Interspecies differences in lactate dehydrogenase and citrate synthase activity among damselfish and cardinalfish

Species with different thermal distributions, life-history traits, and behaviours have evolved physiological processes to suit energetic demands. Previous research has argued that these interspecies differences are often reflected in muscle enzyme activity that serve as proxies for aerobic and anaerobic respiration. Here, we measured the maximal enzyme activity of two enzymes, citrate synthase and lactate dehydrogenase, between two damselfish (Pomacentrus) and cardinalfish (Ostorhinchus) species. Citrate synthase was measured as a proxy for mitochondrial volume density, a marker of aerobic metabolism; lactate dehydrogenase was measured as a proxy for anaerobic energy production, a marker for anaerobic metabolism. Thermal performance curves of maximal enzyme activity were measured from 10 to 50 °C, at 10 °C intervals. Citrate synthase and lactate dehydrogenase both showed a positive correlation with temperature, that was absent of a plateau. Damselfish displayed higher levels of citate synthase maximal enzyme activity, while cardinalfish displayed a higher lactate dehydrogenase to citrate synthase ratio. Ostorhinchus doederleini, a sedentary cardinalfish, displayed higher level of lactate dehydrogenase maximal enzyme activity. Temperature coefficients (Q10) for lactate dehydrogenase showed a curved relationship, peaking at differences between 30 and 40 °C. No differences in Q10 values were observed between species, suggesting no difference in the thermal sensitivity of enzymes. Interspecies differences in maximal enzyme activity identified in this study compliments previous research, whereby more active species require higher levels of citrate synthase to fuel sustained swimming, as well as energetically demanding locomotion behaviours. Alternatively, more sedentary species possessed higher levels of lactate dehydrogenase and reliance on anaerobic metabolism, possibly due to an increased reliance on infrequent burst swimming behaviours.