We measured oxygen consumption rates, the activities of citrate synthase (CS) and lactate dehydrogenase (LDH) and protein contents for over 30 species of deep-sea pelagic Copepoda. The lowest oxygen consumption rates were measured in Euaugaptilus magnus and the highest rates were measured in Paraeuchaeta tonsa. Weight-specific oxygen consumption rates declined significantly with increasing size of the organism. None of the biochemical parameters were particularly good predictors of metabolic rate. Although linear regressions of LDH activity and protein content against oxygen consumption were statistically significant, R-2 values for the relationships were very low. CS activity was not significantly correlated with metabolic rate. The highest CS activities were measured in Pleuromamma abdominalis and Calanus pacificus, which were the 2 smallest and shallowest-living species in our investigation. The lowest CS activities were measured in Euaugaptilus antarcticus and Pachyptilus pacificus. Disseta scopularis had the highest LDH activities and Onchocalanus magnus had the lowest LDH activities. Over all specimens, there were statistically significant increases in weight-specific activities of CS and LDH as a function of body mass. There was much greater variation in glycolytic potential as indicated by LDH activity than in CS activities. Epipelagic copepods apparently rely less on glycolytic energy sources than do mesopelagic and bathypelagic copepods. Higher LDH activities of larger Copepoda may indicate a greater dependence on LDH for burst swimming in large species compared with smaller ones or a reliance on glycolytic abilities for sustained swimming during vertical migrations. Our enzyme data do not support the suggestion that high LDH activities are adaptations to the very low oxygen concentrations found in the oxygen minimum layer. Enzymatic ratios were used to interpret lifestyle, and deep-sea copepods fell into 3 metabolic groups, 'muscular sinkers','thin-muscled floaters' and 'giants', that were related to morphological pattern and behaviour. The effect of hydrostatic pressure on the metabolic rate of an undescribed, but very common, species of Megacalanus was investigated and found to be non-significant. Copepods do not display the depth-related declines in metabolic rates that are found in shrimps, fishes and cephalopods.
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