Intraspecific variation in aerobic and anaerobic locomotion: gilthead sea bream (Sparus aurata) and Trinidadian guppy (Poecilia reticulata) do not exhibit a trade-off between maximum sustained swimming speed and minimum cost of transport Intraspecific variation and trade-off in aerobic and anaerobic traits remain poorly understood in aquatic locomotion. Using gilthead sea bream (Sparus aurata) and Trinidadian guppy (Poecilia reticulata), both axial swimmers, this study tested four hypotheses: (1) gait transition from steady to unsteady (i.e., burst-assisted) swimming is associated with anaerobic metabolism evidenced as excess post exercise oxygen consumption (EPOC); (2) variation in swimming performance (critical swimming speed; U crit) correlates with metabolic scope (MS) or anaerobic capacity (i.e., maximum EPOC); (3) there is a trade-off between maximum sustained swimming speed (U sus) and minimum cost of transport (COT min); and (4) variation in U sus correlates positively with optimum swimming speed (U opt ; i.e., the speed that minimizes energy expenditure per unit of distance traveled). Data collection involved swimming respirometry and video analysis. Results showed that anaerobic swimming costs (i.e., EPOC) increase linearly with the number of bursts in S. aurata, with each burst corresponding to 0.53 mg O 2 kg −1. Data are consistent with a previous study on striped surfperch (Embiotoca lateralis), a labriform swimmer, suggesting that the metabolic cost of burst swimming is similar across various types of locomotion. There was no correlation between U crit and MS or anaerobic capacity in S. aurata indicating that other factors, including morphological or biomechanical traits, influenced U crit. We found no evidence of a trade-off between U sus and COT min. In fact, data revealed significant negative correlations between U sus and COT min , suggesting that individuals with high U sus also exhibit low COT min. Finally, there were positive correlations between U sus and U opt. Our study demonstrates the energetic importance of anaerobic metabolism during unsteady swimming, and provides intraspecific evidence that superior maximum sustained swimming speed is associated with superior swimming economy and optimum speed. Keywords: aerobic metabolic scope, anaerobic capacity, burst swimming, excess post exercise oxygen consumption, intraspecific variation and trade-off, locomotion, maximum sustained swimming speed, minimum cost of transport Abbreviations: EPOC, Excess post exercise oxygen consumption; BL, Body length; COT, Cost of transport; COT min , Minimum cost of transport; Exercise MO 2 , Metabolic rate measured in swimming fish (i.e., instantaneous metabolic rate); MO 2 , Metabolic rate; MO 2active , Active metabolic rate defined as the maximum metabolic rate maintained for 0.5 h; MO 2max , Maximum metabolic rate defined as the maximum metabolic rate measured at increasing swimming speeds; MO 2routine , Routine metabolic rate defined as the average metabolic rate in fish swimming at 0.5 BL s −1 ; MO 2stand , Standard metabolic rate (i.e., a in Equation 2); MO 2sus , Maximum sustained metabolic rate defined as the maximum metabolic rate (over 0.5 h) without any EPOC (i.e., no influence of anaerobic metabolism); Total MO 2 , Exercise MO 2 and EPOC combined as an estimate of the total metabolic swimming cost; U active , Swimming speed associated with the active metabolic rate