Increased temperature tolerance of the air-breathing Asian swamp eel Monopterus albus after high-temperature acclimation is not explained by improved cardiorespiratory performance

Abstract

This study investigated the hypothesis that in the Asian swamp eel Monopterus albus, an air-breathing sh from south-east Asia that uses the buccopharyngeal cavity for oxygen uptake, the upper criti- cal temperature (Tᵤ) is increased by acclimation to higher temperature, and that the increased Tᵤ is associated with improved cardiovascular and respiratory function. Monopterus albus were there- fore acclimated to 27˚C (current average) and 32˚C(current maximum temperature as well as pro- jected average within 100–200 years), and both the effect of acclimation and acute temperature incre- ments on cardiovascular and respiratory functions were investigated. Two weeks of heat acclimation increased upper tolerated temperature (Tᵤ ) by 2˚C from 36⋅9 ± 0⋅1˚C to 38⋅9 ± 0⋅1˚C (mean ± s.e.). Oxygen uptake (Ṁ O₂) increased with acclimation temperature, accommodated by increases in both aerial and aquatic respiration. Overall, Ṁ O₂ from air (Ṁ O₂ₐ) was predominant, representing 85% in 27˚C acclimated sh and 80% in 32˚C acclimated sh. Ṁ O₂ increased with acute increments in temperature and this increase was entirely accommodated by an increase in air-breathing frequency and Ṁ O₂ₐ . Monopterus albus failed to upregulate stroke volume; rather, cardiac output was maintained through increased heart rate with rising temperature. Overall, acclimation of M. albus to 32˚C did not improve its cardiovascular and respiratory performance at higher temperatures, and cardiovascular adaptations, therefore, do not appear to contribute to the observed increase in Tᵤ.