Oxygen Consumption Rates and Metabolic Enzyme Activities of Oceanic California Medusae in Relation to Body Size and Habitat Depth.

Oxygen consumption rates were measured in 14 species of hydromedusae and 5 species of bathypelagic coronate scyphomedusae. Analysis of all individuals of all species of medusae showed the familiar pattern of decreasing specific oxygen consumption rate with increasing wet weight of animals. Citrate synthase (CS), lactate dehydrogenase (LDH), malate dehydrogenase (MDH), and pyruvate kinase (PK) activities were measured in more than 30 species of medusae. Octopine dehydrogenase, strombine dehydrogenase, and alanopine dehydrogenase were not detected in either hydromedusae or scyphomedusae. The allometric scaling phenomenon of decreasing activity in larger individuals was observed in Krebs cycle enzyme activities. LDH activities, on the other hand, increased with increasing wet weight. Most medusae were aerobically poised, with higher CS activities than LDH activities. However, several meso- and bathypelagic medusae, including the coronate scyphozoans Periphylla periphylla and Nausitho{e2dot} rubra, were anaerobically poised, possibly as a mechanism to assist in vertical migrations at low oxygen concentrations in the oxygen minimum layer. There is poor correlation between CS activities and oxygen consumption rates in these medusae when compared to previously investigated animals. To account for this poor correlation, we propose the hypothesis that medusan CS at the periphery of the maximum diffusion distance may be oxygen-limited and does not function at the normal in vivo rate. For pelagic medusae, there is no apparent decline in metabolic rate and metabolic potential, as determined by enzymatic activity, with increasing depth of occurrence, beyond the declines caused by the decrease in temperature with depth. These patterns are in contrast to the rapid declines in metabolic rates and metabolic potentials with depth that have been observed for pelagic fishes and crustaceans. Deep-living medusae have metabolic rates of a magnitude similar to those of bathypelagic fishes and crustaceans.