Morphological correlates of swimming activity in wild largemouth bass (Micropterus salmoides) in their natural environment.

Individual variation in morphology has been linked to organismal performance in numerous taxa. Recently, the relationship between functional morphology and swimming performance in teleost fishes has been studied in laboratory experiments. In this study, we evaluate the relationship between morphology and swimming activity of wild largemouth bass (Micropterus salmoides) during the reproductive period, providing the first data derived on free-swimming fish not exposed to forced swim trials in the laboratory. Sixteen male largemouth bass were angled from their nests, telemetered, and subsequently monitored by a whole-lake acoustic hydrophone array with sub-meter accuracy. Additionally, eleven morphological measurements were taken from digital images of each fish. A principal components analysis of the morphological measurements described 79.8% of the variance. PC1 was characterized by measures of overall body stoutness, PC2 was characterized by measures of the length and depth of the caudal region, and PC3 characterized individuals with relatively large anterior portions of the body and relatively small caudal areas. Of these variables, only PC3 showed significant relationships to swimming activity throughout the parental care period. PC3 was negatively correlated with multiple measures of swimming activity across the parental care period. Furthermore, swimming performance of individual male bass was noted to be repeatable across the parental care period indicating that this phenomenon extends beyond the laboratory.     

Nutritional condition and physiology of paternal care in two congeneric species of black bass (<Emphasis Type="Italic">Micropterus</Emphasis> spp.) relative to stage of offspring development

Parental care requires a complex integration of physiology and behaviour, yet little is known about the physiological and energetic consequences or correlates of these behaviours. Using two species of male black bass (smallmouth bass, Micropterus dolomieu; largemouth bass, M. salmoides) as a model, the focus of this study was to determine the biochemical and hematological indicators of change in nutritional status and potential for chronic stress. This was accomplished by randomly sampling individuals at four stages across parental care. Additionally, a subset of individuals was repeatedly sampled at three brood development stages to track changes in biochemical factors within the individual. Though there were changes in physiological factors across parental care in randomly sampled fish of both species (declines in plasma glucose in largemouth bass; decreases in hematocrit and plasma chloride in smallmouth bass), repeated sampling of individuals was determined to be a more appropriate sampling technique due to natural variability in biochemical factors among individual fish. Repeated sampling of smallmouth bass did not adversely influence physiological metrics or brood abandonment. However, there were higher incidences of nest abandonment in repeatedly sampled largemouth bass. Amongst the repeatedly sampled smallmouth bass, nutritional indicators such as plasma triglyceride levels decreased indicating individual fasting across the majority of parental care. Increases in plasma calcium and magnesium towards the end of care indicated that feeding most likely resumed when the brood was close to independence after ~3 weeks of care. Lastly, several indicators of chronic stress, such as plasma glucose and chloride levels, increased throughout the parental care period. These sublethal stressors are indicative of decreasing body condition associated with prolonged activity and fasting which may have marked impacts on the ability of an individual to continue parental care for the current brood and impact subsequent individual fitness. Further research into the mechanistic relationships between behaviour, physiology, and energetics during the parental care period will provide a better understanding of the decisions by individuals facing multiple trade-offs that ultimately lead to differences in individual fitness.     

Causes and consequences of voluntary anorexia during the parental care period of wild male smallmouth bass (Micropterus dolomieu)

By definition, parental care behaviors increase offspring survival, and individual fitness, at some cost to the parent. In smallmouth bass (Micropterus dolomieu), parental males provide sole care for the developing brood that includes an increase in activity during brood defense and decreased foraging resulting in a decline in endogenous energy reserves. No mechanisms have been proposed for cessation of voluntary foraging, though regulation of appetite hormones such as ghrelin have been documented to affect feeding behavior in other fishes. We documented baseline fluctuations in plasma ghrelin concentrations across parental care. Plasma ghrelin concentrations were lowest during the early stages of parental care before increasing as the brood developed to independence. Additionally, we performed an intervention experiment whereby plasma ghrelin levels were artificially increased through an injection of rodent ghrelin at the onset of parental care. Despite measuring a significant increase in plasma ghrelin approximately 1 week after injection, we noted no differences in plasma-borne indicators of recent foraging activity indicating that voluntary anorexia is possibly reinforced by receptor insensitivity to appetite hormones. Finally, we assessed the ultimate consequences of foraging during parental care by feeding fish to satiation and measuring post-prandial changes in swimming performance and aggression. Fish fed to satiation showed significant decreases in burst swimming ability and aggressiveness towards potential brood predators. Voluntary anorexia during smallmouth bass parental care is an adaptive behavior that avoids potentially deleterious declines in swimming performance and aggression apparently through a modulation of production and reception of appetite hormones including ghrelin.     

