Morphological selection in an extreme flow environment: body shape and waterfall-climbing success in the Hawaiian stream fish Sicyopterus stimpsoni

Flow characteristics are a prominent factor determining body shapes in aquatic organisms, and correlations between body shape and ambient flow regimes have been established for many fish species. In this study, we investigated the potential for a brief period of extreme flow to exert selection on the body shape of juvenile climbing Hawaiian gobiid fishes. Because of an amphidromous life history, juvenile gobies that complete an oceanic larval phase return to freshwater habitats, where they become adults. Returning juveniles often must scale waterfalls (typically with the use of a ventral sucker) in order to reach the habitats they will use as adults, thereby exposing these animals to brief periods of extreme velocities of flow. Hydrodynamic theory predicts that bodies with larger suckers and with lower heights that reduce drag would have improved climbing success and, thus, be well suited to meet the demands of the flows in waterfalls. To test the potential for the flow environment of waterfalls to impose selection that could contribute to differences in body shape between islands, we subjected juvenile Sicyopterus stimpsoni to climbing trials up artificial waterfalls (～100 body lengths) and measured differences in body shape between successful and unsuccessful climbers. Waterfalls appear to represent a significant selective barrier to these fishes, as nearly 30% failed our climbing test. However, the effects of selection on morphology were not straightforward, as significant differences in shape between successful and unsuccessful climbers did not always match hydrodynamic predictions. In both selection experiments and in adult fish collected from habitats with different prevailing conditions of flow (the islands of Hawai'i versus Kaua'i), lower head heights were associated with exposure to high-flow regimes, as predicted by hydrodynamic theory. Thus, a premium appears to be placed on the reduction of drag via head morphology throughout the ontogeny of this species. The congruence of phenotypic selection patterns observed in our experiments, with morphological character divergence documented among adult fish from Hawai'i and Kaua'i, suggests that differences in morphology between subpopulations of adult climbing gobies may result, at least in part, from the selective pressures of high-velocity flows encountered by migrating juveniles.     

Ontogenetic change in novel functions: waterfall climbing in adult Hawaiian gobiid fishes

Juveniles from three species of Hawaiian gobiid fishes climb waterfalls as part of an amphidromous life cycle, allowing them to re-penetrate adult upstream habitats after being swept out to the ocean upon hatching. The importance of climbing for juvenile stream gobies is well established, but adult fish in upstream island habitats also face potential downstream displacement by periodic disturbances. Thus, retention of climbing ability could be advantageous for adult stream gobies. Climbing performance might be expected to decline among adults, however, due to the tendency for mass-specific muscular power production to decrease with body size, and a lack of positively allometric growth among structures like the pelvic sucker that support body weight against gravity. To evaluate changes in waterfall-climbing ability with body size in Hawaiian stream gobies, we compared climbing performance and kinematics between adults and juveniles from three species: Awaous guamensis , Sicyopterus stimpsoni and Lentipes concolor . For species in which juveniles climbed using 'powerbursts' of axial undulation, adult performance and kinematics showed marked changes: adult A. guamensis failed to climb, and adult L. concolor used multiple pectoral fin adductions to crutch up surfaces at slow speeds, rather than rapid powerbursts. Adult S. stimpsoni , like juveniles, still used oral and pelvic suckers to 'inch' up surfaces and climbed at speeds comparable to those of juveniles. However, unlike juveniles, adult S. stimpsoni also add pectoral fin crutching to every climbing cycle. Thus, although powerburst species appear to be particularly susceptible to size-related declines in waterfall-climbing performance, the addition of compensatory mechanisms prevents the loss of this novel function in some species.     

