Metabolic scaling associated with unusual size changes during larval development of the frog, Pseudis paradoxus

The early larvae of P. paradoxus grow large but metamorphose into relatively small frogs, the diminished post-metamorphic growth producing a marked contrast between maximum larval size and adult. Thus, O uptake does not appear to limit the energy expenditure on growth processes, and unlike in other anuran larvae, may not be a surface area-related function in P. paradoxus larvae. The resting rates of metabolism (Ṁ) and partitioning between aquatic (Ṁw) and aerial O uptake (Ṁa) were measured on tadpoles and froglets by closed system respirometry, using water of P ranging from 145 to 40 mmHg. Correlative changes in body glycogen and lactate were examined by standard enzyme assays. Scaling patterns in the growth and degrowth stages were analysed on whole-body, log-transformed data using linear regressions. In normoxia, Ṁ was 2.1-2.5 μmol g h in the early larvae, increasing more than twofold on forelimb emergence and decreasing sharply in the froglets; Ṁ varies in strict proportion to body mass (M), both in the growth (b=1.02) and degrowth (b=0.97) phases, according to the equation Ṁ=aM, where b is the scaling coefficient. Ṁw constitutes >90% of total uptake in the growth stages, increasing with b=1.02 while Ṁa increases with b=1.13; during degrowth there is a change in the pattern related to intensification of metamorphosis. Hypoxic water did not affect Ṁ; however, in all larval stages Ṁw and Ṁa changed with a decrease in P. At 60 mmHg, rates are more severely affected in the largest tadpoles, causing the b values for Ṁw and Ṁa to change to 0.11 and 1.44, respectively, in the growth phase. Glycogen and lactate levels increase out of proportion with body mass increase (b=2.05 and 1.47, respectively) in the growth stages, and increase anaerobic capacity in late metamorphosis. In hypoxic water, glycogen levels decrease in the growth stages and the largest tadpoles accumulate surplus lactate, possibly related to surfacing activity. Our results may reveal the consequences of size on energy demand at the tissue level in P. paradoxus larvae, indicating that air breathing must subsidise energy expenditure during larval development.