Plasticity and evolution shape the scaling of metabolism and excretion along a geothermal temperature gradient

Physiological rates are heavily dependent on temperature and body size. Most current predictions of organisms’ response to environmental warming are based on the assumption that key physiological rates like metabolism and excretion scale independently with body size and temperature and will not evolve. However, temperature is a significant driver for phenotypic variability in the allometric scaling and thermal sensitivity of physiological rates within ectotherm species, suggesting that evolution may play a role in shaping these parameters.

We common-reared six populations of western mosquitofish that have recently established (~100 years ago) in geothermal springs along a broad thermal gradient (19-33°C) to determine whether these scaling parameters are affected by evolutionary and/or plastic responses to warming over ecological timescales. Each population was reared at four different temperatures (23, 26, 30 and 32°C). We measured routine metabolic and nitrogen excretion rates on mosquitofish across a wide body size range.

We found evidence for plasticity, but not evolution, increasing the allometric scaling of metabolic rate with temperature. Plasticity in metabolism allometry reflected a decrease in thermal sensitivity at smaller body sizes.

We found evidence for evolution of phenotypic plasticity on the allometry of excretion rate, reflecting evolutionary differences in how thermal sensitivity varies with body size across different populations.

Evolutionary differences in excretion rate scaling did not influence the relationship between excretion and metabolism across rearing temperatures, suggesting that warming does not affect the balance between mosquitofish energy demands and nutrient recycling rates.

Methods
Data metabolism, excretion and body size data were obtained from introduced mosquitofish populations established in six geothermal springs spanning a broad thermal gradient (19-33°C) t and reared at four different temperatures (23, 26, 30 and 32°C).

Routine metabolic rate was measured as oxygen consumption (MO2) of individual fish for ten to fifteen minutes inside closed-system respirometers. Nitrogen excretion rate was estimated as change in ammonia (NH4+-N) concentrations in the closed respirometers over the metabolism assays. Body size was measured as blotted wet weight.