Eco-physiological properties of Pontogammarus maeoticus (Amphipoda) in a salinity gradient

Quantitative indices of feeding, food assimilation, energy metabolism and growth in Pontogammarus maeoticus (Sow.) from the Azov Sea (12‰S) were studied from the salinity levels. Juveniles and males of different age groups were sampled for experiments. The relationships between feeding rate (C) and energy metabolism (R) on an energy equivalent of w, body weight were approximated by the power equations \(C = C_1 w^{b_c }\) and R=R₁w^b. To express weight growth, the Bertalanffy equation and its solution were used: $$\frac{{dw}}{{dt}} = Nw^q - kw; w(t) = W_\infty (1 - e^{ - mkt} )^{1/m}$$ Under control salinity (12‰S) the following parameters were obtained: C₁=0.73J^(1-b)d^-1; b_c=0.62; R₁=0.53J^(1-b)d^-1; b=0.67. The application of a new method suggested by the author for determining the growth parameters, N, k, m=1-q, revealed their dependence on age. For juveniles N=0.0965, k=0.0046, while for adults the respective values were 0.1540 and 0.0181. The value m=0.39 was the same for both age classes. The individuals were gradually transferred to conditions of changed salinity (～0; 4 and 18‰S). The observations lasted up to two months. Through the entire range of salinity (～0–18‰S), food assimilation alone was constant. In relation to dry matter, food assimilation was 65 and 80% in terms of caloric value. The other indices under study remain at the control level only under desalinization to 4‰S. When P. maeoticus was maintained in fresh water, the juveniles showed enhanced levels of nutrition, energy metabolism and increment, while in mature individuals the respective parameters decreased compared with the control. As a result, the parameters C₁ and R₁ in the functions C(w) and R(w) were ～2 times higher and b_c and b～2 times lower than in 12‰S. An increase in salinity to 18‰S resulted in a decline of all the processes concerned: C₁=0.61, R₁=0.36, b_c and b remained at the control level, while growth rate became 20% lower. A full compensation type acclimation for P. maeoticus is possible with the salinity range (12–4‰S) where the individual can maintain ionic balance.