Morphology,performance, fitness: functional insight into a post-Pleistocene radiation of mosquitofish

Bahamas mosquitofish (Gambusia hubbsi) colonized blue holes during the past approximately 15 000 years and exhibit relatively larger caudal regions in blue holes that contain piscivorous fish. It is hypothesized that larger caudal regions enhance fast-start escape performance and thus reflect an adaptation for avoiding predation. Here I test this hypothesis using a three-pronged, experimental approach. First, G. hubbsi from blue holes with predators were found to possess both greater fast-start performance and greater survivorship in the presence of predatory fish. Second, using individual-level data to investigate the morphology–performance–fitness pathway, I found that (i) fish with larger caudal regions produced higher fast-start performance and (ii) fish with higher fast-start performance enjoyed greater survivorship in the presence of fish predators—trends consistently observed across both predator regimes. Finally, I found that morphological divergence between predator regimes at least partially reflects genetic differentiation, as differences were retained in fish raised in a common laboratory environment. These results suggest that natural selection favours increased fast-start performance in the presence of piscivorous fish, consequently driving the evolution of larger caudal regions. Combined with previous work, this provides functional insight into body shape divergence and ecological speciation among Bahamian blue holes.     

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.     

An interspecific comparison between morphology and swimming performance in cyprinids

Flow regimes are believed to be of major evolutionary significance in fish. The flow regimes inhabited by cyprinids vary extensively from still flow regimes to riptide flow regimes. To test (i) whether flow‐driven swimming performance and relevant morphological differentiation are present among fish species and (ii) whether evolutionary shifts between high‐flow and low‐flow habitats in cyprinids are associated with evolutionary trade‐offs in locomotor performance, we obtained data on both steady and unsteady swimming performance and external body shape for 19 species of cyprinids that typically occur in different flow regimes (still, intermediate and riptide). We also measured the routine energy expenditure (RMR) and maximum metabolic rate (MMR) and calculated the optimal swimming speed. Our results showed that fish species from riptide groups tend to have a higher critical swimming speed (U_crit), maximum linear velocity (V_max) and fineness ratio (FR) than fish from the other two groups. However, there was no correlation between the reconstructed changes in the steady and unsteady swimming performance of the 19 species. According to the phylogenetically independent contrast (PIC) method, the U_crit was actively correlated with the MMR. These results indicated that selection will favour both higher steady and unsteady swimming performance and a more streamlined body shape in environments with high water velocities. The results suggested that steady swimming performance was more sensitive to the flow regime and that for this reason, changes in body shape resulted more from selective pressure on steady swimming performance than on unsteady swimming performance. No evolutionary trade‐off was observed between steady and unsteady swimming performance, although U_crit and MMR were found to have coevolved. However, a further analysis within each typically occurring habitat group suggested that the trade‐off that may exist between steady and unsteady swimming performance may be concealed by the effect of habitat.     

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.     

Shape variation in a benthic stream fish across flow regimes

Evolution of fish body shapes in flowing and non-flowing waters have been examined for several species. Flowing water can select for fish body shapes that increase steady swimming efficiency, whereas non-flowing water can favor shapes that increase unsteady swimming efficiency. Benthic stream fishes often use areas near the substrate that exhibit reduced or turbulent flow, thus it is unclear which swimming forms would be favored in such environments, and how shape might change across flow regimes. To test the relationship between fish body shape and flow regime in a benthic stream fish, we used geometric morphometric techniques to characterize lateral body shape in mountain sucker (Catostomus platyrhynchus) across flow rates, using stream gradient as an indicator of stream flow. Mountain suckers from low-flow environments were more streamlined, consistent with steady swimming body shapes, whereas mountain suckers from high flows had deeper bodies, consistent with unsteady swimming body shapes. In addition, smaller individuals tended to have more robust body shapes. These patterns are opposite to those predicted for stream fishes in the mid-water column. The benthic stream environment represents a distinct selective environment for fish shape that does not appear to conform to the simple dichotomy of flowing versus non-flowing water.     