Muscle fiber type distribution in climbing Hawaiian gobioid fishes: ontogeny and correlations with locomotor performance

Three species of Hawaiian amphidromous gobioid fishes are remarkable in their ability to climb waterfalls up to several hundred meters tall. Juvenile Lentipes concolor and Awaous guamensis climb using rapid bursts of axial undulation, whereas juvenile Sicyopterus stimpsoni climb using much slower movements, alternately attaching oral and pelvic sucking disks to the substrate during prolonged bouts of several cycles. Based on these differing climbing styles, we hypothesized that propulsive musculature in juvenile L. concolor and A. guamensis would be dominated by white muscle fibers, whereas S. stimpsoni would exhibit a greater proportion of red muscle fibers than other climbing species. We further predicted that, because adults of these species shift from climbing to burst swimming as their main locomotor behavior, muscle from adult fish of all three species would be dominated by white fibers. To test these hypotheses, we used ATPase assays to evaluate muscle fiber type distribution in Hawaiian climbing gobies for three anatomical regions (midbody, anal, and tail). Axial musculature was dominated by white muscle fibers in juveniles of all three species, but juvenile S. stimpsoni had a significantly greater proportion of red fibers than the other two species. Fiber type proportions of adult fishes did not differ significantly from those of juveniles. Thus, muscle fiber type proportions in juveniles appear to help accommodate differences in locomotor demands among these species, indicating that they overcome the common challenge of waterfall climbing through both diverse behaviors and physiological specializations.     

Site fidelity and association patterns in a deep-water dolphin: Rough-toothed dolphins (Steno bredanensis) in the Hawaiian Archipelago

In the Pacific, rough-toothed dolphins ( Steno bredanensis ) are typically found in the open ocean and in deep waters around oceanic islands. We examined habitat use, site fidelity, movements, and association patterns of this species in the main Hawaiian Islands. Sighting rates were highest in depths >1,500 m. There were frequent within- and between-year resightings off the island of Hawai'i, indicating a small population size with high site fidelity. Resighting rates were lower off Kaua'i/Ni'ihau, indicating a larger population size, but with some site fidelity. Two individuals were documented moving from Kaua'i to Hawai'i, a distance of 480 km, but were not seen to associate with dolphins off Hawai'i. Observed movements were consistent with at most 2% dispersal per year between these two areas. Differences in group sizes, habitat use, and behavior imply that movements among the islands may be limited. Little is known about the diet of rough-toothed dolphins in Hawai'i, but they are thought to feed primarily on near-surface species. High fidelity to deep-water areas off the island of Hawai'i likely reflects an increase in the predictability of prey associated with upwelling due to the island mass effect, wind stress curl and cyclonic eddies that form off the island.     

Local adaptation despite high gene flow in the waterfall‐climbing Hawaiian goby,Sicyopterus stimpsoni

Environmental heterogeneity can promote the emergence of locally adapted phenotypes among subpopulations of a species, whereas gene flow can result in phenotypic and genotypic homogenization. For organisms like amphidromous fishes that change habitats during their life history, the balance between selection and migration can shift through ontogeny, making the likelihood of local adaptation difficult to predict. In Hawaiian waterfall‐climbing gobies, it has been hypothesized that larval mixing during oceanic dispersal counters local adaptation to contrasting topographic features of streams, like slope gradient, that can select for predator avoidance or climbing ability in juvenile recruits. To test this hypothesis, we used morphological traits and neutral genetic markers to compare phenotypic and genotypic distributions in recruiting juveniles and adult subpopulations of the waterfall‐climbing amphidromous goby, Sicyopterus stimpsoni, from the islands of Hawai'i and Kaua'i. We found that body shape is significantly different between adult subpopulations from streams with contrasting slopes and that trait divergence in recruiting juveniles tracked stream topography more so than morphological measures of adult subpopulation differentiation. Although no evidence of population genetic differentiation was observed among adult subpopulations, we observed low but significant levels of spatially and temporally variable genetic differentiation among juvenile cohorts, which correlated with morphological divergence. Such a pattern of genetic differentiation is consistent with chaotic genetic patchiness arising from variable sources of recruits to different streams. Thus, at least in S. stimpsoni, the combination of variation in settlement cohorts in space and time coupled with strong postsettlement selection on juveniles as they migrate upstream to adult habitats provides the opportunity for morphological adaptation to local stream environments despite high gene flow.     

