Cost of transport is a repeatable trait but is not determined by mitochondrial efficiency in zebrafish (Danio rerio)

Intergenerational plasticity aligns with temperature-dependent selection on offspring metabolic rates

Metabolic rates are linked to key life-history traits that are thought to set the pace of life and affect fitness, yet the role that parents may have in shaping the metabolism of their offspring to enhance survival remains unclear. Here, we investigated the effect of temperature (24°C or 30°C) and feeding frequency experienced by parent zebrafish (Danio rerio) on offspring phenotypes and early survival at different developmental temperatures (24°C or 30°C). We found that embryo size was larger, but survival lower, in offspring from the parental low food treatment. Parents exposed to the warmer temperature and lower food treatment also produced offspring with lower standard metabolic rates-aligning with selection on embryo metabolic rates. Lower metabolic rates were correlated with reduced developmental and growth rates, suggesting selection for a slow pace of life. Our results show that intergenerational phenotypic plasticity on offspring size and metabolic rate can be adaptive when parent and offspring temperatures are matched: the direction of selection on embryo size and metabolism aligned with intergenerational plasticity towards lower metabolism at higher temperatures, particularly in offspring from low-condition parents. These findings provide evidence for adaptive parental effects, but only when parental and offspring environments match. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Variations in cost of transport and their ecological consequences: a review

Movement is essential in the ecology of most animals, and it typically consumes a large proportion of individual energy budgets. Environmental conditions modulate the energetic cost of movement (cost of transport, COT), and there are pronounced differences in COT between individuals within species and across species. Differences in morphology affect COT, but the physiological mechanisms underlying variation in COT remain unresolved. Candidates include mitochondrial efficiency and the efficiency of muscle contraction-relaxation dynamics. Animals can offset increased COT behaviourally by adjusting movement rate and habitat selection. Here, we review the theory underlying COT and the impact of environmental changes on COT. Increasing temperatures, in particular, increase COT and its variability between individuals. Thermal acclimation and exercise can affect COT, but this is not consistent across taxa. Anthropogenic pollutants can increase COT, although few chemical pollutants have been investigated. Ecologically, COT may modify the allocation of energy to different fitness-related functions, and thereby influence fitness of individuals, and the dynamics of animal groups and communities. Future research should consider the effects of multiple stressors on COT, including a broader range of pollutants, the underlying mechanisms of COT and experimental quantifications of potential COT-induced allocation trade-offs.

Physiology can predict animal activity, exploration, and dispersal

Physiology can underlie movement, including short-term activity, exploration of unfamiliar environments, and larger scale dispersal, and thereby influence species distributions in an environmentally sensitive manner. We conducted meta-analyses of the literature to establish, firstly, whether physiological traits underlie activity, exploration, and dispersal by individuals (88 studies), and secondly whether physiological characteristics differed between range core and edges of distributions (43 studies). We show that locomotor performance and metabolism influenced individual movement with varying levels of confidence. Range edges differed from cores in traits that may be associated with dispersal success, including metabolism, locomotor performance, corticosterone levels, and immunity, and differences increased with increasing time since separation. Physiological effects were particularly pronounced in birds and amphibians, but taxon-specific differences may reflect biased sampling in the literature, which also focussed primarily on North America, Europe, and Australia. Hence, physiology can influence movement, but undersampling and bias currently limits general conclusions.

Endocrine disruption from plastic pollution and warming interact to increase the energetic cost of growth in a fish

Energetic cost of growth determines how much food-derived energy is needed to produce a given amount of new biomass and thereby influences energy transduction between trophic levels. Growth and development are regulated by hormones and are therefore sensitive to changes in temperature and environmental endocrine disruption. Here, we show that the endocrine disruptor bisphenol A (BPA) at an environmentally relevant concentration (10 µgl^-1) decreased fish (Danio rerio) size at 30°C water temperature. Under the same conditions, it significantly increased metabolic rates and the energetic cost of growth across development. By contrast, BPA decreased the cost of growth at cooler temperatures (24°C). BPA-mediated changes in cost of growth were not associated with mitochondrial efficiency (P/O ratios (i.e. adenosine diphosphate (ADP) used/oxygen consumed) and respiratory control ratios) although BPA did increase mitochondrial proton leak. In females, BPA decreased age at maturity at 24°C but increased it at 30°C, and it decreased the gonadosomatic index suggesting reduced investment into reproduction. Our data reveal a potentially serious emerging problem: increasing water temperatures resulting from climate warming together with endocrine disruption from plastic pollution can impact animal growth efficiency, and hence the dynamics and resilience of animal populations and the services these provide.