Habitat-specific locomotor variation among Chinese hook snout carp (Opsariichthys bidens) along a river

The Wujiang River is a tributary of the upper Yangtze River that shows great variations in its flow regime and habitat condition. Dams have been built along the Wujiang River and have altered the habitats profoundly enough that they may give rise to reproductive isolation. To test whether the swimming performance and morphology of the Chinese hook snout carp (Opsariichthys bidens), varied among habitats and whether the possible differences had a genetic basis, we measured the steady and unsteady swimming performance, external body shape and genetic distance among fish collected from both the main and tributary streams of the upper, middle and lower reaches along the river. We also measured the routine energy expenditure (RMR), maximum metabolic rate (MMR), cost of transport (COT) and calculated the optimal swimming speed. The steady swimming capacity, RMR, MMR and optimal swimming speed were all higher and the COT was lower in the upper reach or tributary streams compared with the lower reach or main stream. However, unsteady swimming performance showed no variation among collecting sites. Flow regimes as suggested by river slope and water velocity were positively correlated with steady swimming performance but not with unsteady swimming performance. Predation stress were significantly related with body morphology and hence energy cost during swimming but not U(crit) value. The fish from only one population (Hao-Kou) showed relatively high genetic differentiation compared with the other populations. Fish from the upper reach or tributary streams exhibited improved steady swimming performance through improved respiratory capacity and lower energy expenditure during swimming at the cost of higher maintenance metabolism. There was no correlation between the steady and unsteady swimming performance at either the population or the individual levels. These results suggest that a trade-off between steady and unsteady swimming does not occur in O. bidens.     

Locomotor Patterns in the Evolution of Actinopterygian Fishes

SYNOPSIS. Locomotor adaptations in actinopterygian fishes aredescribed for (a) caudal propulsion, used in cruising and sprintswimming, acceleration, and fast turns and (b) median and pairedfin propulsion used for slow swimming and in precise maneuver.Caudal swimming is subdivided into steady (time independent)and unsteady (time dependent acceleration and turning) locomotion. High power caudal propulsion is the major theme in actinopterygianswimming morphology because of its role in predator evasionand food capture. Non-caudal slow swimming appears to be secondaryand is not exploited before the Acanthopterygii. Optimal morphological requirements for unsteady swimming are(a) large caudal fin and general body area, (b) deep caudalpeduncle, often enhanced by posterior dorsal and anal fins,(c) an anterior stabilizing body mass and\or added mass, (d)flexible body and (e) large ratio of muscle mass to body mass.Optimal morphological requirements for steady swimming are (a)high aspect ratio caudal fin, (b) narrow caudal peduncle, (c)small total caudal area, (d) anterior stabilizing body massand added mass, and (e) a stiff body. Small changes in morphologycan have large effects on performance. Exclusive morphological requirements for steady versus unsteadyswimming are partially overcome using collapsible fins, butcompromises remain necessary. Morphologies favoring unsteadyperformance are a recurring theme in actinopterygian evolution.Successive radiations at chondrostean, halecostome and teleosteanlevels are associated with modifications in the axial and caudalskeleton. Strength of ossified structures probably limited maximum propulsionforces early in actinopterygian evolution, so that specializationsfor fast cruising (carangiform and thunmform modes) followedstructural advances especially in the caudal skeleton. No suchlimits apply to eel-like forms which consequently recur in successiveactinopterygian radiations. Slow swimming using mainly non-caudal propulsion probably firstoccurred among neopterygians in association with reduced andneutral buoyancy. Slow swimming adaptations can add to and extendthe scope of caudal swimming, but specialization is associatedwith reduced caudal swimming performance. Marked exploitationof slow swimming opportunities does not occur prior to the anterodorsallocation of pectoral and pelvic girdles and the vertical rotationof the base of the pectoral fin, as found in the Acanthopterygii.     

The swimming endurance of threespine sticklebacks, Gasterosteus aculeatus L., from the Afon Rheidol, Wales

The endurance of threespine sticklebacks, Gasterosteus aculeatus , swimming with pectoral fin locomotion at 20° C in a laboratory flume was measured. Each trial lasted a maximum of 480 min. At a speed of 4 body lengths per sec (L s⁻¹) all fish were still swimming at the end of the trial, but endurance decreased at higher speeds. At speeds of 5 or 6 L s⁻¹ (20–30 cm s⁻¹) a few fish still maintained labriform locomotion for the 480 min. However, at a speed of 7 L s⁻¹ all fish furled their pectoral fins and used body and caudal fin propulsion but fatigued rapidly. During sustained swimming, fish could cover distances of 6 km or more. No significant differences between males and females were found.     