Effects of nutritional status on metabolic rate, exercise and recovery in a freshwater fish

The influence of feeding on swimming performance and exercise recovery in fish is poorly understood. Examining swimming behavior and physiological status following periods of feeding and fasting is important because wild fish often face periods of starvation. In the current study, researchers force fed and fasted groups of largemouth bass (Micropterus salmoides) of similar sizes for a period of 16 days. Following this feeding and fasting period, fish were exercised for 60 s and monitored for swimming performance and physiological recovery. Resting metabolic rates were also determined. Fasted fish lost an average of 16 g (nearly 12%) of body mass, while force fed fish maintained body mass. Force fed fish swam 28% further and required nearly 14 s longer to tire during exercise. However, only some physiological conditions differed between feeding groups. Resting muscle glycogen concentrations was twofold greater in force fed fish, at rest and throughout recovery, although it decreased in both feeding treatments following exercise. Liver mass was nearly three times greater in force fed fish, and fasted fish had an average of 65% more cortisol throughout recovery. Similar recovery rates of most physiological responses were observed despite force fed fish having a metabolic rate 75% greater than fasted fish. Results are discussed as they relate to largemouth bass starvation in wild systems and how these physiological differences might be important in an evolutionary context.     

Parental care and the ontogeny of predator-avoidance in two species of centrarchid fish

Stamps (1978) suggested that the type and extent of parental care can influence the behavioural ontogeny of the offspring. Largemouth bass (Micropterus salmoides) males guard the fry for up to a month after they leave the nest while rock bass (Ambloplites rupestris) males guard the fry only until they leave the nest. In laboratory experiments, naive largemouth fry displayed reduced predator-avoidance responses to large and small predators during their first 3 weeks of free-swimming. Rock bass fry, on the other hand, avoided predators throughout the study period. From 6 weeks (largemouth) and 5 weeks (rock bass) of age the fry showed a significantly greater response to the large predator than the small predator. The responses of laboratory-reared largemouth bass fry to the small predator changed from avoidance to association as the fry grew. The size of the fry relative to predator size was found to be important.     

Predation drives morphological convergence in the Gambusia panuco species group among lotic and lentic habitats

Fish morphology is often constrained by a trade‐off between optimizing steady vs. unsteady swimming performance due to opposing effects of caudal peduncle size. Lotic environments tend to select for steady swimming performance, leading to smaller caudal peduncles, whereas predators tend to select for unsteady swimming performance, leading to larger caudal peduncles. However, it is unclear which aspect of performance should be optimized across heterogeneous flow and predation environments and how this heterogeneity may affect parallel phenotypic evolution. We investigated this question among four Gambusia species in north‐eastern Mexico, specifically the riverine G. panuco, the spring endemics G. alvarezi and G. hurtadoi, and a fourth species, G. marshi, found in a variety of habitats with varying predation pressure in the Cuatro Ciénegas Basin and Río Salado de Nadadores. We employed a geometric morphometric analysis to examine how body shapes of both male and female fish differ among species and habitats and with piscivore presence. We found that high‐predation and low‐predation species diverged morphologically, with G. marshi exhibiting a variable, intermediate body shape. Within G. marshi, body morphology converged in high‐predation environments regardless of flow velocity, and fish from high‐predation sites had larger relative caudal peduncle areas. However, we found that G. marshi from low‐predation environments diverged in morphology between sub‐basins of Cuatro Ciénegas, indicating other differences among these basins that merit further study. Our results suggest that a morphological trade‐off promotes parallel evolution of body shape in fishes colonizing high‐predation environments and that changing predation pressure can strongly impact morphological evolution in these species.     

