Collective movement of schooling fish reduces the costs of locomotion in turbulent conditions

The ecological and evolutionary benefits of energy-saving in collective behaviors are rooted in the physical principles and physiological mechanisms underpinning animal locomotion. We propose a turbulence sheltering hypothesis that collective movements of fish schools in turbulent flow can reduce the total energetic cost of locomotion by shielding individuals from the perturbation of chaotic turbulent eddies. We test this hypothesis by quantifying energetics and kinematics in schools of giant danio (Devario aequipinnatus) and compared that to solitary individuals swimming under laminar and turbulent conditions over a wide speed range. We discovered that, when swimming at high speeds and high turbulence levels, fish schools reduced their total energy expenditure (TEE, both aerobic and anaerobic energy) by 63% to 79% compared to solitary fish (e.g., 228 versus 48 kj kg-1). Solitary individuals spend approximately 22% more kinematic effort (tail beat amplitude•frequency: 1.7 versus 1.4 BL s-1) to swim in turbulence at higher speeds than in laminar conditions. Fish schools swimming in turbulence reduced their three-dimensional group volume by 41% to 68% (at higher speeds, approximately 103 versus 33 cm3) and did not alter their kinematic effort compared to laminar conditions. This substantial energy saving highlights that schooling behaviors can mitigate turbulent disturbances by sheltering fish (within schools) from the eddies of sufficient kinetic energy that can disrupt locomotor gaits. Therefore, providing a more desirable internal hydrodynamic environment could be one of the ecological drivers underlying collective behaviors in a dense fluid environment.

Abiotic drivers of the space use and activity of gray reef sharks Carcharhinus amblyrhynchos in a dynamic tidal environment

Predators display rhythms in behavior and habitat use, often with the goal of maximizing foraging success. The underlying mechanisms behind these rhythms are generally linked to abiotic conditions related to diel, lunar, or seasonal cycles. To understand their effects on the space use, activity, and swimming depth of gray reef sharks (Carcharhinus amblyrhynchos), we tagged 38 individuals with depth and accelerometer sensors in a French Polynesian atoll channel exposed to strong tidal flow, and monitored them over a year. C. amblyrhynchos used a larger space during nighttime and were more active at night and during outgoing currents. Shark activity also peaked during the full and new moons. The swimming depth of sharks was mostly influenced by diel cycles, with sharks swimming deeper during the day compared to nighttime. The dynamic energyscape may promote the emergence of discrete behavioral strategies in reef sharks that use the south channel of Fakarava for resting and foraging purposes. Turbulence imposed by outgoing tides induces additional foraging cost on sharks, shifting their hunting areas to the southern part of the channel, where turbulence is less pronounced. Understanding when and where sharks are active and foraging is important for our understanding of predator-prey dynamics and ecosystem dynamics. This study highlights how abiotic rhythms in a highly dynamic environment likely generate spatiotemporal heterogeneity in the distribution of predation pressure.

Multiple environmental stressors affect predation pressure in a tropical freshwater system

Environmental change can alter predator-prey dynamics. However, studying predators in the context of co-occurring environmental stressors remains rare, especially under field conditions. Using in situ filming, we examined how multiple stressors, including temperature and turbidity, impact the distribution and behaviour of wild fish predators of Trinidadian guppies (Poecilia reticulata). The measured environmental variables accounted for 17.6% of variance in predator species composition. While predator species differed in their associations with environmental variables, the overall prevalence of predators was greatest in slow flowing, deeper, warmer and less turbid habitats. Moreover, these warmer and less turbid habitats were associated with earlier visits to the prey stimulus by predators, and more frequent predator visits and attacks. Our findings highlight the need to consider ecological complexity, such as co-occurring stressors, to better understand how environmental change affects predator-prey interactions.

