Mass-Scaled Rates of Respiration and Intrinsic Growth in Very Small Invertebrates

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Abstract
Rates of O2 uptake and intrinsic growth of very small invertebrate species near 20.degree. C were compared with those of larger invertebrate and unicellular animals by means of the allometric relation (rate .alpha. Mb where M = mass). Respiration rates of small species of major invertebrate taxa are low than those extrapolated for larger invertebrates but generally higher than those for protozoans of the same mass. Mass-specific rates of small metazoans and protozoans are lowered accordingly; their total food and O2 consumption is likely to be small relative to that of large metazoans in average pelagic and benthic communities where most of the biomass is in large animals. The exponent b for each of the major taxa treated here tends to be near 0.75 rather than close to unity. The respiration of these taxa is about as mass dependent as that of large animals. Metabolic reduction is set in relation to small adult size as such, and previous inferences about the phylogeny of metazoan metabolism do not seem warranted any more. Intrinsic growth rates of benthic rotifers and marine free-living nematodes are not only lower than those extrapolated for larger invertebrates but fall below those for protozoans of the same mass. Among benthic harpacticoid copepods, a slight reduction relative to mass-scaled large invertebrates is indicated. Pelagic freshwater rotifers do not exhibit this reduction. Intrinsic growth rates of pelagic freshwater cladocerans are independent of adult mass and not clearly higher than those of the few pelagic copepods studied. Future research into the physiological reasons for mass dependence of metabolism and growth should focus on unicellular or small multicellular species because of their short generation times.