Predation‐associated divergence of male genital morphology in a livebearing fish

Male genital morphology is remarkably diverse across internally fertilizing animals, a phenomenon largely attributed to sexual selection. Ecological differences across environments can alter the context of sexual selection, yet little research has addressed how this may influence the rapid, divergent evolution of male genitalia. Using the model system of Bahamas mosquitofish (Gambusia hubbsi) undergoing ecological speciation across blue holes, we used geometric morphometric methods to test (i) whether male genital shape (the small, approximately 1 mm long, distal tip of the sperm‐transfer organ, the gonopodium) has diverged between populations with and without predatory fish and (ii) whether any observed divergence has a genetic basis. We additionally examined the effects of genetic relatedness and employed model selection to investigate other environmental factors (i.e. interspecific competition, adult sex ratio and resource availability) that could potentially influence genital shape via changes in sexual selection. Predation regime comprised the most important factor associated with male genital divergence in this system, although sex ratio and some aspects of resource availability had suggestive effects. We found consistent, heritable differences in male genital morphology between predation regimes: Bahamas mosquitofish coexisting with predatory fish possessed more elongate genital tips with reduced soft tissue compared with counterparts inhabiting blue holes without predatory fish. We suggest this may reflect selection for greater efficiency of sperm transfer and fertilization during rapid and often forced copulations in high‐predation populations or differences in sexual conflict between predation regimes. Our study highlights the potential importance of ecological variation, particularly predation risk, in indirectly generating genital diversity.     

Simulated trapping and trawling exert similar selection on fish morphology

Commercial fishery harvest can influence the evolution of wild fish populations. Our knowledge of selection on morphology is however limited, with most previous studies focusing on body size, age, and maturation. Within species, variation in morphology can influence locomotor ability, possibly making some individuals more vulnerable to capture by fishing gears. Additionally, selection on morphology has the potential to influence other foraging, behavioral, and life‐history related traits. Here we carried out simulated fishing using two types of gears: a trawl (an active gear) and a trap (a passive gear), to assess morphological trait‐based selection in relation to capture vulnerability. Using geometric morphometrics, we assessed differences in shape between high and low vulnerability fish, showing that high vulnerability individuals display shallower body shapes regardless of gear type. For trawling, low vulnerability fish displayed morphological characteristics that may be associated with higher burst‐swimming, including a larger caudal region and narrower head, similar to evolutionary responses seen in fish populations responding to natural predation. Taken together, these results suggest that divergent selection can lead to phenotypic differences in harvested fish populations.     

Shared and unique features of morphological differentiation between predator regimes in Gambusia caymanensis

When multiple groups of organisms experience similar environmental gradients, their patterns of differentiation might exhibit both shared and unique features. Here, we investigated the relative importance of three factors in generating body shape variation in a livebearing fish, Gambusia caymanensis, inhabiting the Cayman Islands: (i) shared patterns of divergent selection between predator regimes (presence/absence of piscivorous fish) driving replicated morphological differentiation, (ii) historical island effects yielding different morphologies across the three islands and (iii) unique effects of predation on morphological differentiation within each island. Shared effects of predation proved much more important than historical or unique effects. Populations coexisting with piscivorous fish exhibited larger caudal regions and smaller heads than conspecifics found in the absence of predatory fish. These results match a priori predictions, and mirror recent findings in a number of fish species, suggesting predation might often drive predictable morphological trends in disparate fishes. However, interestingly, the sexes achieved this morphological pattern through different means: head depth, caudal peduncle length and depth in males; head length, caudal peduncle depth in females. In G. caymanensis, we quantitatively confirmed that predation intensity represents a primary driver of body shape differentiation.     

