The morphology of an individual can affect functional performance and, ultimately, survival and fitness. To study these links, a first step is to evaluate the relationship between morphology and performance. Sicydiine fishes are an interesting model for such studies, because successful completion of their life cycle depends on a functionally demanding task: climbing waterfalls when they return to freshwater from the ocean. Previous studies identified two different climbing modes among sicydiines: ‘inching’ and ‘powerburst’. Relationships between morphology and climbing performance have been found for inching and powerburst species present on different islands from distant oceans (Pacific and Caribbean), but species from the same island have not been compared. In this study, we challenged two different sicydiine species from Réunion Island to an artificial climbing ramp: the inching climber Sicyopterus lagocephalus and the powerburst climber Cotylopus acutipinnis. For each species, we evaluated differences in morphology between successful and unsuccessful climbers. We predicted that species with similar climbing styles (S. lagocephalus and S. stimpsoni) would show differences between successful and unsuccessful climbers that more closely resembled each other than differences between successful and unsuccessful climbers across species from the same island (S. lagocephalus and C. acutipinnis). This prediction was only partially met. S. lagocephalus individuals with a streamlined body showed better climbing performance, as observed for S. stimpsoni. However, the size of the pelvic sucker, which fish use to attach to the substrate while resting, appeared less important for S. lagocephalus than for S. stimpsoni and C. acutipinnis. Instead, the size of the mouth, which inching species use to attach to the substrate while moving up, seemed important for S. lagocephalus but not for other species. Thus, differences in the morphology–performance relationship among sicydiines may relate not only to differences in climbing mode, but also to species‐specific specializations within each climbing mode.

Fish locomotor performance depends on inter‐individual morphological differences and influences the response of populations to anthropogenic impacts. Amphidromous gobies, Sicyopterus lagocephalus and Cotylopus acutipinnis can climb up obstacles several metres high, after their arrival in rivers from the ocean. A previous study demonstrated that juveniles of S. lagocephalus performed better than C. acutipinnis when challenged with an artificial climbing ramp, and that individual morphology explained climbing performance for both species. This present study was focused on the effects of two 10‐m high dams on the individual morphological selection of these species. We hypothesized that, compared with C. acutipinnis, (a) S. lagocephalus juveniles will reach the dams faster after they arrive in freshwater and (b) their morphology will play a lesser role in their capacity to climb up the dams. Anatomical traits were measured for 186 and 201 S. lagocephalus and 179 and 221 C. acutipinnis captured downstream and upstream of the dams, respectively. The time they spent in freshwater was estimated by examining the microstructures of their otoliths. As expected, S. lagocephalus individuals reached the dams 10–20 days faster than C. acutipinnis, and their morphology upstream and downstream of the dams did not differ, in contrast to C. acutipinnis. Upstream of the dam, C. acutipinnis had a larger pelvic sucker than downstream, confirming experimental observations. It suggests that juveniles of the cosmopolitan S. lagocephalus have more efficient upstream migration capacities than the endemic C. acutipinnis. Moreover, differences in individual morphologies above and below barriers appear efficient for quantifying their impact on upstream migration of amphidromous species.

The life cycle of gobies of the Sicydiinae subfamily depends on climbing waterfalls. Two sympatric sicydiines species from Reunion Island, Sicyopterus lagocephalus (SIL) and Cotylopus acutipinnis (COA), employ different climbing modes. SIL uses a steady "inching" mode interrupted by short rest periods, whereas COA exhibits short "power-burst" undulatory movements punctuated by longer rest periods. Consequently, we explored the relationship between climbing performance and metabolic activity in these two species. We demonstrated that the two climbing modes are supported by different ecophysiological profiles that promote the interspecific variability of locomotor performance. More specifically, SIL performed better than COA during a climbing experiment because of its inching climbing mode, supported by a generally greater metabolic capacity and a higher potential for oxidative metabolism. Interestingly, we did not detect any difference in metabolic fuel storage and lactate production during climbing in either species, suggesting that these species can maintain fuel reserves and limit lactate accumulation through extensive rest times. Overall, this study provides new insights into the ecophysiology of these two emblematic species and suggests that the better climbing capacity of SIL is supported by its muscular metabolic capacity.