Blind cavefish evolved higher foraging responses to chemo- and mechanostimuli

In nature, animals must navigate to forage according to their sensory inputs. Different species use different sensory modalities to locate food efficiently. For teleosts, food emits visual, mechanical, chemical, and/or possibly weak-electrical signals, which can be detected by optic, auditory/lateral line, and olfactory/taste buds sensory systems. However, how fish respond to and use different sensory inputs when locating food, as well as the evolution of these sensory modalities, remain unclear. We examined the Mexican tetra, Astyanax mexicanus, which is composed of two different morphs: a sighted riverine (surface fish) and a blind cave morph (cavefish). Compared with surface fish, cavefish have enhanced non-visual sensory systems, including the mechanosensory lateral line system, chemical sensors comprising the olfactory system and taste buds, and the auditory system to help navigate toward food sources. We tested how visual, chemical, and mechanical stimuli evoke food-seeking behavior. In contrast to our expectations, both surface fish and cavefish did not follow a gradient of chemical stimulus (food extract) but used it as a cue for the ambient existence of food. Surface fish followed visual cues (red plastic beads and food pellets), but, in the dark, were likely to rely on mechanosensors-the lateral line and/or tactile sensor-as cavefish did. Our results indicate cavefish used a similar sensory modality to surface fish in the dark, while affinity levels to stimuli were higher in cavefish. In addition, cavefish evolved an extended circling strategy to forage, which may yield a higher chance to capture food by swimming-by the food multiple times instead of once through zigzag motion. In summary, we propose that ancestors of cavefish, similar to the modern surface fish, evolved extended food-seeking behaviors, including circling motion, to adapt to the dark.

Environmental impact on visual perception modulates behavioral responses of schooling fish to looming predators

Aggregation in social fishes has evolved to improve safety from predators. The individual interaction mechanisms that govern collective behavior are determined by the sensory systems that translate environmental information into behavior. In dynamic environments, shifts in conditions impede effective visual sensory perception in fish schools, and may induce changes in the collective response. Here, we consider whether environmental conditions that affect visual contrast modulate the collective response of schools to looming predators. By using a virtual environment to simulate four contrast levels, we tested whether the collective state of minnow fish schools was modified in response to a looming optical stimulus. Our results indicate that fish swam slower and were less polarized in lower contrast conditions. Additionally, schooling metrics known to be regulated by non-visual sensory systems tended to correlate better when contrast decreased. Over the course of the escape response, schools remained tightly formed and retained the capability of transferring social information. We propose that when visual perception is compromised, the interaction rules governing collective behavior are likely to be modified to prioritize ancillary sensory information crucial to maximizing chance of escape. Our results imply that multiple sensory systems can integrate to control collective behavior in environments with unreliable visual information.

Blind cavefish evolved food-searching behavior without changing sensory modality compared with sighted conspecies in the dark

In nature, animals must navigate to forage according to their sensory inputs. Different species use different sensory modalities to locate food efficiently. For teleosts, food emits visual, mechanical, chemical, and/or possibly weak-electrical signals, which can be detected by optic, auditory/lateral line, and olfactory/taste buds sensory systems. However, how fish respond to and use different sensory inputs when locating food, as well as the evolution of these sensory modalities, remain unclear. We examined the Mexican tetra, Astyanax mexicanus, which is composed of two different morphs: a sighted riverine (surface fish) and a blind cave morph (cavefish). Compared with surface fish, cavefish have enhanced non-visual sensory systems, including the mechanosensory lateral line system, chemical sensors comprising the olfactory system and taste buds, and the auditory system to help navigate toward food sources. We tested how visual, chemical, and mechanical stimuli evoke food-seeking behavior. In contrast to our expectations, both surface fish and cavefish did not follow a gradient of chemical stimulus (food extract) but used it as a cue for the ambient existence of food. Surface fish followed visual cues (red plastic beads and food pellets), but, in the dark, were likely to rely on mechanosensors-the lateral line and/or tactile sensor-as cavefish did. Our results indicate cavefish used similar sensory modality to surface fish in the dark, while adherence levels to stimuli were higher in cavefish. In addition, cavefish evolved an extended circling strategy to capture food, which may yield a higher chance to capture food by swimming-by the food multiple times instead of once through zigzag motion. In summary, we propose ancestors of cavefish similar to surface fish may have needed little modification in food-seeking strategy to adapt to the dark.

The influence of temperature on performance, biological indices, composition, and nutrient retention of juvenile Chinook salmon (Oncorhynchus tshawytscha) reared in freshwater

This study investigated the effects of different temperatures on the performance, composition, and nutrient retention of Chinook salmon reared in freshwater. Individuals (187.6 ± 27.1 g) were distributed into twelve tanks of 8000 L each (155 to 157 fish per tank) at a temperature of 14 °C. The tanks were transitioned from 14 °C (hatchery temperature) to 8, 12, 16, and 20 °C over seven days. Three fish assessments were performed, the first (initial) when the fish were distributed in the tanks, a second (interim) between days 9 and 16 at the start of the experiment, and a third (final) after 41 to 49 days at the target temperature. At the end of the trial, performance parameters, proximate composition, amino acid, and fatty acid composition, and nutrient retention were evaluated. Better growth performance was observed in fish at 16 °C and 20 °C compared to the lower temperatures. Fish at higher temperatures had higher levels of saturated fatty acids (SFA), while at lower temperatures fish had higher levels of n-3 and n-6 polyunsaturated fatty acids (PUFA), especially eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). A polynomial relationship between nutrient retention and temperature showed that fish from all treatments retained more lipids than proteins, with higher retention of MUFA compared to the other fatty acid classes. Additionally, DHA retention was approximately three times higher than EPA retention. The results showed that 16 to 20 °C was the optimum temperature range for Chinook salmon, and the performance differences were mainly modulated by lipid retention/catabolism.