饥饿对南方鲇幼鱼游泳能力个体变异和重复性的影响

为考察肉食性鱼类有氧和无氧运动能力的种内个体变异、重复性及其对饥饿的响应, 研究以南方鲇（Silurus meridionalis Chen）幼鱼为实验对象, 在（250.5）℃条件下测定对照组（n=28）和饥饿组（n=29）的临界游泳速度（Critical swimming speed, Ucrit）、暴发游泳速度（Constant acceleration speed, Ucat）和固定流速耐受时间（Endurance）, 分析游泳能力的个体变异、稳定性及饥饿的影响。结果显示: （1）饥饿组的体重、体长和肥满度均分别显著下降了（15.100.86）%、（2.570.40）%和（7.941.59）%（P0.05）, 而对照组无明显变化; （2）对照组的Ucat和耐受时间无明显变化（P0.05）, 但Ucrit下降（6.632.25）%（P=0.031）, 而除耐受时间外饥饿组其Ucrit和Ucat分别显著下降了（26.002.76）%和（13.681.86）%（P0.001）, 并饥饿组Ucrit的下降比例显著大于其Ucat（P0.001）; （3）并且对照组三个指标的变异系数（Coefficient of variation, CV）变化方向和程度不尽相同, 饥饿组的Ucrit、Ucat和耐受时间的CV全部增加; （4）南方鲇幼鱼Ucrit和Ucat呈正相关且2周的饥饿并未改变此正相关。饥饿明显降低南方鲇幼鱼两种游泳能力并导致游泳能力的个体变异变大, 但没有改变该种鱼的有氧运动能力和无氧运动能力之间的内在正相关关系。研究表明无氧运动能力在环境改变后显得更为保守, 种内个体变异的变动可能有利于在相同自然选择压力下种内个体采取不同的捕食和避敌对策。       

EVOLUTION OF INTRINSIC GROWTH AND ENERGY ACQUISITION RATES. I. TRADE-OFFS WITH SWIMMING PERFORMANCE IN MENIDIA MENIDIA

Abstract Latitudinal populations of the Atlantic silverside, Menidia menidia , show substantial genetic variation in rates of energy acquistion and allocation. Reared in common environments, silversides from northern latitudes consume more food, grow faster and more efficiently, store more energy, and produce greater quantities of eggs than their southern conspecifics. The persistence of seemingly inferior southern genotypes in the face of ostensibly superior northern genotypes suggest that there are hidden evolutionary trade-offs associated with these elevated acquisition and allocation rates. We tested the hypothesis that rapid growth and high levels of food consumption trade-off against locomotory performance in M. menidia . We compared both aerobic (prolonged and endurance) and anaerobic (burst) swimming capacities between intrinsically fast-growing fish from the north (Nova Scotia, NS) and intrinsically slow-growing fish from the south (South Carolina, SC) and between growth-manipulated phenotypes within each population. We also compared swimming speeds and endurance between fasted and recently fed fish within populations. Maximum prolonged and burst swimming speeds of NS fish were significantly lower than those of SC fish, and swimming speeds of fast-growing phenotypes were lower than those of slow-growing phenotypes within populations. Fed fish had lower burst speeds and less endurance than fasted fish from the same population. Thus, high rates of growth and the consumption of large meals clearly diminish swimming performance, which likely increases vulnerability to predation and decreases survival and relative fitness. The submaximal growth rate of southern M. menidia appears to be adaptive, resulting from balancing selection on rates of somatic growth.     

Repeat UCrit and endurance swimming in juvenile shortnose sturgeon (Acipenser brevirostrum)

Previous results show that juvenile shortnose sturgeon are steady swimmers and, compared with salmonids, generally have low critical swimming (UCrit) and endurance swimming capacities. Most studies on swimming capacities of sturgeon, and other fishes, include those where fish have only been swum once and the metrics of swimming performance are assessed (e.g., time swum, speed achieved). Under natural conditions, there are ample instances where fish undergo multiple swimming cycles when traversing fish ways, culverts and other sources of fast water flow. While some evidence exists for salmonids, the effects of repeat swimming are not well known for sturgeon. The current study consisted of two experiments. The first examined the UCrit of juvenile shortnose sturgeon following three consecutive swimming trials with a 30 min recovery period between subsequent tests. The second examined the endurance swimming capacities of juvenile shortnose sturgeon following three consecutive swimming trials with a 60 min recovery period between subsequent tests. Our findings indicate that (i) UCrit was consistent (~2 body lengths/s) among swimming trials; (ii) significant individual variation exists between individuals in the endurance swimming trials; and (iii) consistent results exist for individuals across swimming trials in both the UCrit and the endurance swimming tests. These results suggest that juvenile shortnose sturgeon have a high recovery capacity, and their behaviour and morphology likely reflect aspects of their swimming capacities.     