Finding paradise: cues directing the migration of the waterfall climbing Hawaiian gobioid Sicyopterus stimpsoni

A series of waterfall-climbing trials were conducted to identify cues that direct the climbing of juvenile Sicyopterus stimpsoni. In the first experiment, whether climbing juveniles preferentially ascend water sources with conspecifics or whether the presence of just stream water is sufficient to attract fish to ascend a climbing path were assessed. In the second experiment, whether climbing juveniles create a trail of mucus that facilitates the ability of conspecifics to follow their lead was determined. The results indicate that juvenile S. stimpsoni are less likely to climb in waters devoid of organic cues but are strongly attracted to stream water with or without the odour of conspecifics. Once climbing, performance did not differ for juveniles climbing in differing water choices, suggesting an all-or-nothing commitment once climbing commences. Climbing S. stimpsoni did produce a mucous trail while climbing that was associated with a mucous gland that dramatically increases in size just prior to juveniles gaining the ability to climb. The trail was not followed closely by subsequent juveniles traversing the same channel, however, suggesting only weak trail-following in waterfall climbing S. stimpsoni. Previous genetic studies suggest that juvenile S. stimpsoni do not home to natal streams in the face of strong near-shore oceanic currents. Instead, these fish appear primarily to rely on cues that suggest the presence of organic growth in streams, a factor that may indicate suitable habitat in an ever-changing stream environment but which may also be vulnerable to interference through human activity.     

Population structure of island-associated dolphins: Evidence from photo-identification of common bottlenose dolphins (Tursiops truncatus) in the main Hawaiian Islands

Management agencies often use geopolitical boundaries as proxies for biological boundaries. In Hawaiian waters a single stock is recognized of common bottlenose dolphins, Tursiops truncatus , a species that is found both in open water and near-shore among the main Hawaiian Islands. To assess population structure, we photo-identified 336 distinctive individuals from the main Hawaiian Islands, from 2000 to 2006. Their generally shallow-water distribution, and numerous within-year and between-year resightings within island areas suggest that individuals are resident to the islands, rather than part of an offshore population moving through the area. Comparisons of identifications obtained from Kaua'i/Ni'ihau, O'ahu, the "4-island area," and the island of Hawai'i showed no evidence of movements among these island groups, although movements from Kaua'i to Ni'ihau and among the "4-islands" were documented. A Bayesian analysis examining the probability of missing movements among island groups, given our sample sizes for different areas, indicates that interisland movement rates are less than 1% per year with 95% probability. Our results suggest the existence of multiple demographically independent populations of island-associated common bottlenose dolphins around the main Hawaiian islands.     

Evolutionary relationships, interisland biogeography, and molecular evolution in the Hawaiian violets (Viola: Violaceae)

The endemic Hawaiian flora offers remarkable opportunities to study the patterns of plant morphological and molecular evolution. The Hawaiian violets are a monophyletic lineage of nine taxa distributed across six main islands of the Hawaiian archipelago. To describe the evolutionary relationships, biogeography, and molecular evolution rates of the Hawaiian violets, we conducted a phylogenetic study using nuclear rDNA internal transcribed spacer sequences from specimens of each species. Parsimony, maximum likelihood (ML), and Bayesian inference reconstructions of island colonization and radiation strongly suggest that the Hawaiian violets first colonized the Maui Nui Complex, quickly radiated to Kaua'i and O'ahu, and recently dispersed to Hawai'i. The lineage consists of "wet" and "dry" clades restricted to distinct precipitation regimes. The ML and Bayesian inference reconstructions of shifts in habitat, habit, and leaf shape indicate that ecologically analogous taxa have undergone parallel evolution in leaf morphology and habit. This parallel evolution correlates with shifts to specialized habitats. Relative rate tests showed that woody and herbaceous sister species possess equal molecular evolution rates. The incongruity of molecular evolution rates in taxa on younger islands suggests that these rates may not be determined by growth form (or lifespan) alone, but may be influenced by complex dispersal events.     

