The effect of hypoxia and hyperoxia on the growth and metabolic rate of <i>Rhinella marina</i> tadpoles

The gill-oxygen limitation theory (GOLT) hypothesises that specific growth rate slows as water-breathing ectotherms increase in size because their two-dimensional respiratory surfaces cannot keep up with the growth of their three-dimensional bodies. Thus, a declining relative oxygen supply causes the slowing and ultimately the cessation of growth. Here, we tested this hypothesis by rearing tadpoles Rhinella marina at four levels of aquatic oxygen (4, 10, 21 and 40 kPa) and measuring their growth rate and resting metabolic rate. We found that growth rates are positively related to environmental oxygen earlier in development, in support of GOLT, but that the difference in size among treatments disappears as animals continue to grow. At the time when among-treatment differences in growth are large, animals reared in hypoxia have elevated metabolic rate. This difference in metabolic rate is hypothesised to arise as a result of osmoregulatory costs associated with gill hypertrophy in hypoxia. We conclude that growth trajectories in tadpoles are shaped by allocation trade-offs among energy-demanding processes, operating within resource availability and supply constraints imposed by the environment and the physical geometry of exchange and transport systems.