Mass scaling of standard metabolic rate within and among individuals in Western mosquitofish (Gambusia affinis).

This study investigates the scaling exponent (b) of standard metabolic rate (SMR) in western mosquitofish, focusing on both among- and within-individual levels while exploring the influences of growth and sex on within-individual b values. For group-reared fish, the intraspecific b value was determined by analyzing the relationship between SMR and body mass across individuals of varying sizes and ages. For individually-reared fish, the SMR of each fish was measured five times between 60 and 150 days of age, and both among- and within-individual b values were calculated. The among- and within-individual b values of the individually-reared fish across ages did not differ significantly from the intraspecific b value of the group-reared fish. The among-individual b values showed a nonsignificant V-shaped change with increasing age in the individually-reared fish, suggesting that the ontogenetic decrease of b value related to growth slowdown can be reversed post-maturation. Although there was no significant difference in within-individual b values between females and males, greater variability was observed among females. Furthermore, the relationship between within-individual b values and specific growth rate displayed sexual dimorphism, with females exhibiting a stronger positive correlation, suggesting higher energy demands associated with both somatic and gonadal development compared to males.