摄食和饥饿对大口黑鲈游泳运动能力和低氧耐受的影响

为了研究摄食和饥饿对鱼类游泳运动能力和低氧耐受的影响; 以大口黑鲈(Micropterus salmoides)为对象, 在25℃下, 测定对照组(禁食2d)、摄食组(摄食后3h)和饥饿组(禁食16d)实验鱼的日常代谢率(RMR)、活跃代谢率(AMR)、代谢范围(MS)、临界游泳速度(U_crit)、临界氧压(P_crit)和失去平衡点(LOE)。研究显示摄食后实验鱼RMR显著提升, AMR没有显著变化, 而MS和U_crit显著下降(P<0.05); 饥饿后实验鱼RMR、AMR和MS均没有显著变化, 而U_crit显著下降(P<0.05); 摄食后实验鱼P_crit显著上升, 溶解氧(DO)高于P_crit时的代谢率(MR)与DO之间的关系的斜率显著大于对照组所对应的斜率, 而LOE没有变化(P<0.05); 饥饿后实验鱼P_crit和LOE均没有显著变化, 而DO 低于P_crit时的MR与DO之间的关系的斜率显著小于对照组所对应的斜率(P<0.05)。结果表明, 摄食削弱大口黑鲈游泳运动能力是因为“心鳃”系统对其有氧代谢能力的限制; 饥饿后大口黑鲈游泳运动能力下降可能与其无氧代谢能力下降相关; 摄食削弱大口黑鲈的低氧耐受, 而饥饿后其低氧耐受有所增强, 但大口黑鲈低氧耐受总体趋于保守。     

Sub-lethal ammonia toxicity in largemouth bass

Guidelines for ammonia toxicity in fish are often determined using static exposure tests with immature fish over a 96-h period. These results may not be relevant to aquaculture, hauling or angling tournament scenarios where mature fish can be exposed to ammonia for shorter durations, often following additional stressors such as handling. The current study sought to quantify (1) the impact of ambient ammonia on the ability of largemouth bass to recover from exercise, (2) the behavioural response of largemouth bass to elevated ambient ammonia and (3) the concentration of ammonia that can accumulate in a live-release vessel at an angling tournament. After approximately 3 h, total ammonia (T(amm)) concentrations in a live-release vessel at an angling tournament were almost 200 muM. Exposure of fish to 1000 microM T(amm) (a value approximately 80% below the criteria maximum concentration for largemouth bass) caused significant reductions in ventilation rates, and increases in erratic swimming and irregular ventilation. Exposure to 100 microM T(amm) impaired the ability of largemouth bass to recover from exercise relative to fish recovering in fresh water. Therefore, sub-lethal ambient ammonia concentrations cause physiological disturbances that can impair the recovery of largemouth bass from exercise.     

Physiological and behavioral consequences of long-term artificial selection for vulnerability to recreational angling in a teleost fish

Few studies have examined the physiological and behavioral consequences of fisheries-induced selection. We evaluated how four generations of artificial truncation selection for vulnerability to recreational angling (i.e., stocks selected for high and low vulnerability [HVF and LVF, respectively]) affected cardiovascular physiology and parental care behavior in the teleost fish largemouth bass Micropterus salmoides. Where possible, we compared artificially selected fish to control fish (CF) collected from the wild. Although, compared to control fish, resting cardiac activity was approximately 18% lower for LVF and approximately 20% higher for HVF, maximal values did not vary among treatments. As a result, the HVF had less cardiac scope than either LVF or CF. Recovery rates after exercise were similar for HVF and CF but slower for LVF. When engaged in parental care activities, nesting male HVF were captured more easily than male LVF. During parental care, HVF also had higher turning rates and pectoral and caudal fin beat rates, increased vigilance against predators, and higher in situ swimming speeds. Energetics simulations indicated that to achieve the same level of growth, the disparity in metabolic rates would require HVF to consume approximately 40% more food than LVF. Selection for angling vulnerability resulted in clear differences in physiological and energetic attributes. Not only is vulnerability to angling a heritable trait, but high vulnerability covaries with factors including higher metabolic rates, reduced metabolic scope, and increased parental care activity. Despite these energetically costly differences, HVF and LVF of the same age were of similar size, suggesting that heightened food consumption in HVF compensated for added costs in experimental ponds. Ultimately, angling vulnerability appears to be a complex interaction of numerous factors leading to selection for very different phenotypes. If HVF are selectively harvested from a population, the remaining fish in that population may be less effective in providing parental care, potentially reducing reproductive output. The strong angling pressure in many freshwater systems, and therefore the potential for this to occur in the wild, necessitate management approaches that recognize the potential evolutionary consequences of angling.     