Water temperature and precipitation stimulate small-sized Japanese eels to climb a low-height vertical weir

Although river-crossing structures can have a detrimental effect on the movement and distribution of anguillid eels (genus Anguilla), they have inhabited locations upstream of river-crossing structures in many rivers. However, the growth stage in which the eels start to climb river-crossing structures remains unclear. In this study, we directly observed, using infrared video camera systems, that the Japanese eel (Anguilla japonica) climbs a low-height vertical weir as a migratory barrier, confirming the ability of eels to climb over a low-height weir within a river. All eels climbed the weir at night, between sunset and sunrise. We observed that the total length of Japanese eels that climbed the weir ranged from 60 to 140 mm, suggesting that eels inhabiting the upstream area of a weir climbed it when they were small and then lived and grew upstream. Moreover, the general additive model showed considerable effects of water temperature and precipitation on eel climbing, suggesting that water temperature and precipitation are important for its activation. The results of this study also show the effectiveness of infrared video cameras in monitoring eel climbing, which could be applied to monitor fish migratory behavior through fish passages. The findings of this study are useful for the comprehensive management and conservation of wild eel stocks.

The Evolution of Feeding Mechanics in the Danioninae, or Why Giant Danios Don't Suck Like Zebrafish

By linking anatomical structure to mechanical performance we can improve our understanding of how selection shapes morphology. Here we examined the functional morphology of feeding in fishes of the subfamily Danioninae (order Cypriniformes) to determine aspects of cranial evolution connected with their trophic diversification. The Danioninae comprise three major lineages and each employs a different feeding strategy. We gathered data on skull form and function from species in each clade, then assessed their evolutionary dynamics using phylogenetic-comparative methods. Differences between clades are strongly associated with differences in jaw protrusion. The paedomorphic Danionella clade does not use jaw protrusion at all, members of the Danio clade use jaw protrusion for suction production and prey capture, and members of the sister clade to Danio (e.g., Devario and Microdevario) use jaw protrusion to retain prey after capture. The shape of the premaxillary bone is a major determinant of protrusion ability, and premaxilla morphology in each of these lineages is consistent with their protrusion strategies. Premaxilla shapes have evolved rapidly, which indicates that they have been subjected to strong selection. We compared premaxilla development in giant danio (Devario aequipinnatus) and zebrafish (Danio rerio) and discuss a developmental mechanism that could shift danionine fishes between the feeding strategies employed by these species and their respective clades. We also identified a highly integrated evolutionary module that has been an important factor in the evolution of trophic mechanics within the Danioninae.

Differences among reciprocal hybrids of Labeotropheus

Current evidence suggests that hybridization played a crucial role in the early evolution and diversification of the species flocks of cichlid fishes in the African Great Lakes. Nonetheless, evidence for hybridization in the extant cichlid fauna is scant, suggesting that hybridization is rare in the modern era, perhaps enforced by natural or sexual selection acting against F1 hybrids. Additionally, most experimental studies of hybridization perform a hybrid cross in one direction, ignoring the reciprocal hybrid. In this study, we perform reciprocal crosses between sympatric congeners from Lake Malaŵi, Labeotropheus fuelleborni and L. trewavasae, in order to compare the body shape and coloration of males of both of these hybrids, as well as to examine how these hybrids fare during both inter- and intrasexual interactions. We found that L. trewavasae-sired hybrid males are intermediate to the parental species both morphologically and chromatically, while the reciprocal L. fuelleborni-sired hybrids are likely transgressive hybrids. Males of these transgressive hybrids also fare poorly during our mate choice experiments. While female L. trewavasae reject them as possible mates, male L. trewavasae do not make a distinction between them and conspecific males. Selection against transgressive F1 hybrids as observed in our crossing experiments may help explain why contemporary hybridization in Lake Malaŵi cichlids appears to be rare.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10750-022-05092-4.

