Adaptation of rainbow fish to lake and stream habitats. Evolution 57

Abstract. Fish occupy a range of hydrological habitats that exert different demands on locomotor performance. We examined replicate natural populations of the rainbow fishes Melanotaenia eachamensis and M. duboulayi to determine if colonization of low-velocity (lake) habitats by fish from high-velocity (stream) habitats resulted in adaptation of locomotor morphology and performance. Relative to stream conspecifics, lake fish had more posteriorly positioned first dorsal and pelvic fins, and shorter second dorsal fin bases. Habitat dimorphism observed between wild-caught fish was determined to be heritable as it was retained in M. eachamensis offspring raised in a common garden. Repeated evolution of the same heritable phenotype in independently derived populations indicated body shape divergence was a consequence of natural selection. Morphological divergence between hydrological habitats did not support a priori expectations of deeper bodies and caudal peduncles in lake fish. However, observed divergence in fin positioning was consistent with a family-wide association between habitat and morphology, and with empirical studies on other fish species. As predicted, decreased demand for sustained swimming in lakes resulted in a reduction in caudal red muscle area of lake fish relative to their stream counterparts. Melanotaenia duboulayi lake fish also had slower sustained swimming speeds (U crit ) than stream conspecifics. In M. eachamensis, habitat affected U crit of males and females differently. Specifically, females exhibited the pattern observed in M. duboulayi (lake fish had faster U crit than stream fish), but the opposite association was observed in males (stream males had slower U crit than lake males). Stream M. eachamensis also exhibited a reversed pattern of sexual dimorphism in U crit (males slower than females) relative to all other groups (males faster than females). We suggest that M. eachamensis males from streams responded to factors other than water velocity. Although replication of muscle and U crit phenotypes across same habitat populations within and/or among species was suggestive of adaptation, the common garden experiment did not confirm a genetic basis to these associations. Kinematic studies should consider the effect of the position and base length of dorsal fins. Key words. Burst speed, common garden experiment, Melanotaeniidae, morphology, natural selection, red muscle, U crit . Received April 18, 2002. Accepted September 11, 2002 Locomotion is a complex, whole animal function, crucial for activities that have deterministic effects on fitness. For example, differential locomotor performance has been demonstrated to result in both differential survival (e.g., Beamish 1978; Swain 1993) and feeding efficiency (e.g., An extensive array of theoretical and empirical literature underpins our current understanding of fish locomotion (e.g., 3 Present address: Museum of Vertebrate Zoology, 3101 Life Sciences Building, University of California, Berkeley, California 94720-3160. mum performance at one can preclude maximum performance at the other (e.g., Morphologically, maximum sustained swimming performance depends on a high aspect ratio caudal fin, a narrow caudal peduncle, and an inflexible, streamlined body Another factor that deterministically affects swimming performance and is of interest in this paper are the muscles that power swimming. Sustained swimming is powered by red (slow twitch, oxidative) muscle and burst swimming by white (fast twitch, glycolytic) muscle The effect of water velocity on swimming performance and associated traits has received limited attention, despite the known ecological effects and potential role of water velocity in natural selection. Morphologically, fish from fas