Angling-induced cardiac disturbance of free-swimming largemouth bass (Micropterus salmoides) monitored with heart rate telemetry

The sub‐lethal effects of catch‐and‐release angling have been poorly studied because of the difficulties in monitoring physiological parameters in free‐swimming fish. Laboratory studies provide the opportunity to examine sub‐lethal effects in controlled environments, but do not incorporate site‐specific characteristics. In this study we angled free‐swimming largemouth bass (Micropterus salmoides) equipped with heart rate transmitters to exhaustion using rod and reel, and exposed fish to air for 30 s. Experiments were repeated at four water temperatures (13, 17, 21, and 25°C). These field data were compared with published findings from largemouth bass collected at the same water temperatures in a controlled laboratory setting using Doppler flow probes. Field collected heart rate data increased with increasing water temperatures (Q₁₀ values 1.30–1.37). Pre‐disturbance heart rates were ～30% higher for free‐swimming fish in the field than previously collected laboratory data at the same water temperatures. Fish angled in the field exhausted ～40% more rapidly than fish chased in the laboratory. Maximal heart rate was ～15% higher for free‐swimming fish in the field than for data collected from laboratory restrained fish, but scope for heart rate was reduced by up to 20% in the field, especially at higher water temperatures. Heart rate in free‐swimming fish was highly variable at all times, obscuring clear recovery patterns. Conversely, laboratory cardiac parameters exhibited less variable patterns, peaking clearly following disturbances and recovering in about 135 min, independent of water temperature. Based upon these findings, we suggest that comprehensive studies incorporating both laboratory and field experiments are needed for truly understanding the effect of catch‐and‐release angling on fish.     

在低蛋白质饲料中补充必需氨基酸对大口黑鲈生长、体组成和免疫指标的影响

为了研究在低蛋白质饲料中补充晶体必需氨基酸对大口黑鲈生长、体组成和免疫指标的影响,根据鱼体的必需氨基酸组成模式设计了7种等能的试验饲料。其中4种饲料（45CP、40CP、35CP和30CP）的粗蛋白质水平分别为45%、40%、35%和30%,另3种饲料（40AA、35AA和30AA）是在低蛋白质饲料（40CP、35CP和30CP）的基础上添加必需氨基酸,使它们的必需氨基酸含量与45CP（对照组）相一致。用上述饲料对初始体重为（10.13 0.01） g的大口黑鲈进行了89d的饲养试验。饲养试验在室内循环水养殖系统中进行,每种饲料设3个重复,每重复放养30尾鱼。方差分析显示:试验鱼的生长性能、饲料效率、全鱼和肌肉的粗蛋白质含量、成活率以及免疫指标均随着饲料蛋白质含量的降低而显著降低（P0.05）。添加必需氨基酸的35AA和30AA的饲料效率和蛋白质保留率分别显著高于对应的未添加必需氨基酸的35CP和30CP组（P0.05）,但仍显著低于45CP组（P0.05）。40AA的试验鱼血清溶菌酶活性和血清补体活性与45CP组差异不显著（P0.05）。35AA和30AA组的头肾白细胞呼吸爆发活性显著高于35CP和30CP组（P0.05）。30AA组的全鱼粗蛋白质含量以及肥满度显著高于30CP（P0.05）。各组试验鱼的水分和灰分均无显著差异（P0.05）。回归分析显示:在低蛋白质饲料中补充晶体必需氨基酸对大口黑鲈幼鱼的生长、饲料效率和蛋白质保留率所产生的影响与其引起增加了的饲料蛋白质水平而不是饲料的必需氨基酸水平正相关。研究表明,在低蛋白质饲料中补充的晶体必需氨基酸对大口黑鲈的生长、体组成和免疫指标产生的有益作用不及等量的以蛋白质为来源的必需氨基酸。       