Ecomechanics and the Rules of Life: a Critical Conduit Between the Physical and Natural Sciences

Nature provides the parameters, or boundaries, within which organisms must cope in order to survive. Therefore, ecological conditions have an unequivocal influence on the ability of organisms to perform the necessary functions for survival. Biomechanics brings together physics and biology to understand how an organism will function under a suite of conditions. Despite a relatively rich recent history linking physiology and morphology with ecology, less attention has been paid to the linkage between biomechanics and ecology. This linkage, however, could provide key insights into patterns and processes of evolution. Ecomechanics, also known as ecological biomechanics or mechanical ecology, is not necessarily new, but has received far less attention than ecophysiology or ecomorphology. Here, we briefly review the history of ecomechanics, and then identify what we believe are grand challenges for the discipline and how they can inform some of the most pressing questions in science today, such as how organisms will cope with global change.

Habitat transitions by a large coastal sciaenid across life history stages, resolved using otolith chemistry

Many coastal species move between estuarine and coastal environments throughout their life. Migration patterns develop as a result of ecology and evolution and must be understood to effectively manage harvested stocks. This study examined movements across estuarine and coastal marine habitats in adult Mulloway (Argyrosomus japonicus); a commercially, indigenous and recreationally harvested sciaenid of south-eastern Australia. Chemical profiles across the otolith (ear bone) were used to examine transitions between estuary and marine habitats over life history. Patterns in otolith Ba:Ca concentrations indicated that the majority of fish migrated between estuary and marine habitats, but a small proportion of fish appeared to remain in either the estuary or the marine habitat. Such movements may potentially be driven by a range of biological and environmental factors. This approach allows questions about the life history and habitat use of Mulloway to be addressed, which will aid management and provide a platform for future research on Mulloway, other sciaenid's and coastal migratory species.

Riverscape Genomics Clarifies Neutral and Adaptive Evolution in an Amazonian Characin Fish (Triportheus albus)

Understanding the role of natural selection in the evolution of wild populations is challenging due to the spatial complexity of natural systems. The richest diversity of freshwater fishes in the world is found in the Amazon Basin, a system where marked hydrochemical differences exist at the interface of major rivers with distinct “water colors” (i.e., black, white, and clear water). We hypothesize that divergent natural selection associated with these “aquatic ecotones” influences population-level adaptive divergence in the non-migratory Amazonian fish fauna. This hypothesis was tested using a landscape genomics framework to compare the relative contribution of environmental and spatial factors to the evolutionary divergence of the Amazonian characin fish Triportheus albus. The framework was based on spatial data, in situ hydrochemical measurements, and 15,251 filtered SNPs (single nucleotide polymorphisms) for T. albus sampled from three major Amazonian rivers. Gradient Forest, redundancy analysis (RDA) and BayPass analyses were used to test for signals of natural selection, and model-based and model-free approaches were used to evaluate neutral population differentiation. After controlling for a signal of neutral hierarchical structure which was consistent with the expectations for a dendritic system, variation in turbidity and pH were key factors contributing to adaptive divergence. Variation in genes involved in acid-sensitive ion transport pathways and light-sensitive photoreceptor pathways was strongly associated with pH and turbidity variability. This study improves our understanding of how natural selection and neutral evolution impact on the distribution of aquatic biodiversity from the understudied and ecologically complex Amazonia.

Pursuit and evasion strategies in the predator-prey interactions of fishes

Predator-prey interactions are critical to the biology of a diversity of animals. Although prey capture is determined by the direction, velocity, and timing of motion by both animals, it is generally unclear what strategies are employed by predators and prey to guide locomotion. Here we review our research on fishes that tests the pursuit strategy of predators and the evasion strategy of prey through kinematic measurements and agent-based models. This work demonstrates that fish predators track prey with variations on a deviated-pursuit strategy that is guided by visual cues. Fish prey employ a mixed strategy that varies with factors such as the direction of a predator's approach. Our models consider the stochastic nature of interactions by incorporating measured probability distributions to accurately predict measurements of survivorship. A sensitivity analysis of these models shows the importance of the response distance of prey to their survival. Collectively, this work demonstrates how strategy affects the outcome of predator-prey interactions and articulates the roles of sensing, control, and propulsion. The research program that we have developed has the potential to offer a framework for the study of strategy in the predator-prey interactions of a variety of animals.