Stairway to Heaven: Evaluating Levels of Biological Organization Correlated with the Successful Ascent of Natural Waterfalls in the Hawaiian Stream Goby Sicyopterus stimpsoni

Selective pressures generated by locomotor challenges act at the level of the individual. However, phenotypic variation among individuals that might convey a selective advantage may occur across any of multiple levels of biological organization. In this study, we test for differences in external morphology, muscle mechanical advantage, muscle fiber type and protein expression among individuals of the waterfall climbing Hawaiian fish Sicyopterus stimpsoni collected from sequential pools increasing in elevation within a single freshwater stream. Despite predictions from previous laboratory studies of morphological selection, few directional morphometric changes in body shape were observed at successively higher elevations. Similarly, lever arm ratios associated with the main pelvic sucker, central to climbing ability in this species, did not differ between elevations. However, among climbing muscles, the adductor pelvicus complex (largely responsible for generating pelvic suction during climbing) contained a significantly greater red muscle fiber content at upstream sites. A proteomic analysis of the adductor pelvicus revealed two-fold increases in expression levels for two respiratory chain proteins (NADH:ubiquinone reductase and cytochrome b) that are essential for aerobic respiration among individuals from successively higher elevations. Assessed collectively, these evaluations reveal phenotypic differences at some, but not all levels of biological organization that are likely the result of selective pressures experienced during climbing.     

Auditory sensitivity of Hawaiian moths (Lepidoptera: Noctuidae) and selective predation by the Hawaiian hoary bat (Chiroptera: Lasiurus cinereus semotus).

The islands of Hawai'i offer a unique opportunity for studying the auditory ecology of moths and bats since this habitat has a single species of bat, the Hawaiian hoary bat (Lasiurus cinereus semotus), which exerts the entire predatory selection pressure on the ears of sympatric moths. I compared the moth wings discarded by foraging bats with the number of surviving moths on the island of Kaua'i and concluded that the endemic noctuid Haliophyle euclidias is more heavily preyed upon than similar-sized endemic (e.g. Agrotis diplosticta) and adventive (Agrotis ipsilon and Pseudaletia unipuncta) species. Electrophysiological examinations indicated that, compared with species less preyed upon, H. euclidias has lower auditory sensitivities to the bat's social and echolocation calls, which will result in shorter detection distances of the bat. The poor ears of H. euclidias suggest that this moth coevolved with the bat using non-auditory defences that resulted in auditory degeneration. This moth now suffers higher predation because it is drawn away from its normal habitat by the man-made lights that are exploited by the bat.     

Rapid acquisition of directional preferences by migratory juveniles of two amphidromous Hawaiian gobies, Awaous guamensis and Sicyopterus stimpsoni

The native freshwater fish fauna of the Hawaiian Islands is composed of four gobiid species and one eleotrid. All five species are amphidromous. They spawn in freshwater, go to sea as free embryos and, as juveniles (sometimes referred to as post-larvae), enter fresh water again. Juveniles of two of these species, Awaous guamensis and Sicyopterus stimpsoni, had been observed maintaining directed migratory activity in the absence of water flow during the early freshwater phase of their upstream migration. Our experiments demonstrate that these juveniles are strongly attracted to flowing water. But, when water flow stops the fish persist in migratory behaviour in the same compass direction even when the tank is rotated, controlling for local landmarks and chemical cues, or when the trap that was the original water source is moved. This indicates a rapid (10 min or less) learning of directional cues. Preliminary data indicates a similar ability in a third species, Lentipes concolor. Rapid acquisition of directional preferences may be critical during upstream migration, when these tiny fish traverse water falls and other barriers by climbing in intermittent water flows. We suggest that rapid directional learning may have evolved in gobies as part of their predator avoidance repertoire and been subsequently applied to migration in streams.     

