Metabolism of deep-sea zooplankton

Because of logistic difficulties and technical complexities, experimental study on the metabolism of deep-sea zooplankton had been lagged far behind those of epipelagic zooplankton until the observation of lower oxygen consumption rates of deeper-living mysids, euphausiids, decapods and micronektonic fishes in early 1970s. Since then, many studies followed and confirmed the pattern. In situ observations with submersibles revealed that many deep-sea pelagic fishes and large crustaceans are 'lethargic', which was postulated to be due to extremely higher water contents or lower muscle (protein or nitrogen) contents in their bodies. According to 'Visual Interactions Hypothesis (VIH)', reduction in metabolism seen in deep-sea pelagic crustaceans, fishes and cephalopods with image-forming eyes are a result from the relaxation of selection for strong swimming abilities (which are accompanied with high metabolic rates) for both predators and prey as the visual reactive distances shorten with increasing depth. For zooplankton taxa lack image-forming eyes (copepods, chaetognaths, medusae etc.) the VIH predicts neither depth-related changes in the metabolism nor chemical composition. However, recent studies on metabolism, chemical composition (N contents) and protein synthesis activity (RNA:DNA ratios) of pelagic copepods revealed the depth-related pattern similar to that of taxa with image-forming eyes, leading an alternative 'Predation-Mediated Selection Hypothesis (PMSH)'. PMSH explains depth-related reduction in metabolism to be a result of lowered selective pressure of high activity toward depth in all taxa with and without eyes. Differences between VIH and PMSH are discussed in the light of rational of the definition of the depth of occurrence of zooplankton ('the minimum depth of occurrence' or 'mid-capture depth') and expression of metabolic rates (wet mass specific rate or N-mass specific rate). In terms of metabolic rate of animals standardized to a body mass of 1 mg body N at 1degC (assumed Qsub(10) = 2), oxygen consumption rates of diverse pelagic taxa including large crustaceans, fishes, cephalopods and copepods from 500-2,000 m depth are shown to converge at 2-3 micro l O2/mg N**0.8/h. As an extension of PMSH, a global-bathymetric model for oxygen consumption by copepods with four parameters (body mass, temperature, depth, and oxygen saturation) was shown to be superior to casual global model for general epipelagic zooplankton with two parameters (body mass and temperature) to predict observed oxygen consumption rates of diverse deep-sea zooplankton species. Considering large regional variations in the deep-sea environments of the world oceans, the data of deep-sea zooplankton without eyes presently available represent only from two regions (off south California and Oyashio region, western subarctic Pacific). For the validation of VIH or PMSH, there is an urgent need to collect more metabolic data on zooplankton from diverse deep-sea environments in future study.