Effects of wave-driven water flow on the fast-start escape response of juvenile coral reef damselfishes

Fish often evade predators with a fast-start escape response. Studies typically examine this behaviour in still water despite water motion being an inherent feature of aquatic ecosystems. In shallow habitats, waves create complex flows that likely influence escape performance, particularly in small fishes with low absolute swimming speeds relative to environmental flows. I examined how wave-driven water flow affects the behaviour and kinematics of escape responses in juveniles of three coral reef damselfishes (Pomacentridae) with different body morphologies. Tropical damselfishes have similar fin and body shapes during early development, with the exception of body depth, a trait deemed important for postural control and stability. Wave-driven flow increased response latency in two of the three species tested: fish with a fusiform body responded 2.9 times slower in wave-driven flow than in still water, whereas this difference was less pronounced in fish with an intermediate body depth (1.9 times slower response) and absent in fish with a laterally compressed body. The effect of wave-driven flow on swimming performance (cumulative escape distance and turning rate) was variable and depended on the timing and trajectory of escape responses in relation to the wave phase. Given intense predation pressure on juvenile coral reef fishes during settlement, interspecific differences in how wave-driven flow affects their ability to escape predators could influence the distribution and abundance of species across spatial and temporal scales.

Temperature-Dependent Biomarkers of Estrogenic Exposure in a Piscivore Freshwater Fish

The biological effects of endocrine-active compounds and increasing water temperatures as a result of climate change have been studied extensively and independently, but there is a dearth of research to examine the combined effect of these factors on exposed organisms. Recent data suggest that estrogenic exposure and rising ambient temperatures independently impact predator-prey relationships. However, establishing these connections in natural settings is complex. These obstacles can be circumvented if biomarkers of estrogenic exposure in resident fish can predict changes in predator-prey relationships. To test the effects of estrone and temperature, the piscivore bluegill sunfish (Lepomis macrochirus) was exposed for 30 days to estrone at concentrations (90 ± 17.6 ng/L [mean ± standard deviation] and 414 ± 146 ng/L) previously shown to reduce prey-capture success. Exposures were conducted at four temperatures (15 °C, 18 °C, 21 °C, 24 °C) to simulate breeding season ambient temperatures across the natural range of this species. A suite of morphological and physiological biomarkers previously linked to estrogenic exposures were examined. Biomarkers of estrone exposure were more commonly and severely impacted in male fish than in female fish. Notably, the gonadosomatic index was lower and gonads were less mature in exposed males. Additionally, temperature modulated the effects of estrone similarly in males and females with fish exposed at higher temperatures typically exhibiting a decreased morphological index. This study provides evidence that alterations in hepatic function and gonadal function may cause shifts in metabolism and energy allocation that may lead to declining prey capture performance.

Functional effect of vaterite - the presence of an alternative crystalline structure in otoliths alters escape kinematics of the brown trout

The fast-start escape response is the main locomotor behaviour observed in fish to evade predatory attacks and thereby increase their probability of survival. Thus far, this high-speed sensory motor control has been extensively studied in relation to extrinsic factors. In contrast, there has been surprisingly little consideration of intrinsic individual factors that can mediate sensorial perception, such as inter-individual variability in mechanosensory systems. The inner ear of teleost fishes is composed of otoliths that play an important role in hearing and balance functions. While sagittal otoliths are normally composed of aragonite in many fish species, the inclusion of vaterite (an abnormal crystalline structure) has been reported in a number of individuals from different environments. There is currently strong theoretical and empirical evidence that vaterite deposition has a negative impact on auditory sensitivity in fishes. While the functional/behavioural implications of this defect on otolith-related hearing function has been hypothesised, it has remained largely untested experimentally. Here, using juvenile (0+ years) Salmo trutta originating from the wild in experimental conditions, we report for the first time that the deposition of calcium carbonate in its crystalline vateritic polymorph has significant pervasive effects on the escape kinematics of fish. The presence of an alternative crystalline structure in otoliths is likely to alter fish behaviour in ways that decrease survival. We also report that altered behaviour in individuals with vateritic otoliths is partially compensated for by the presence of a functional lateral line. Such functional compensation suggests more slight consequences, if any, in the wild.

Effects of temperature on feeding and digestive processes in fish

As most fish are ectotherms, their physiology is strongly affected by temperature. Temperature affects their metabolic rate and thus their energy balance and behavior, including locomotor and feeding behavior. Temperature influences the ability/desire of the fish to obtain food, and how they process food through digestion, absorb nutrients within the gastrointestinal tract, and store excess energy. As fish display a large variability in habitats, feeding habits, and anatomical and physiological features, the effects of temperature are complex and species-specific. The effects of temperature depend on the timing, intensity, and duration of exposure as well as the speed at which temperature changes occur. Whereas acute short-term variations of temperature might have drastic, often detrimental, effects on fish physiology, long-term gradual variations might lead to acclimation, e.g. variations in metabolic and digestive enzyme profiles. The goal of this review is to summarize our current knowledge on the effects of temperature on energy homeostasis, with specific focus on metabolism, feeding, digestion, and how fish are often able to "adapt" to changing environments through phenotypic and physiological changes.

