Tropical coral reef teleosts are exclusively ectotherms and their capacity for physical and physiological performance is therefore directly influenced by ambient temperature. This study examined the effect of increased water temperature to 3 °C above ambient on the swimming and metabolic performance of 10 species of damselfishes (Pomacentridae) representing evolutionary lineages from two subfamilies and four genera. Five distinct performance measures were tested: (a) maximum swimming speed (U_crit), (b) gait‐transition speed (the speed at which they change from strictly pectoral to pectoral‐and‐caudal swimming,U_p−c), (c) maximum aerobic metabolic rate (MO_2−MAX), (d) resting metabolic rate (MO_2−REST), and (e) aerobic scope (ratio of MO_2−MAXto MO_2−REST, A_SC). Relative to the control (29 °C), increased temperature (32 °C) had a significant negative effect across all performance measures examined, with the magnitude of the effect varying greatly among closely related species and genera. Specifically, five species spanning three genera (Dascyllus, NeopomacentrusandPomacentrus) showed severe reductions in swimming performance withU_critreduced in these species by 21.3–27.9% andU_p−cby 32.6–51.3%. Furthermore, five species spanning all four genera showed significant reductions in metabolic performance with aerobic scope reduced by 24.3–64.9%. Comparisons of remaining performance capacities with field conditions indicate that 32 °C water temperatures will leave multiple species with less swimming capacity than required to overcome the water flows commonly found in their respective coral reef habitats. Consequently, unless adaptation is possible, significant loss of species may occur if ocean warming of ≥3 °C arises.

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