Tissue-specific allometry of an aerobic respiratory enzyme in a large and a small species of cyprinid (Teleostei)

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Abstract
The aerobic enzyme cytochrome-c oxidase (CCO) was used as a measure of tissue-specific metabolic capacity. Changes in tissue CCO activity with increased body size were combined with allometric relationships for tissue weight to describe changes in energy use with increased body size. A large (common carp, Cyprinus carpio) and a small (common shiner, Notropis cornutus) species of cyprinid were used to examine differences between ontogenetic and phylogenetic allometry. With increased size, shifts occurred in energy use from viscera (high metabolic rate) to muscle tissue (low metabolic rate) which would account for the negative allometry of whole-body metabolic rate. This shift was more severe for the larger (i.e., faster growing) species, common carp. Percent muscle mass was fairly constant in the shiner, but increased from 42 to 62% of total body mass in the carp. For both species, the greatest allometry in total tissue CCO activity occurred in the brain and intestine. Total intestinal CCO activity scaled as weight to the exponents 0.70 and 0.54 for the carp and shiner, respectively. The greater proportion of metabolically active visceral tissue in young individuals is apparently not an energetic disadvantage, because these fish often have the highest growth efficiencies. These efficiencies may be due to the more favorable ratio of food "processing" capacity to target growth tissue (muscle) in small fish. The negative allometry in processing ability is likely responsible for the decreases in ingestion and growth rates with increased size. The influence of growth on ectotherm metabolism is large and, because growth is allometric, it can affect the exponent for metabolic rate in ontogenetic allometry studies.