Microfluidic Transportation Control of Larval Zebrafish through Optomotor Regulations under a Pressure-Driven Flow

To perform zebrafish larvae-related experiments within a microfluidic environment, the larvae need to be anesthetized and subsequently transported into respective test sections through mechanical or manual means. However, anesthetization tends to affect larval sensory perceptions, hindering their natural behaviors. Taking into account that juvenile larvae move naturally within their environment by accessing visual as well as hydromechanical cues, this work proposes an experimental framework to transport nonanesthetized larvae within a microfluidic environment by harmonically tuning both of the aforementioned cues. To provide visual cues, computer-animated moving gratings were provided through an in-house-developed control interface that drove the larval optomotor response. In the meantime, to provide hydromechanical cues, the flow rate was tuned using a syringe pump that affected the zebrafish larvae’s lateral line movement. The results obtained (corresponding to different test conditions) suggest that the magnitude of both modalities plays a crucial role in larval transportation and orientation control. For instance, with a flow rate tuning of 0.1 mL/min along with grating parameters of 1 Hz temporal frequency, the average transportation time for larvae that were 5 days postfertilization was recorded at 1.29 ± 0.49 s, which was approximately three times faster than the transportation time required only in the presence of hydromechanical cues.

Short-Term Interactive Effects of Experimental Heat Waves and Turbidity Pulses on the Foraging Success of a Subtropical Invertivorous Fish

Sudden increases in temperature and turbidity in aquatic ecosystems are expected for different regions in the future, as a result of the more frequent extreme climatic events that are predicted. The consequences of these abrupt changes in the outcomes of predator–prey interactions are unknown. Here, we tested the effects of a heat wave and a turbidity pulse on the foraging success of a subtropical cichlid fish (Gymnogeophagus terrapurpura) on amphipods (Hyalella curvispina). We carried out a short-term experiment combining treatments of turbidity (3 and 100 nephelometric turbidity units [NTU]) and water temperature (19.2, 22.2, 25.2 and 27.0 °C), considering potential differences given by fish length. Changes in water temperature did not promote significant changes in prey consumption. Higher turbidity, in contrast, decreased prey consumption. Also, we found that fish with different body lengths consumed a similar amount of prey under clear waters, but, in turbid waters, bigger individuals were more efficient than the smaller individuals. This finding is an empirical demonstration that the effect of increased turbidity on predation rate depends upon predator body size, and it suggests that bigger body sizes may help overcome turbidity-associated limitations in finding and capturing prey. Our short-term results suggest that, if turbidity pulses and heat waves become more frequent in the future, the outcome of fish–invertebrate interaction can be affected by local characteristics such as fish population size distribution.

Fish Response to Turbulence Generated Using Multiple Randomly Actuated Synthetic Jet Arrays

Hydroelectric power stations generate turbulent flow conditions, which represent a potentially significant hydraulic stressor to fish passing through the turbine system. A test facility has been developed using two randomly actuated synthetic jet arrays (RASJAs) of 25 independent submersible pumps to generate a turbulent flow field for biological dose-response testing. The novel elements of this approach include the ability to control the exposure duration within a test volume due to low mean flow velocity as well as the capacity to scale the turbulence levels as a function of pump capacity. Juvenile Chinook salmon (Oncorhynchus tshawytscha) were subjected to the turbulent flow regime with average turbulence kinetic energy per unit mass of 0.089 m 2 / s 2 for periods of 2 min and 10 min. No significant loss of equilibrium or disorientation was observed after exposure for either duration at the level of turbulence achieved in this prototype. Further scaling of this approach is required to generate a complete dose-response relationship.

