Repeated evolution of local adaptation in swimming performance: population-level trade-offs between burst and endurance swimming inBrachyrhaphisfreshwater fish

Relationship between caudal fin closing motion and acceleration capability of Rudarius ercodes balistiform locomotion

Fish leverage the interaction of multiple fins to improve their swimming ability. Acceleration performance, in particular, is a key aspect of locomotion that directly affects survival through predator evasion and prey capture. Most of these studies have focused on body and caudal fin (BCF) swimmers, which are specialized for high-speed locomotion. In contrast, relatively few studies have examined median and paired fin (MPF) swimmers, and fins' functional roles under different locomotor modes remain insufficiently understood. Therefore, MPF swimmers may employ acceleration strategies distinct from those of BCF swimmers. Understanding the effects of fin interaction on acceleration performance in MPF swimming can contribute to a unified understanding of how such interactions influence swimming performance across fishes with different locomotor modes. A type of MPF swimmer is the balistiform, which uses its dorsal and anal fins as the main propulsion organs. We observed the closing of the caudal fins in Rudarius ercodes in balistiform locomotion during acceleration. We hypothesized that R. ercodes increased their average acceleration (the time taken to reach the experimentally observed velocity) by closing their caudal fins. We performed water tank observations to capture swimming behavior and three-dimensional computational fluid dynamics (3D-CFD) analysis to clarify the impact of caudal fin opening and closing on acceleration capability in balistiform locomotion. For example, parameters such as swimming speed and caudal fin spreading angle were measured in the swimming observations. In contrast, the fluid dynamic analysis computed the vortex structures, propulsive efficiency (the ratio of input energy contributing to thrust), and the cost of transport (the energy required to travel a unit distance). Our analysis showed that closing the caudal fin increased the average acceleration by 30%, increased the cost of transport, and decreased the dimensionless Froude efficiency. We also clarified the role of the caudal fin in MPF locomotion straight-line swimming. Our findings will help us better understand how fin interactions affect fish's swimming ability.

Does predation environment affect repeated responses to predation cues in the fish Brachyrhaphis rhabdophora?

More time spent by prey avoiding predators often results in less time allocated to energy acquisition and reproductive-related activities. Thereby, individuals that optimize this trade-off and respond appropriately to current risk levels in their environment should be at an advantage. But how does this tradeoff change when individuals repeatedly encounter predation threats? There may be advantages to prey by modulating behavior in response to repeated exposures to a threat. Moreover, it is unclear how evolutionary history of a population might affect such individual responses. Our study addressed two questions: (1) how does the fish Brachyrhaphis rhabdophora respond to repeated predation cues; and (2) do responses to repeated cues differ based on predation environment? To answer these questions, we repeatedly exposed B. rhabdophora individuals from high- and low-predation populations to a chemical predation cue. We measured the change in total distance traveled during 15-minutes before and after each cue exposure and compared the proportional change in response of each successive cue. We found that fish from both populations responded consistently to each of the four successive cue exposures. These results provide insights in understanding how both recent risk exposure and evolutionary history of risk influence individual response to threats over time.

Great power comes at a high (locomotor) cost: the role of muscle fascicle length in the power versus economy performance trade-off

Muscle design constraints preclude simultaneous specialization of the vertebrate locomotor system for explosive and economical force generation. The resulting performance trade-off between power and economy has been attributed primarily to individual differences in muscle fiber type composition. While certainly crucial for performance specialization, fiber type likely interacts with muscle architectural parameters, such as fascicle length, to produce this trade-off. Longer fascicles composed of more serial sarcomeres can achieve faster shortening velocities, allowing for greater power production. Long fascicles likely reduce economy, however, because more energy-consuming contractile units are activated for a given force production. We hypothesized that longer fascicles are associated with both increased power production and locomotor cost. In 11 power-trained and 13 endurance-trained recreational athletes, we measured (1) muscle fascicle length via ultrasound in the gastrocnemius lateralis, gastrocnemius medialis and vastus lateralis, (2) maximal power during cycling and countermovement jumps, and (3) running cost of transport. We estimated muscle fiber type non-invasively based on the pedaling rate at which maximal cycling power occurred. As predicted, longer gastrocnemius muscle fascicles were correlated with greater lower-body power production and cost of transport. Multiple regression analyses revealed that variability in maximal power was explained by fiber type (46% for cycling, 24% for jumping) and average fascicle length (20% for cycling, 13% for jumping), while average fascicle length accounted for 15% of the variation in cost of transport. These results suggest that, at least for certain muscles, fascicle length plays an important role in the power versus economy performance trade-off.

