The Maintenance of Different Enzyme Activity Levels in Congeneric Fishes Living at Different Depths

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Abstract
To determine the influence of depth-related factors on enzymic activity and, hence, on metabolism, we measured enzymic activities of white skeletal muscle and brain in two teleost fishes of the genus Sebastolobus (Scorpaenidae). These species are genetically close and have similar life histories but differ in their depth distributions. White skeletal muscle of the shallow-living species, S. alascanus, contained approximately twice the activity (measured as units per g wet weight of tissue) of lactate dehydrogenase (LDH), pyruvate kinase (PK), malate dehydrogenase (MDH), creatine phosphokinase (CPK), and citrate synthase (CS) as the white muscle of the deep-living species, S. altivelis. The CS and LDH activities exhibited body-sizerelated scaling; CS activity decreased and LDH activity rose in larger-sized specimens. The interspecific activity differences may derive either from differences in substrate turnover number (e.g., LDH) or from differences in enzyme concentration (e.g., PK). Through these two activity-regulating mechanisms, constant ratios of activity of different enzymes within the muscles of each species are maintained, while approximately twofold interspecific differences are maintained. The interspecific differences in enzymic activity are attributed to the importance of depth-related environmental factors, e.g., food availability, lack of light, and a low-energy physical environment, which may permit reduced levels of metabolism at depth. Correlated with the reduced capacity for muscle metabolism in S. altivelis is a small (9%) reduction in muscle-buffering capacity. Protein and water content did not differ in the muscle tissues of the two species. Unlike muscle, in brain tissue no interspecific differences in enzymic activity (LDH, PK, and CS) were found. This suggests that the differences in enzymic activity noted for muscle tissue reflect muscle-specific changes associated with reduced metabolic and locomotory demands at depth. Comparisons of congeneric species experiencing similar thermal regimes and having similar life history and ecological characteristics, but having different depth distributions, permit sensitive analyses of depth-related biochemical adaptations.