Constraints on adaptive evolution: the functional trade-off between reproduction and fast-start swimming performance in the Trinidadian guppy (Poecilia reticulata)

The empirical study of natural selection reveals that adaptations often involve trade-offs between competing functions. Because natural selection acts on whole organisms rather than isolated traits, adaptive evolution may be constrained by the interaction between traits that are functionally integrated. Yet, few attempts have been made to characterize how and when such constraints are manifested or whether they limit the adaptive divergence of populations. Here we examine the consequences of adaptive life-history evolution on locomotor performance in the live-bearing guppy. In response to increased predation from piscivorous fish, Trinidadian guppies evolve an increased allocation of resources toward reproduction. These populations are also under strong selection for rapid fast-start swimming performance to evade predators. Because embryo development increases a female's wet mass as she approaches parturition, an increased investment in reproductive allocation should impede fast-start performance. We find evidence for adaptive but constrained evolution of fast-start swimming performance in laboratory trials conducted on second-generation lab-reared fish. Female guppies from high-predation localities attain a faster acceleration and velocity and travel a greater distance during fast-start swimming trials. However, velocity and distance traveled decline more rapidly over the course of pregnancy in these same females, thus reducing the magnitude of divergence in swimming performance between high- and low-predation populations. This functional trade-off between reproduction and swimming performance reveals how different aspects of the phenotype are integrated and highlights the complexity of adaptation at the whole-organism level.     

The influence of swimming demand on phenotypic plasticity and morphological integration: a comparison of two polymorphic charr species

In northern freshwater lakes, several fish species have populations composed of discrete morphs, usually involving a divergence between benthic and limnetic morphs. Although it has been suggested that swimming demand plays an important role in morphological differentiation, thus influencing habitat selection, it is unclear how it affects reaction norms, patterns in character correlation, and levels of morphological integration. We examined whether swimming demand could induce morphological plasticity in the directions expected under divergent habitat selection, and evaluated its influence on the morphological integration in Arctic charr (Salvelinus alpinus) and brook charr (S. fontinalis), two congeneric species exhibiting conspicuous and subtle resource polymorphism, respectively. We found that changes in morphology were induced by differential swimming demands in both species. The length of the pectoral fin was the character that responded most strongly according to the predicted morphological expectations under divergent habitat selection. High levels of morphological plasticity, relatively low levels of integration, and differences found in the morphological correlation structure among water velocity treatments suggest that constraints on morphological change are unlikely in either species, thus allowing great potential for phenotypic flexibility in both species. The magnitude of character integration, however, was larger for Arctic charr than for brook charr. This latter result is discussed in the light of the differences in the level of polymorphism between the two species in the wild. The results of the present study indicate that swimming demand alone may not be sufficient to generate the polymorphism encountered in nature. Given that both diet and swimming demands can induce morphological changes, it would be important to conduct experiments targeting the interaction between the morphological modules related to trophic and swimming demands.     

A hydro-mechanical approach to the scaling of swimming performance in the sand flathead Platycephalus bassensis Cuvier: effects of changes in morphological features based on fish size

The swimming performance of Platycephalus bassensis at steady speed was assessed with an emphasis on hydrodynamics. The minimum swimming speed to maintain hydrostatic equilibrium for P. bassensis of 0·271 m total length ( L _T) was calculated to be 1·06 L _T s⁻¹. At this speed, the required lift to support the mass of the fish was equivalent to 6·6% of the fish mass; 82·7% of which was created by the body as a hydrofoil, and the rest of which was created by the pelvic fins as hydrofoils. The minimum swimming speed decreased with the L _T of the fish and ranged from 1·15 L _T s⁻¹ for a fish of 0·209 m to 0·89 L _T s⁻¹ for a fish of 0·407 m. The forward movement per tail-beat cycle ( i.e. stride length) was described with an equation including quantities of morphological and hydro-mechanical relevance. This equation explained that stride length was increased by the effect of turbulence characterized by the Reynolds number and demonstrated the morphological and hydro-mechanical functional design of the fish for maximizing thrust and minimizing drag. The larger span of the caudal fin and caudal tail-beat amplitude was associated with larger stride length, whereas greater frictional drag was associated with smaller stride length.     

