Prolonged survival out of water is linked to a slow pace of life in a selfing amphibious fish

Metabolic rate and life history traits vary widely both among and within species reflecting trade-offs in energy allocation, but the proximate and ultimate causes of variation are not well understood. We tested the hypothesis that these trade-offs are mediated by environmental heterogeneity, using isogenic strains of the amphibious fish Kryptolebias marmoratus that vary in the amount of time each can survive out of water. Consistent with pace of life theory, the strain that survived air exposure the longest generally exhibited a “slow” phenotype including the lowest metabolic rate, largest scope for metabolic depression, slowest consumption of energy stores, and least investment in reproduction under standard conditions. Growth rates were fastest in the otherwise “slow” strain, however. We then tested for fitness trade-offs between “fast” and “slow” strains using microcosms where fish were held with either constant water availability or under fluctuating conditions where water was absent for half of the experiment. Under both conditions the “slow” strain grew larger and was in better condition, and under fluctuating conditions the “slow” strain produced more embryos. However, the “fast” strain had larger adult population sizes under both conditions, indicating that fecundity is not the sole determinant of population size in this species. We conclude that genetically based differences in pace of life of amphibious fish determine survival duration out of water. Relatively “slow” fish tended to perform better under conditions of limited water availability, but there was no detectable cost under control conditions. Thus, pace of life differences may reflect a conditionally neutral instead of antagonistic trade-off.