Geological age, ecosystem development, and local resource constraints on arthropod community structure in the Hawaiian Islands

An ongoing debate in evolutionary ecology concerns the relative role of contemporary vs. historical processes in determining local species richness and community structure. At sites along a 4 Mya geological chronosequence on Hawai'i, Moloka'i and Kaua'i, numerous extrinsic factors can be held constant, but ecosystem fertility and nutrient availability are low, both very young and very old sites, peaking at intermediate geological age across islands. Thus, contemporary resource traits are similar among sites with different biogeographical legacies, and these opposing gradients allowed a test of their relative importance for arboreal arthropod community structure. Pyrethrum knockdown was used to sample arboreal arthropods from Metrosideros polymorpha (Myrtaceae), the dominant tree throughout the Hawaiian Islands. Arthropod abundances and sample-based species richness peaked at more productive, intermediate-aged sites, but did not correlate with geological age. The proportions of individuals and biomass in trophic groups and in different taxonomic orders differed widely across sites, but proportions of species in trophic groups were more regular than the chance expectation. Species richness in local communities did not accumulate or pack more tightly with increasing geological age to the oldest island. Intermediate-aged islands may be contemporary peaks of richness, mediated by ecosystem development and senescence. Although historical and evolutionary processes generate diversity at broad scales, local communities converged in trophic structure and composition, and ecosystem resource availability constrained arthropod numbers and richness at local scales.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 551–570.     

Predator-driven phenotypic diversification in Gambusia affinis

Predation is heterogeneously distributed across space and time, and is presumed to represent a major source of evolutionary diversification. In fishes, fast-starts--sudden, high-energy swimming bursts--are often important in avoiding capture during a predator strike. Thus, in the presence of predators, we might expect evolution of morphological features that facilitate increased fast-start speed. We tested this hypothesis using populations of western mosquitofish (Gambusia affinis) that differed in level of predation by piscivorous fish. Body morphology of G. affinis males, females, and juveniles diverged in a consistent manner between predatory environments. Fish collected from predator populations exhibited a larger caudal region, smaller head, more elongate body, and a posterior, ventral position of the eye relative to fish from predator-free populations. Divergence in body shape largely matched a priori predictions based on biomechanical principles, and was evident across space (multiple populations) and time (multiple years). We measured maximum burst-swimming speed for male mosquitofish and found that individuals from predator populations produced faster bursts than fish from predator-free populations (about 20% faster). Biomechanical models of fish swimming and intrapopulation morphology-speed correlations suggested that body shape differences were largely responsible for enhanced locomotor performance in fish from predator populations. Morphological differences also persisted in offspring raised in a common laboratory environment, suggesting a heritable component to the observed morphological divergence. Taken together, these results strongly support the hypothesis that divergent selection between predator regimes has produced the observed phenotypic differences among populations of G. affinis. Based on biomechanical principles and recent findings in other species, it appears that the general ecomorphological model described in this paper will apply for many aquatic taxa, and provide insight into the role of predators in shaping the body form of prey organisms.     

Population genetic structure of two rare tree species (Colubrina oppositifolia and Alphitonia ponderosa,Rhamnaceae) from Hawaiian dry and mesic forests using random amplified polymorphic DNA markers

Hawaiian dry and mesic forests contain an increasingly rare assemblage of species due to habitat destruction, invasive alien weeds and exotic pests. Two rare Rhamnaceae species in these ecosystems, Colubrina oppositifolia and Alphitonia ponderosa, were examined using random amplified polymorphic DNA (RAPD) markers to determine the genetic structure of the populations and the amount of variation relative to other native Hawaiian species. Relative variation is lower than with other Hawaiian species, although this is probably not a consequence of genetic bottleneck. Larger populations of both species contain the highest levels of genetic diversity and smaller populations generally the least as determined by number of polymorphic loci, estimated heterozygosity, and Shannon's index of genetic diversity. Populations on separate islands were readily discernible for both species as were two populations of C. oppositifolia on Hawai'i island (North and South Kona populations). Substructure among Kaua'i subpopulations of A. ponderosa that were ecologically separated was also evident. Although population diversity is thought to have remained at predisturbance levels, population size continues to decline as recruitment is either absent or does not keep pace with senescence of mature plants. Recovery efforts must focus on control of alien species if these and other endemic dry and mesic forest species are to persist.     