水流速度对黑鲷和美国红鱼续航游泳能力及生理代谢的影响

为了确保黑鲷(Acanthopagrus schlegeli)和美国红鱼(Sciaenops ocellatus)在开放海域的养殖产量和鱼类养殖福利,在20℃下,对体长差异性不显著(P>0.05)的两种鱼进行续航游泳能力测试。首先,确定不同流速下的耐力游泳时间,然后选择耐力游泳时间为150min时的速度进行续航游泳实验。其中黑鲷和美国红鱼分别被迫以3.15和4.32 BL/s的恒定游泳速度,进行0、30min、60min、90min、120min和150min的测试,解剖鱼获得肌肉、血液和肝脏,测定样品在6个时间点的代谢物浓度,每个时间点保证3组有效数据。对0和150min的实验组对比,结果显示,两种鱼肝糖原、背肌乳酸和血糖浓度差异显著(P<0.05),肌肉糖原浓度差异不显著(P>0.05)。双变量相关分析显示,随着疲劳程度增加,肝糖原浓度下降,背肌乳酸和血糖上升。灰度关联分析和主成分分析显示,血糖和肝糖原浓度是影响疲劳的主要因素,但黑鲷相比美国红鱼,其浓度变化范围更大。综上:(1)美国红鱼比黑鲷拥有更强的游泳能力,而且黑鲷和美国红鱼不适合养殖在流速超过3.15和4....     

Integrating environmental variation, predation pressure, phenotypic plasticity and locomotor performance

The Wujiang River, a tributary of the Three Gorges Reservoir, has many dams along its length. These dams alter the river's natural habitat and produce various flow regimes and degrees of predator stress. To test whether the swimming performance and external body shape of pale chub (Zacco platypus) have changed as a result of alterations in the flow regime and predator conditions, we measured the steady (U _crit) and unsteady (fast-start) swimming performances and morphological characteristics of fish collected from different sites along the Wujiang River. We also calculated the maximum respiratory capacity and cost of transport (COT). We demonstrated significant differences in swimming performance and morphological traits among the sampling sites. Steady swimming performance was positively correlated with water velocity and negatively correlated with the abundance of predators, whereas unsteady swimming performance was negatively correlated with water velocity. The body shape was significantly correlated with both swimming performance and ecological parameters. These findings suggested that selection pressure on swimming performance results in a higher U _crit and a more streamlined body shape in fast-flow and (or) in habitats with low predator stress and subsequently results in a lower COT. These characteristics were accompanied by a poorer fast-start performance than that of the fish from the slow-flow and (or) high-predator habitats. The divergence in U _crit may also be due in part to variation in respiratory capacity.     

Competition as a selective mechanism for larger offspring size in guppies

Highly competitive environments are predicted to select for larger offspring. Guppies Poecilia reticulata from low-predation populations have evolved to make fewer, larger offspring than their counterparts from high-predation populations. As predation co-varies with the strength of competition in natural guppy populations, here I present two laboratory experiments that evaluate the role of competition in selecting for larger offspring size. In the first experiment, paired groups of large and small newborns from either a high- or a low-predation population were reared in mesocosms under a high- or a low-competition treatment. While large newborns retained their size advantage over small newborns in both treatments, newborn size increased growth only in the high-competition treatment. Moreover, the increase in growth with size was greater in guppies derived from the low-predation population. In the second experiment, pairs of large and small newborns were reared in a highly competitive environment until reproductive maturity. Small size at birth delayed maturation and the effect of birth size on male age of maturity was greater in the low-predation population. These results support the importance of competition as a selective mechanism in offspring size evolution.