Externally attached radio transmitters do not affect the parental care behaviour of rock bass

Turning, pectoral fin and caudal fin rates and time spent on the nest of male rock bass Ambloplites rupestris , engaged in parental care, were not affected after the attachment of external radio transmitters. Reproductive success was similar between treatment and control fish. Micro external radio transmitters can be used on small fishes for studying parental care duration and post-care movement without altering their behaviour.     

Predation of Japanese dace, Tribolodon hakonensis, by largemouth bass, Micropterus salmoides, in experimental aquaria

 To test the size range of prey fish that largemouth bass, Micropterus salmoides, can successfully consume, live Japanese dace, Tribolodon hakonensis, were given as prey fish to individual largemouth bass in aquaria. The ratio of maximum standard length (SL) of the Japanese dace consumed by largemouth bass was 46–69% of bass SL. The maximum length of Japanese dace consumed did not differ significantly between largemouth bass and smallmouth bass (M. dolomieu) previously studied, although largemouth bass have relatively larger mouth sizes than smallmouth bass. Largemouth bass occasionally injured and killed Japanese dace larger than the limit that could be consumed.     

Does nest predation pressure influence the energetic cost of nest guarding in a teleost fish?

The energetic costs of providing parental care are widely documented, but rarely do studies consider the role of environmental variation (e.g., predation pressure) in this context. Here, we tested if variation in nest predation pressure influenced the energetic costs of parental care in smallmouth bass (Micropterus dolomieu), a teleost fish species that provides lengthy paternal care. First, we documented that nest predation pressure varied among the six lakes studied and the relative predation pressure ranking was consistent across a three year period. We used a combination of traditional proximate body composition (PBC) analyses and electromyogram (EMG) telemetry to quantify activity costs of nesting fish across these populations. The traditional approach revealed declines in energy stores across the parental care period but showed no evidence of an increased energetic cost to parents from populations with higher nest predation pressure. Comparing the distribution of EMG data from the two extremes of predation pressure revealed that males from the site of highest predation spent more time at higher EMG levels relative to the parents from the lake of lowest predation pressure. Although not statistically significant, males from the site of highest predation pressure also spent 21?C24?% of their time burst swimming when guarding young offspring compared to 10?C11?% for males at the site of lowest predation pressure. These differences in overall activity, a large contributor to the energy use of fish, may translate into longer recovery times and decreased future reproductive opportunities.     

Reproductive success and the energetic cost of parental care in male smallmouth bass

Direct field measurements of the energetic expenditure on parental care and within-nest reproductive success of individual male smallmouth bass Micropterus dolomieui were determined by measuring the change in total body mass as well as by total body electroconductivity analysis (TOBEC™). With TOBEC, the change in total body lean mass of the same live individual was measured non-destructively at the beginning and end of the parental care period. Lean mass was the primary source of energy utilized during parental care indicating starvation and potential loss of future reproduction. Individual loss in lean mass was related positively to reproductive success suggesting that the energy expended during parental care does affect individual fitness.     

The energetic impact of overwinter prey assemblages on age-0 largemouth bass, <Emphasis Type="Italic">Micropterus salmoides</Emphasis>

Synopsis We compared survival, growth, and swimming performance of two size classes of age-0 largemouth bass, Micropterus salmoides, in the spring after being fed diets of bluegill, Lepomis macrochirus, fathead minnows, Pimephales promelas, or invertebrate prey during the winter. Regardless of prey assemblage, survival was uniformly high and independent of size. Length, wet- and dry-mass, and condition was also similar among treatments for both size classes. However, variation in individual performance differed, with the lowest variability in growth occurring among small age-0 largemouth bass in the invertebrate only treatment. Absolute and length corrected swimming speeds of largemouth bass were highest for invertebrate prey assemblages, intermediate for fathead minnow prey, and lowest for bluegill prey. The patterns in growth and spring swimming performance likely reflect the varied nutritive quality of different prey, the ability of largemouth bass to capture different prey, and competition with the piscine prey.     

