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Morphological responses to flowing water have been documented in many fishes, but 1 rarely reported in species with gibbose body types, despite their use of riffles in rivers and 2 streams. In this study, we compared morphological responses to water flow in three gibbose 3 centrarchids; the bluegill Lepomis macrochirus (Rafinesque, 1819), pumpkinseed Lepomis 4 gibbosus (L., 1758), and rock bass Ambloplites rupestris (Rafinesque, 1817). Flow-through 5 plexiglass units, set to either a sustained flow or static conditions, were used to rear juveniles 6 from each species for an 80-day period. All three species developed more streamlined body 7 forms, longer and thinner caudal peduncles and longer pectoral fins when reared in sustained 8 current. The pumpkinseed exhibited the highest level of phenotypic plasticity in response to 9 water flow, followed closely by bluegill; rock bass was the least plastic species. Rock bass 10 developed longer pre-dorsal and pre-pectoral lengths, which differ from the morphological 11 changes observed in the bluegill and pumpkinseed. The findings not only suggest that some 12 gibbose fishes are capable of exhibiting strong phenotypic responses to water flow, but that 13 many morphological changes are species specific, even within the same taxonomic family. 14 Lastly, all three species developed shorter dorsal fin base lengths when reared under flowing 15 condition, which differs from some previously documented work on fusiform fishes. The results 16 of this study provide a better understanding of how morphology in three closely related species 17 varies in response to environmental conditions. 18 19

Phenotypic plasticity can contribute to the proliferation and invasion success of nonindigenous species by promoting phenotypic changes that increase fitness, facilitate range expansion and improve survival. In this study, differences in phenotypic plasticity were investigated using young-of-year pumpkinseed sunfish from colonies established with lentic and lotic populations originating in Canada (native) and Spain (non-native). Individuals were subjected to static and flowing water treatments for 80 days. Interand intra-population differences were tested using ANCOVA and discriminant function analysis, and differences in phenotypic plasticity were tested through a MANOVA of discriminant function scores. Differences between Iberian and North American populations were observed in dorsal fin length, pectoral fin position and caudal peduncle length. Phenotypic plasticity had less influence on morphology than genetic factors, regardless of population origin. Contrary to predictions, Iberian pumpkinseed exhibited lower levels of phenotypic plasticity than native populations, suggesting that canalization may have occurred in the non-native populations during the processes of introduction and range expansion.