Are the surface areas of the gills and body involved with changing metabolic scaling with temperature?

The metabolic-level boundaries (MLB) hypothesis proposes that metabolic level mediates the relative influence of surface area (SA) vs. volume related metabolic processes on the body-mass scaling of metabolic rate in organisms. The variation in the scaling of SA may affect how metabolic level affects the metabolic scaling exponent. This study aimed to determine the influence of increasing metabolic level at a higher temperature on the metabolic scaling exponent of the crucian carp and determine the link between metabolic scaling exponents and SA parameters of both gills and body. The SA of gills and body and the resting metabolic rate (RMR) of the crucian carp were assessed at 15oC and 25oC, and their mass scaling exponents were analyzed. The results showed a significantly higher RMR, with a lower scaling exponent, in the crucian carp at a higher temperature. The gills SA and the total SA of the fish (TSA) are reduced with the increasing temperature. The bRMR tends to be close to that of the TSA at a higher temperature. This suggests that temperature positively affects metabolic level but negatively affects the scaling exponent of RMR (bRMR). The findings support the MLB hypothesis. The lower scaling exponent at a higher temperature can be alternatively explained as that the higher viscosity of cold water impedes respiratory ventilation and oxygen uptake and reduces MR more in smaller individuals than in larger individuals at lower temperature, thus resulting in a negative association between temperature and bRMR.