Annual cycles of growth and reproduction in hatchery–reared Florida largemouth bass, Micropterus salmoides floridanus, raised on forage or pelleted diets

Annual cycles of growth and reproduction of hatchery–reared Florida largemouth bass, Micropterus salmoides floridanus , were investigated. Animals were raised on either forage (goldfish, Carassius auratus ) or a pelleted salmon feed. Male and female year–class 1 largemouth bass were sampled throughout one complete yearly cycle (January–December). A biphasic growth cycle was observed in both forage–fed and pellet–fed fish. No increase in body length or weight was observed until approximately midway through the spawning period (May), after which fish grew at a consistent rate for the remainder of the study. The reproductive cycle of forage–fed fish was characterized by a rapid increase in gonadosomatic index (GS1) between January and April, followed by a prolonged spawning period (April–July) during which GSI progressively declined. Fully regressed gonads were observed in September and October, and a resumption of gonadal recrudescence was observed between October and December. Visceral adipose deposits (expressed as mesenteric fat index; MFI) were resorbed during gonadal growth and the initial stages of the spawning period, and restored during the post–spawning phase. Fish raised on pelleted feed had growth and reproductive cycles that parallelled those of forage–fed fish, but several significant differences were observed between the two diet groups. During the growth phase of the cycle, pellet–fed largemouth bass grew significantly faster than forage–fed largemouth bass, and had significantly larger MFIs than forage–fed largemouth bass at all times of the year. Pellet–fed fish also had significantly larger GSIs than forage–fed fish. These data indicate that diet composition may be an important determinant of growth and reproductive function in this species.     

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.     

Gene regulatory divergence among species estimated by altered developmental patterns in interspecific hybrids

Disturbances in the schedules of gene expression in developing interspecific fish hybrids have been used to draw inferences about the extent of gene regulatory divergence between species and about the degree to which this gene regulatory divergence is correlated with structural gene divergence, as estimated by genetic distance. Sperm from each of 10 different species representing six genera within the family Centrarchidae was used to fertilize eggs of the Florida largemouth bass (Micropterus salmoides floridanus). The genetic distances (D; Nei 1978) between the parental species used to form the hybrids ranged from 0.133 to 0.974. The developmental success and temporal patterns of gene expression of each of the hybrids were compared with those of the Florida largemouth bass. As the genetic distance between the paternal species and the Florida largemouth bass increased, there was a general decline in developmental success in the hybrid embryos as demonstrated by the observed reductions in the percentage of hatching and by progressively earlier and more extensive morphological abnormalities. Concomitantly, progressively more marked alterations in developmental schedules of expression of 15 enzyme loci occurred in the hybrids as the genetic distance between parental species increased. However, observed deviations from this trend for a few species may represent an uncoupling of the rates and modes of evolution of structural genes from those for genes regulating developmental processes.      

Trade-off between steady and unsteady swimming underlies predator-driven divergence in Gambusia affinis

Differences in predation intensity experienced by organisms can lead to divergent natural selection, driving evolutionary change. Western mosquitofish (Gambusia affinis) exhibit larger caudal regions and higher burst-swimming capabilities when coexisting with higher densities of predatory fish. It is hypothesized that a trade-off between steady (constant-speed cruising; important for acquiring resources) and unsteady (rapid bursts and turns; important for escaping predators) locomotion, combined with divergent selection on locomotor performance (favouring steady swimming in high-competition scenarios of low-predation environments, but unsteady swimming in high-predation localities) has caused such phenotypic divergence. Here, I found that morphological differences had a strong genetic basis, and low-predation fish required less hydromechanical power during steady swimming, leading to increased endurance. I further found individual-level support for cause-and-effect relationships between morphology, swimming kinematics and endurance. Results indicate that mosquitofish populations inhabiting low-predation environments have evolved increased steady-swimming abilities via stiffer bodies, larger anterior body/head regions, smaller caudal regions and greater three-dimensional streamlining.