A Hierarchical View of Gecko Locomotion: Photic Environment, Physiological Optics, and Locomotor Performance

Terrestrial animals move in complex habitats that vary over space and time. The characteristics of these habitats are not only defined by the physical environment, but also by the photic environment, even though the latter has largely been overlooked. For example, numerous studies of have examined the role of habitat structure, such as incline, perch diameter, and compliance, on running performance. However, running performance likely depends heavily on light level. Geckos are an exceptional group for analyzing the role of the photic environment on locomotion as they exhibit several independent shifts to diurnality from a nocturnal ancestor, they are visually-guided predators, and they are extremely diverse. Our initial goal is to discuss the range of photic environments that can be encountered in terrestrial habitats, such as day versus night, canopy cover in a forest, fog, and clouds. We then review the physiological optics of gecko vision with some new information about retina structures, the role of vision in motor-driven behaviors, and what is known about gecko locomotion under different light conditions, before demonstrating the effect of light levels on gecko locomotor performance. Overall, we highlight the importance of integrating sensory and motor information and establish a conceptual framework as guide for future research. Several future directions, such as understanding the role of pupil dynamics, are dependent on an integrative framework. This general framework can be extended to any motor system that relies on sensory information, and can be used to explore the impact of performance features on diversification and evolution.

The effect of climate change on the escape kinematics and performance of fishes: implications for future predator-prey interactions

Climate change can have a pronounced impact on the physiology and behaviour of fishes. Notably, many climate change stressors, such as global warming, hypoxia and ocean acidification (OA), have been shown to alter the kinematics of predator-prey interactions in fishes, with potential effects at ecological levels. Here, we review the main effects of each of these stressors on fish escape responses using an integrative approach that encompasses behavioural and kinematic variables. Elevated temperature was shown to affect many components of the escape response, including escape latencies, kinematics and maximum swimming performance, while the main effect of hypoxia was on escape responsiveness and directionality. OA had a negative effect on the escape response of juvenile fish by decreasing their directionality, responsiveness and locomotor performance, although some studies show no effect of acidification. The few studies that have explored the effects of multiple stressors show that temperature tends to have a stronger effect on escape performance than OA. Overall, the effects of climate change on escape responses may occur through decreased muscle performance and/or an interference with brain and sensory functions. In all of these cases, since the escape response is a behaviour directly related to survival, these effects are likely to be fundamental drivers of changes in marine communities. The overall future impact of these stressors is discussed by including their potential effects on predator attack behaviour, thereby allowing the development of potential future scenarios for predator-prey interactions.

Selection, hybridization, and the evolution of morphology in the Lake Malaŵi endemic cichlids of the genus Labeotropheus

The cichlid fishes of Lake Malaŵi are the paramount example of adaptive radiation in vertebrates. Evidence of their astounding diversity is perhaps most visible in their adaptations for obtaining food; the genus Labeotropheus, due to their prominent snouts, are an interesting example of an extreme adaptation for feeding. Two different body types are found in this genus: a deep-bodied form (e.g., L. fuelleborni) found most often in turbulent shallow water; and a slender bodied form (e.g., L. trewavasae) found in structurally-complex deep water habitats. Here we test the hypothesis that L. trewavasae should suffer a loss in fitness, measured as growth rate, if raised in turbulence; additionally, we examined growth and morphology of L. fuelleborni and L. fuelleborni x L. trewavasae hybrids under these conditions. We did find the predicted loss of fitness in turbulent-raised L. trewavasae, but found no loss of fitness for L. fuelleborni in either condition; hybrids, due to an unusual morphology, performed better in turbulent as opposed to control conditions. Fitness in turbulent conditions was dependent upon morphology, with deeper bodies and upturned neurocrania allowing a greater growth rate under these conditions. Directional selection on morphology was crucial in the evolution of morphology in the Labeotropheus.

Role of water flow regime in the swimming behaviour and escape performance of a schooling fish

Animals are exposed to variable and rapidly changing environmental flow conditions, such as wind in terrestrial habitats and currents in aquatic systems. For fishes, previous work suggests that individuals exhibit flow-induced changes in aerobic swimming performance. Yet, no one has examined whether similar plasticity is found in fast-start escape responses, which are modulated by anaerobic swimming performance, sensory stimuli and neural control. In this study, we used fish from wild schools of the tropical damselfish Chromis viridis from shallow reefs surrounding Lizard Island in the Great Barrier Reef, Australia. The flow regime at each site was measured to ascertain differences in mean water flow speed and its temporal variability. Swimming and escape behaviour in fish schools were video-recorded in a laminar-flow swim tunnel. Though each school's swimming behaviour (i.e. alignment and cohesion) was not associated with local flow conditions, traits linked with fast-start performance (particularly turning rate and the distance travelled with the response) were significantly greater in individuals from high-flow habitats. This stronger performance may occur due to a number of mechanisms, such as an i n s itu training effect or greater selection pressure for faster performance phenotypes in areas with high flow speed.This article has an associated First Person interview with the first author of the paper.