Countergradient variation in body shape between two populations of Atlantic cod (Gadus morhua)

Variation in morphological traits is generally thought to be cogradient, with environmental effects on phenotypic expression reinforcing genetic differences between populations. We compared body shape between two populations of Atlantic cod (Gadus morhua). Striking shape differences occurred between juveniles from the two populations when reared in a common laboratory environment. However, no difference in body shape occurred between wild-reared juveniles from the two populations, suggesting that the genetic differences between populations were obscured by opposing effects of the environmental differences experienced in the wild. We suggest that much of the genetic diversity in body shape of fishes may be cryptic, with stabilizing selection for the same optimal phenotype resulting in genetic divergence between populations subject to contrasting environmental influences.     

Recent parallel divergence in body shape and diet source of alewife life history forms

Recent work suggests that juvenile alewives (Alosa pseudoharengus) share similar phenotypes among independently derived landlocked (freshwater resident) populations. Based on this observation, it is possible that the alewife life history forms represent a case of parallel adaptive divergence. To further evaluate this hypothesis, we describe patterns of body shape divergence between anadromous and landlocked alewife life history forms using geometric morphometrics. Our results suggest that body shape differs significantly between juveniles of the alewife forms: anadromous fish were more robust, with larger heads and deeper caudal peduncles, while landlocked fish from three independently isolated populations were more fusiform with thinner caudal peduncles and smaller heads. These differences matched population level dietary patterns, which suggest that anadromous fish consumed more littoral resources than landlocked fish. Finding consistent differences across populations of the same form supports the notion that landlocked alewives have diverged from their anadromous ancestors in a parallel manner, in response to pressures associated with being isolated in freshwater lakes. Comparing alewife phenotypes to expectations from the literature suggests that neither migration distance of the population, nor the relative availability of habitats in each lake, are likely drivers of the pattern we report. Instead, the pattern is consistent with the hypothesis that divergence between alewife forms results from the distinct effects of each form on its zooplankton prey.     

Contrasting post-settlement selection results in many-to-one mapping of high performance phenotypes in the Hawaiian waterfall-climbing goby <Emphasis Type="Italic">Sicyopterus stimpsoni</Emphasis>

Natural selection drives adaptive evolution, but contrasting environmental pressures may lead to trade-offs between phenotypes that confer different performances. Such trade-offs may weaken the strength of selection and/or generate complex fitness surfaces with multiple local optima that correspond to different selection regimes. We evaluated how differences in patterns of phenotypic selection might promote morphological differences between subpopulations of the amphidromous Hawaiian waterfall-climbing goby, Sicyopterus stimpsoni. We conducted laboratory experiments on fish from the islands of Kaua‘i and Hawai‘i (the “Big Island”) to compare patterns of linear and nonlinear selection, and the opportunity for selection, that result from two contrasting pressures, predator evasion and waterfall climbing, which vary in intensity between islands. We found directional and nonlinear selection were strongest when individuals were exposed to their primary selective pressures (predator evasion on Kaua‘i, waterfall climbing on the Big Island). However, the opportunity for selection was greater for the non-primary pressure: climbing on Kaua‘i, predator evasion on the Big Island. Canonical rotation of the nonlinear gamma matrix demonstrated that individuals from Kaua‘i and the Big Island occupy regions near their local fitness peaks for some traits. Therefore, selection for predator evasion on Kaua‘i and climbing on the Big Island may be less effective in promoting morphological changes in this species, because variation of functionally important traits in their respective environments may have been reduced by directional or stabilizing selection. These results demonstrate that despite constraints on the opportunities for selection, population differences in phenotypic traits can arise due to differences in selective regimes. For S. stimpsoni, sufficient variation exists in other locomotor traits, allowing for necessary levels of performance in the contrasting selective regime (i.e., climbing on Kaua‘i and predator evasion on the Big Island) through many-to-one-mapping, which may be essential for the survival of local populations in an evanescent island environment.     

