The increase in salinity, in proportion to the increased viscosity of solution, is accompanied by an increase in the cost of movement for all animals. Hence, the costs of movement will tend to the maximum possible, sharply limiting the possibilities of locomotor activity. This reduces the possibility of obtaining the necessary diet. Thus, empirical generalization is as follows: as salinity increases above the optimal level, the total metabolic costs will increase and the resulting diet will decrease. The analysis of this generalization based on the energy balance approach leads to certain conclusions: (1) the possible definitive size will decrease with increasing salinity, as is observed in nature and in experiments; (2) the higher the concentration of food, the less pronounced is this effect; (3) a decrease in body weight allows a species to exist in wider ranges of salinity and food concentration. This, first of all, can explain the fact that an increase in salinity and other unfavorable/stress conditions result in “dropping out” of large-sized animal species from the community in the first place, as is observed in nature. The energy balance approach can be used to understand the revealed empirical patterns. At the same time, it only outlines the areas of the possible and defined the trends, not to mention specific realizations of those possibilities and their mechanisms. It cannot provide understanding of the uniqueness of specific complex processes and systems.

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