The role of physiological traits in assortment among and within fish shoals

Individuals of gregarious species often group with conspecifics to which they are phenotypically similar. This among-group assortment has been studied for body size, sex and relatedness. However, the role of physiological traits has been largely overlooked. Here, we discuss mechanisms by which physiological traits—particularly those related to metabolism and locomotor performance—may result in phenotypic assortment not only among but also within animal groups. At the among-group level, varying combinations of passive assortment, active assortment, phenotypic plasticity and selective mortality may generate phenotypic differences among groups. Even within groups, however, individual variation in energy requirements, aerobic and anaerobic capacity, neurological lateralization and tolerance to environmental stressors are likely to produce differences in the spatial location of individuals or associations between group-mates with specific physiological phenotypes. Owing to the greater availability of empirical research, we focus on groups of fishes (i.e. shoals and schools). Increased knowledge of physiological mechanisms influencing among- and within-group assortment will enhance our understanding of fundamental concepts regarding optimal group size, predator avoidance, group cohesion, information transfer, life-history strategies and the evolutionary effects of group membership. In a broader perspective, predicting animal responses to environmental change will be impossible without a comprehensive understanding of the physiological basis of the formation and functioning of animal social groups.

This article is part of the themed issue ‘Physiological determinants of social behaviour in animals’.

Three-dimensional migration behavior of juvenile salmonids in reservoirs and near dams

To acquire 3-D tracking data on juvenile salmonids, Juvenile Salmon Acoustic Telemetry System (JSATS) cabled hydrophone arrays were deployed in the forebays of two dams on the Snake River and at a mid-reach reservoir between the dams. The depth distributions of fish were estimated by statistical analyses performed on large 3-D tracking data sets from ~33,500 individual acoustic tagged yearling and subyearling Chinook salmon and juvenile steelhead at the two dams in 2012 and subyearling Chinook salmon at the two dams and the mid-reach reservoir in 2013. This research investigated the correlation between vertical migration behavior and passage routes. The depth distributions of fish within the forebays of the dams were significantly different from fish passing the mid-reach reservoir. Fish residing deeper in the forebay tended to pass the dam using deeper powerhouse routes. This difference in depth distributions indicated that the depth distribution of fish at the mid-reach reservoir was not related to behaviors of fish passing through certain routes of the adjacent dams. For fish that were detected deeper than 17.5 m in the forebays, the probability of powerhouse passage (i.e., turbine) increased significantly. Another important finding was the variation in depth distributions during dam passage associated with the diel period, especially the crepuscular periods.

Speciation through the lens of biomechanics: locomotion, prey capture and reproductive isolation

Speciation is a multifaceted process that involves numerous aspects of the biological sciences and occurs for multiple reasons. Ecology plays a major role, including both abiotic and biotic factors. Whether populations experience similar or divergent ecological environments, they often adapt to local conditions through divergence in biomechanical traits. We investigate the role of biomechanics in speciation using fish predator-prey interactions, a primary driver of fitness for both predators and prey. We highlight specific groups of fishes, or specific species, that have been particularly valuable for understanding these dynamic interactions and offer the best opportunities for future studies that link genetic architecture to biomechanics and reproductive isolation (RI). In addition to emphasizing the key biomechanical techniques that will be instrumental, we also propose that the movement towards linking biomechanics and speciation will include (i) establishing the genetic basis of biomechanical traits, (ii) testing whether similar and divergent selection lead to biomechanical divergence, and (iii) testing whether/how biomechanical traits affect RI. Future investigations that examine speciation through the lens of biomechanics will propel our understanding of this key process.

Field swimming behavior in largemouth bass deviates from predictions based on economy and propulsive efficiency

Locomotion is energetically expensive. This may create selection pressures that favor economical locomotor strategies, such as the adoption of low-cost speeds and efficient propulsive movements. For swimming fish, the energy expended to travel a unit distance, or cost of transport (COT), has a U-shaped relationship to speed. The relationship between propulsive kinematics and speed, summarized by the Strouhal number (St=fA/U, where f is tail beat frequency, A is tail tip amplitude in m and U is swimming speed in m s−1), allows for maximal propulsive efficiency where 0.2<St<0.4. Largemouth bass adopted field speeds that were generally below the range predicted to minimize their COT. This may reflect speed modulation to meet competing functional demands such as enabling effective prey detection and capture. St exceeded the optimal range for the lowest observed swimming speeds. Mechanical and physiological constraints may prevent adoption of efficient St during low-speed swimming.