Morphological correlates of river velocity and reproductive development in an ornamented stream fish

Adaptive phenotypic divergence can arise when environments vary in ways favoring alternative phenotypic optima. In aquatic habitats, the costs of locomotion are expected to increase with water velocity, generally favoring a more streamlined body and the reduction of traits that produce drag. However, because streamlining in fish may come at the cost of maneuverability, the net benefits of drag reduction can differ not only among habitats, but also among individuals (or classes of individuals) that rely on locomotion for different uses (e.g., males vs. females or adults vs. juveniles). We tested these predictions by exploring relationships among river velocity, body streamlining, ornamental fin size, and male reproductive condition in the steelcolor shiner (Cyprinella whipplei), a small-bodied North American cyprinid. Overall, males in peak reproductive condition (defined by the development of sexually dimorphic tubercles) had less streamlined bodies and larger ornamental fins than males in lower reproductive condition or individuals lacking these secondary sexual characters (females and immature males). There was a relationship between river velocity and body streamlining only for males in peak reproductive condition, but it was in the opposite direction of our predictions: these males were less streamlined in faster rivers. We found only weak support for the prediction that ornamental fin size would be negatively associated with river velocity. Overall, these results suggest either that drag is not an important selective pressure in these habitats, or that the sexual selection advantages of a deep body and large fin compensate any natural selection costs for C. whipplei males. This study highlights the often overlooked diversity of selective pressures acting on streamlining in fishes, and can offer novel insights and predictions allowing a more nuanced understanding of fish ecomorphology.     

Differences in locomotor behavior correspond to different patterns of morphological selection in two species of waterfall-climbing gobiid fishes

Behavior plays an important role in mediating relationships between morphology and performance in animals and, thus, can influence how selection operates. However, to what extent can the use of specific behaviors be associated with particular types of selection on morphological traits? Laboratory selection analyses on waterfall-climbing gobiid fishes were performed to investigate how behavioral variations in locomotion can affect patterns of linear and nonlinear morphological selection. Species from sister genera (Sicyopterus stimpsoni and Sicydium punctatum) that use different climbing behaviors were exposed to similar artificial waterfalls to simulate a controlled selective regime involving the climbing of a nearly vertical slope against flowing water. Juvenile S. stimpsoni “inch up” waterfalls by alternate attachment of oral and pelvic suckers with little axial or fin movement, leading to straightforward expectations that climbing selection should favor morphologies that improve drag reduction and substrate adhesion. In contrast, juvenile S. punctatum climb using substantial axial and fin movements, complicating expectations for selection patterns and potentially promoting correlational selection. Comparisons of directional, quadratic and correlational selection coefficients for various morphological traits and trait interactions indicated that these species showed different selection patterns that generally fit these predictions. Both directional and correlational selection patterns were different between the species, and on average were stronger in S. punctatum compared to S. stimpsoni. Stronger selection in S. punctatum may be related to its climbing style that requires more integrated movement of the fins and body axis than S. stimpsoni, promoting dynamic interactions among body regions within a complicated hydrodynamic environment.     

Population structure and comparative phylogeography of jack species (Caranx ignobilis and C. melampygus) in the high Hawaiian Islands

Members of the family Carangidae are top-level predators and highly prized food and sport fishes. Although ecologically and economically important, little is known about the biology of numerous species in the family. This is particularly true of the jacks Caranx ignobilis and C. melampygus, which have experienced recent population reductions around the high Hawaiian Islands due to overfishing. Previous studies have documented territorial tendencies as well as cases of long-distance excursions in both species, suggesting populations may exhibit a range of structure at the genetic level. To explore this possibility, mitochondrial DNA ATPase6 and ATPase8 gene sequence variation was assessed from 91 individuals (33 C. ignobilis and 58 C. melampygus) spanning the islands of Kaua'i, O'ahu, Moloka'i, Maui, and Hawai'i. Although a total of 20 distinct haplotypes (8 for C. ignobilis; 12 for C. melampygus) were recovered, no evidence of population structure was found for either species across the examined geographic range. However, distinct demographic patterns were identified, implying differing evolutionary histories and/or population dynamics. Additionally, ～ 6% of the examined C. ignobilis were C. ignobilis × C. melampygus hybrids because they harbored mitochondrial haplotypes typical of C. melampygus. These hybrids contribute to measurable gene flow between the species and may play a significant role in the evolution of the genus.