Interactions among multiple selective pressures on the form–function relationship in insular stream fishes

Relationships between body shape and escape performance are well established for many species. However, organisms can face multiple selection pressures that might impose competing demands. Many fishes use fast starts for escaping predator attacks, whereas some species of gobiid fishes have evolved the ability to climb waterfalls out of predator-dense habitats. The ancestral ‘powerburst’ climbing mechanism uses lateral body undulations to move up waterfalls, whereas a derived ‘inching’ mechanism uses rectilinear locomotion. We examined whether fast-start performance is impacted by selection imposed from the new functional demands of climbing. We predicted that non-climbing species would show morphology and fast-start performance that facilitate predator evasion, because these fish live consistently with predators and are not constrained by the demands of climbing. We also predicted that, by using lateral undulations, powerburst climbers would show escape performance superior to that of inchers. We compared fast starts and body shape across six goby species. As predicted, non-climbing fish exhibited distinct morphology and responded more frequently to an attack stimulus than climbing species. Contrary to our predictions, we found no differences in escape performance among climbing styles. These results indicate that selection for a competing pressure need not limit the ability of prey to escape predator attacks.

Predator Environment Does Not Predict Life History in the Morphologically Constrained Fish Alfaro cultratus (Cyprinodontiformes: Poeciliidae)

Predation is known to have a significant effect on life history diversification in a variety of species. However, physical constraints of body shape and size can sometimes limit life history divergence. We test this idea in the Costa Rican livebearing fishAlfaro cultratus. Individuals in this species have a narrow body and keeled ventral surface, and females do not develop a distended abdomen when pregnant like other livebearing fishes. Here, we describe the life history ofA. cultratusfrom 20 different populations across both high-predation and low-predation environments. We found significantly lower reproductive allotment in females from high-predation environments than in females from low-predation environments, but no significant difference in female or male size at maturity, number of offspring produced by females, or size of offspring. We found thatA. cultratusexhibit isometric patterns of allocation for clutch dry mass in relation to female dry mass in high-predation and low-predation environments. Our results suggest that body shape constraints in this species limit the life history divergence we typically see between populations from high-predation and low-predation environments in other species.

Morphological predictors of swimming speed performance in river and reservoir populations of Australian smelt Retropinna semoni

Dam construction is a major driver of ecological change in freshwater ecosystems. Fish populations have been shown to diverge in response to different flow velocity habitats, yet adaptations of fish populations to river and reservoir habitats created by dams remains poorly understood. We aimed to evaluate divergence of morphological traits and prolonged swimming speed performance between lotic and lentic populations of Australian smelt Retropinna semoni and quantify the relationship between prolonged swimming speed performance and morphology. Prolonged swimming speed performance was assessed for 15 individuals from each of three river and two reservoir populations of R. semoni using the critical swimming speed test (U_crit ). Body shape was characterized using geometric morphometrics, which was combined with fin aspect ratios and standard length to assess morphological divergence among the five populations. Best subsets model-selection was used to identify the morphological traits that best explain U_crit variation among individuals. Our results indicate R. semoni from river populations had significantly higher prolonged swimming speed performance (U_crit = 46.61 ± 0.98 cm s^-1 ) than reservoir conspecifics (U_crit = 35.57 ± 0.83 cm s^-1 ; F_1,74 = 58.624, Z = 35.938, P < 0.001). Similarly, R. semoni sampled from river populations had significantly higher fin aspect ratios (AR_caudal = 1.71 ± 0.04 and 1.29 ± 0.02 respectively; F_(1,74) = 56.247, Z = 40.107, P < 0.001; AR_pectoral = 1.85 ± 0.03 and 1.33 ± 0.02 respectively; F_(1,74) = 7.156, Z = 4.055, P < 0.01). Best-subset analyses revealed U_crit was most strongly correlated with pectoral and caudal fin aspect ratios (R² _adj = 0.973, AIC_c = 41.54). Body shape, however, was subject to a three-way interaction among population, habitat and sex effects (F_3,74 = 1.038. Z = 1.982; P < 0.05). Thus sexual dimorphism formed a significant component of unique and complex variation in body shape among populations from different habitat types. This study revealed profound effects of human-altered flow environments on locomotor morphology and its functional link to changes in swimming performance of a common freshwater fish. While past studies have indicated body shape may be an important axis for divergence between lotic and lentic populations of several freshwater fishes, fin aspect ratios were the most important predictor of swimming speed in our study. Differences in body morphology here were inconsistent between river and reservoir populations, suggesting this aspect of phenotype may be more strongly influenced by other factors such as predation and sexual dimorphism.

