Comparison of some epigean and troglobiotic animals regarding their metabolism intensity. Examination of a classical assertion

This study determines oxygen consumption (R), electron transport system (ETS) activity and R/ETS ratio in two pairs of epigean and hypogean crustacean species or subspecies. To date, metabolic characteristics among the phylogenetic distant epigean and hypogean species (i.e., species of different genera) or the epigean and hypogean populations of the same species have been studied due to little opportunity to compare closely related epigean and hypogean species. To fill this gap, we studied the epigean Niphargus zagrebensis and its troglobiotic relative Niphargus stygius, and the epigean subspecies Asellus aquaticus carniolicus in comparison to the troglobiotic subspecies Asellus aquaticus cavernicolus. We tested the previous findings of different metabolic rates obtained on less-appropriate pairs of species and provide additional information on thermal characteristics of metabolic enzymes in both species or subspecies types. Measurements were done at four temperatures. The values of studied traits, i.e., oxygen consumption, ETS activity, and ratio R/ETS, did not differ significantly between species or subspecies of the same genus from epigean and hypogean habitats, but they responded differently to temperature changes. Higher Q10-values for oxygen consumption of N. stygius than N. zagrebensis in the temperature range 10-20°C and higher Ea indicated higher thermal sensitivity in the subterranean species. On the other hand, lower Q10 and Ea-values for ETS activity of N. stygius than N. zagrebensis indicated more thermally stable enzymatic machinery in N. stygius than N. zagrebensis. In Asellus, we observed a similar trend of lower Ea for oxygen consumption and higher Ea for ETS activity in epigean than the troglomorphic subspecies, but the values did not differ significantly between the two. Our most important conclusion is that the low metabolic rate is not a universal property of troglobiotic animals, and the degree of metabolic adaptation is not necessarily in agreement with the degree of morphological adaptation (troglomorphy)