Routine metabolic rates of pelagic marine fishes and cephalopods as a function of body mass, habitat temperature and habitat depth

• A predictive model of pelagic marine fish and cephalopod respiration is constructed.
• Body mass, habitat temperature, habitat depth and taxon are the model parameters.
• The resultant models explain 95.6–96.4% of the variance of the respiration rates.
• Cephalopods show higher (× 1.5–1.7) C or N-specific respiration rates than fishes.

Cephalopods and fishes are major components of marine micronekton and nekton, so an understanding of their physiology and roles in ocean biogeochemistry is important. I compiled the routine respiration rates (50 datasets on 41 cephalopod species; 102 datasets on 90 fish species) from various depth horizons (< 1300 m) of the world's oceans and analyzed these rates as a function of body mass [wet mass (WM), dry mass (DM), carbon (C) or nitrogen (N)], habitat temperature and habitat depth using multiple regression. Stepwise-regression analyses revealed that body mass was the most important parameter, followed by habitat temperature and habitat depth, and these variables explained 89.7–93.8% and 94.7–95.8%, respectively, of the variance in the respiration data of fishes and cephalopods. The addition of a taxon category (order or family) as a fourth variable improved these correlations only slightly (95.6–95.7% and 95.7–96.2%, respectively). The resultant regression equation showed higher respiration rates in cephalopods than fishes relative to the DM, C or N body mass (by a factor 1.5- to 1.7-fold), but not to the WM body mass. The O:N ratios (respiration/ammonia excretion, by atoms) reported for 6 cephalopods (median: 13.2) and 35 fishes (24.2) suggested the predominance of protein as a metabolite in the former and carbohydrate or lipid in the latter. The present results are discussed in light of the methodological constraints and standing hypothesis for the relationship between the metabolic rate and temperature. The empirical models established in the present study can be used to assess the roles of cephalopods and fishes in C and N cycles in pelagic ecosystems based on the organisms' body mass spectra, ambient temperatures and depth distributions.