Morphological determinants of jumping performance in the Iberian green frog

Predation is one of the main selective forces in nature, frequently selecting potential prey for developing escape strategies. Escape ability is typically influenced by several morphological parameters, such as morphology of the locomotor appendices, muscular capacity, body mass, or fluctuating asymmetry, and may differ between sexes and age classes. In this study, we tested the relationship among these variables and jumping performance in 712 Iberian green frogs Pelophylax perezi from an urban population. The results suggest that the main determinant of jumping capacity was body size (explaining 48% of variance). Larger frogs jumped farther, but jumping performance reached an asymptote for the largest frogs. Once controlled by structural body size, the heaviest frogs jumped shorter distances, suggesting a trade-off between fat storage and jumping performance. Relative hind limb length also determined a small but significant percentage of variance (2.4%) in jumping performance—that is, the longer the hind limbs, the greater the jumping capacity. Juveniles had relatively shorter and less muscular hind limbs than adults (for a given body size), and their jumping performance was poorer. In our study population, the hind limbs of the frogs were very symmetrical, and we found no effect of fluctuating asymmetry on jumping performance. Therefore, our study provides evidence that jumping performance in frogs is not only affected by body size, but also by body mass and hind limb length, and differ between age classes.

Hindlimb abnormality reduces locomotor performance in Pelobates cultripes metamorphs but is not predicted by larval morphometrics

Locomotor performance is a fundamental feature commonly related to many animals’ fitness. In most cases, locomotor performance is closely related to morphology of the structures responsible for it, which is therefore under strong selective pressure. Hence, limb abnormality could hinder locomotion and, for that reason, be eradicated by selection, which could explain its overall low prevalence that makes proper research difficult. Here, we took advantage of the moderately high prevalence of hindlimb abnormality in a sample of Iberian spadefoot (Pelobatescultripes) metamorphs developed from tadpoles captured and transferred to the laboratory before selection could act against metamorph abnormality. We tested the hypothesis that limb abnormality impairs locomotor performance. Moreover, we measured several larval and metamorph morphometrics, and checked for differences between normal and abnormal-limbed individuals. We also assessed correlations between hindlimb ratio (hindlimb length/SVL) and jumping performance in normal and abnormal-limbed metamorphs. Larval traits measured could not predict hindlimb abnormality. In metamorphs, only hindlimb ratio differed between normal and abnormal-limbed individuals, being shorter in the latter. Abnormal-limbed metamorphs jumped considerably shorter distances than normal-limbed conspecifics. Therefore, selection against reduced locomotor performance could eliminate limb abnormality from populations. Hindlimb ratio was included in the model as a covariable, and thus controlled for. Consequently, other factors besides shorter hindlimbs, probably hindlimb abnormality itself, could play a role in worse jumping capability of abnormal-limbed individuals. Hindlimb ratio was positively related to jumping distance in both groups, although the relationship was weaker in abnormal-limbed metamorphs.

Latitudinal phenotypic variation in the southernmost trichomycterid, the catfish Hatcheria macraei: an amalgam of population divergence and environmental factors

Body shape and meristic characters are highly variable phenotypic aspects in fish, and in most cases are related to phylogeography, environmental factors and life history patterns. Our main goals here were to evaluate morphological and meristic characters in five populations of the catfish Hatcheria macraei living at different latitudes across Patagonia, and to assess the importance of environmental and phylogenetic variables in determining body shape. The present study reveals great morphological variation among populations distributed along the latitudinal gradient. We found that the highest levels of variation in external morphological features were in peduncle height, dorsal fin length and anus position. This variation in body shape, quantified by geometric morphometrics, was mostly explained by the phylogenetic relationship between populations, stream gradient and spawning temperature. In contrast, the meristic characters, such as vertebral and fin ray numbers, except for dorsal fin ray number, were negatively related to latitude and positively to spawning temperature.