Metabolism of mesopelagic and bathypelagic copepods in the western North Pacific Ocean

Respiration (= oxygen consumption) rates and electron transport system (ETS) enzyme activities in conjunction with body carbon and nitrogen composition (for respiration) or protein (for ETS) were determined for over 50 copepod species from the mesopelagic (M; 500 to 1000 m), upper-bathypelagic (UB; 1000 to 2000 m) and lower-bathypelagic (LB; 2000 to 3000 m) zones of the western subarctic Pacific. Calculated specific respiration rates (SR, a fraction of body carbon respired) at in situ temperatures (3, 2 and 1.5 degrees C for the M, UB and LB zones, respectively) were greater for the M zone copepods (mean: 1.1% body C d(-1)) than that for the UB and LB zone copepods (both 0.6% body C d(-1)). Respiration rates adjusted to those at 1 degrees C by using a Q(10) value (2.0), and to those of specimens with 1 mg body nitrogen by using a body mass exponent (0.8) (e.g. adjusted metabolic rates, AMR, in mu l O-2 (mg body N)(-0.8) h(-1)) were also greater for the M zone copepods than for the UB and LB zone copepods. ETS activities, measured as mu l O-2 (Mg protein)(-0.8) h(-1), showed the same depth-related decline from the M zone to the LB zone. Stepwise regression analysis revealed that stage/sex, feeding type and/or reaction speeds (as judged by the presence/absence of myelin sheath enveloping axons) of copepods were possible additional variables affecting their respiration rates and ETS activities. The reduction in respiration rates and ETS activities from the M zone to the UB or LB zone is more pronounced when respiration rate data on Arctic/Antarctic epipelagic copepods is added; the same is true for ETS activities when respiration rate data is added from copepods dominant in the subarctic Pacific. The present results are compared with those of micronektonic crustaceans and fishes reported for specimens collected from 500 to 2000 m in other regions and discussed in the light of the 'visual interactions' hypothesis.