Diet and temperature modify the relationship between energy use and ATP production to influence behavior in zebrafish (Danio rerio)

Food availability and temperature influence energetics of animals and can alter behavioral responses such as foraging and spontaneous activity. Food availability, however, is not necessarily a good indicator of energy (ATP) available for cellular processes. The efficiency of energy transduction from food-derived substrate to ATP in mitochondria can change with environmental context. Our aim was to determine whether the interaction between food availability and temperature affects mitochondrial efficiency and behavior in zebrafish (Danio rerio). We conducted a fully factorial experiment to test the effects of feeding frequency, acclimation temperature (three weeks to 18 or 28°C), and acute test temperature (18 and 28°C) on whole-animal oxygen consumption, mitochondrial bioenergetics and efficiency (ADP consumed per oxygen atom; P:O ratio), and behavior (boldness and exploration). We show that infrequently fed (once per day on four days per week) zebrafish have greater mitochondrial efficiency than frequently fed (three times per day on five days per week) animals, particularly when warm-acclimated. The interaction between temperature and feeding frequency influenced exploration of a novel environment, but not boldness. Both resting rate of producing ATP and scope for increasing it were positively correlated with time spent exploring and distance moved in standardized trials. In contrast, behavior was not associated with whole-animal aerobic (oxygen consumption) scope, but exploration was positively correlated with resting oxygen consumption rates. We highlight the importance of variation in both metabolic (oxygen consumption) rate and efficiency of producing ATP in determining animal performance and behavior. Oxygen consumption represents energy use, and P:O ratio is a variable that determines how much of that energy is allocated to ATP production. Our results emphasize the need to integrate whole-animal responses with subcellular traits to evaluate the impact of environmental conditions on behavior and movement.

The relationship between membrane fatty acid content and mitochondrial efficiency differs within- and between- omega-3 dietary treatments

An important, but underappreciated, consequence of climate change is the reduction in crucial nutrient production at the base of the marine food chain: the long-chain omega-3 highly unsaturated fatty acids (n-3 HUFA). This can have dramatic consequences on consumers, such as fish as they have limited capacity to synthesise n-3 HUFA de novo. The n-3 HUFA, such as docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), are critical for the structure and function of all biological membranes. There is increasing evidence that fish will be badly affected by reductions in n-3 HUFA dietary availability, however the underlying mechanisms remain obscure. Hypotheses for how mitochondrial function should change with dietary n-3 HUFA availability have generally ignored ATP production, despite its importance to a cell's total energetics capacity, and in turn, whole-animal performance. Here we (i) quantified individual variation in mitochondrial efficiency (ATP/O ratio) of muscle and (ii) examined its relationship with content in EPA and DHA in muscle membrane of a primary consumer fish, the golden grey mullet Chelon auratus, receiving either a high or low n-3 HUFA diet. Mitochondria of fish fed on the low n-3 HUFA diet had higher ATP/O ratio than those of fish maintained on the high n-3 HUFA diet. Yet, mitochondrial efficiency varied up about 2-fold among individuals on the same dietary treatment, resulting in some fish consuming half the oxygen and energy substrate to produce the similar amount of ATP than conspecific on similar diet. This variation in mitochondrial efficiency among individuals from the same diet treatment was related to individual differences in fatty acid composition of the membranes: a high ATP/O ratio was associated with a high content in EPA and DHA in biological membranes. Our results highlight the existence of interindividual differences in mitochondrial efficiency and its potential importance in explaining intraspecific variation in response to food chain changes.

The impacts of climate change on the biomechanics of animals: Themed Issue Article: Biomechanics and Climate Change

Anthropogenic climate change induces unprecedented variability in a broad range of environmental parameters. These changes will impact material properties and animal biomechanics, thereby affecting animal performance and persistence of populations. Climate change implies warming at the global level, and it may be accompanied by altered wind speeds, wave action, ocean circulation, acidification as well as increased frequency of hypoxic events. Together, these environmental drivers affect muscle function and neural control and thereby movement of animals such as bird migration and schooling behaviour of fish. Altered environmental conditions will also modify material properties of animals. For example, ocean acidification, particularly when coupled with increased temperatures, compromises calcified shells and skeletons of marine invertebrates and byssal threads of mussels. These biomechanical consequences can lead to population declines and disintegration of habitats. Integrating biomechanical research with ecology is instrumental in predicting the future responses of natural systems to climate change and the consequences for ecosystem services such as fisheries and ecotourism.

Increased physical activity does not improve obesity-induced decreases in muscle quality in zebrafish (Danio rerio)

Obesity has a negative effect on muscle contractile function, and the effects of obesity are not reversed by weight loss. It is therefore important to determine how muscle function can be restored, and exercise is the most promising approach. We tested the hypothesis (in zebrafish, Danio rerio) that moderate aerobic exercise (forced swimming for 30 min/day, raising metabolic rates to at least twice resting levels) will alleviate the negative effects of obesity on muscle function. We allocated zebrafish randomly to experimental treatments in a fully factorial design with diet treatment [three levels: lean control, diet-induced obese, obese followed by weight loss (obese-lean)], and exercise (exercise and sedentary control) as independent factors. Treatments were conducted for 10 wk, and we measured locomotor performance, isolated muscle mechanics, and myosin heavy chain composition. Obesity led to decreased muscle force production per unit area ( P = 0.01), and slowed muscle contraction ( P = 0.004) and relaxation rates ( P = 0.02). These effects were not reversible by weight loss or exercise. However, at the level implemented in our experimental animals, neither diet nor exercise affected swimming performance or myosin heavy chain concentrations. The moderate levels of exercise we implemented therefore are not sufficient to reverse the effects of obesity on muscle function, and higher intensity or a combination of modes of exercise may be necessary to improve muscle quality during obesity and following weight loss.

NEW & NOTEWORTHY Obesity can have a negative effect on muscle function and thereby compromise mobility. Even though aerobic exercise has many physiological benefits in obese and normal-weight individuals, we show that in zebrafish aerobic exercise does not improve obesity-induced reductions in muscle contractile function. A combination between different modes of exercise may be more effective than aerobic exercise alone.