Tracing exploitation of egg boons: an experimental study using fatty acids and stable isotopes.

Coordinated spawning of marine animals releases millions of planktonic eggs into the environment, known as egg boons. Eggs are rich in essential fatty acids and may be an important lipid subsidy to egg consumers. Our aim was to validate the application of fatty acid and stable isotope tracers of egg consumption to potential egg consumers and to confirm egg consumption by the selected species. We conducted feeding experiments with ctenophores, crustaceans and fishes. We fed these animals a common diet of Artemia or a commercial feed (Otohime) and simulated egg boons for half of them by intermittently supplementing the common diet with red drum (Sciaenops ocellatus) eggs for 10-94 days. Controls did not receive eggs. Fatty acid profiles of consumers fed eggs were significantly different from those of controls 24 h after the last egg-feeding event. Consumers took on fatty acid characteristics of eggs. In fishes and ctenophores, fatty acid markers of egg consumption did not persist 2-5 days after the last egg-feeding event, but markers of egg consumption persisted in crustaceans for at least 5-10 days. Additionally, consumption of eggs, which had high values of δ15N, led to δ15N enrichment in crustaceans and a fish. We conclude that fatty acids and nitrogen stable isotope can be used as biomarkers of recent egg consumption in marine animals, validating their use for assessing exploitation of egg boons in nature.

Gene expression underlying variation in the survival skills of red drum larvae (Sciaenops ocellatus).

Mortality rates of marine fish larvae are incredibly high and can determine year-class strength. The major causes of larval mortality are predation and starvation, and the performance of larvae in survival skills that can mitigate this mortality (predator evasion, foraging) varies among individuals and cohorts, but the causes of the variation are not known. Transcriptomics can link gene expression variation to phenotypic variation at the whole-system level to investigate the molecular basis of behavioural variation. We used tag-based RNA-sequencing to examine the molecular basis of variation in predator evasion and routine swimming (trait related to foraging efficiency) in the larval red drum, Sciaenops ocellatus. We looked for functional gene networks in which interindividual variation would explain variation in larval behavioural performance. We identified co-expressed gene groups ("modules") associated with predator evasion traits and found enrichment of motor, neural and energy metabolism pathways. These functional associations and pattern of correlations between modules and traits suggest that energy availability and allocation were responsible for the magnitude of startle responses, while differential neural and motor activation were associated with differences in response latency.

Nutritional programming by maternal diet alters offspring lipid metabolism in a marine teleost.

Nutritional programming - the association between the early nutritional environment and long-term consequences for an animal - is an emerging area of research in fish biology. Previous studies reported correlations between maternal provisioning of essential fatty acids to eggs and the whole-body fatty acid composition of larvae reared under uniform conditions for red drum, Sciaenops ocellatus. This study aimed to further investigate the nutritional stimulus and the consequences of nutritional programming by feeding adult red drum several distinct diets and rearing larvae under uniform conditions until 21 days post-hatching when larval lipid and fatty acid compositions were assessed. Different maternal diets produced eggs with distinctive lipid and fatty acid compositions, and despite receiving the same larval diet for almost 3 weeks, larvae showed differences in total fatty acid accumulation and in retention of highly unsaturated fatty acids (HUFA). Specifically, larvae reared from a maternal diet of shrimp generally showed elevated levels of fatty acids in the initial steps of the n-3 and n-6 HUFA biosynthetic pathways and reduced levels of fatty acid products of the same pathways, especially in triglyceride. Furthermore, the variations in larval fatty acid accumulation induced by maternal diet varied among females. Lipid metabolism altered by parental diet may have consequences for larval physiological processes and behavioral performance, which may ultimately influence larval survival.

FishSizer: Software solution for efficiently measuring larval fish size.

Length and depth of fish larvae are part of the fundamental measurements in many marine ecology studies involving early fish life history. Until now, obtaining these measurements has required intensive manual labor and the risk of inter- and intra-observer variability.We developed an open-source software solution to semi-automate the measurement process and thereby reduce both time consumption and technical variability. Using contrast-based edge detection, the software segments images of a fish larva into "larva" and "background." Length and depth are extracted from the "larva" segmentation while taking curvature of the larva into consideration. The graphical user interface optimizes workflow and ease of usage, thereby reducing time consumption for both training and analysis. The software allows for visual verification of all measurements.A comparison of measurement methods on a set of larva images showed that this software reduces measurement time by 66%-78% relative to commonly used software.Using this software instead of the commonly used manual approach has the potential to save researchers from many hours of monotonous work. No adjustment was necessary for 89% of the images regarding length (70% for depth). Hence, the only workload on most images was the visual inspection. As the visual inspection and manual dimension extraction works in the same way as currently used software, we expect no loss in accuracy.

Incorporation of dietary lipids and fatty acids into red drum Sciaenops ocellatus eggs.

Embryonic and early larval development and metabolism are fueled entirely by maternally derived nutritional resources (yolk and oil) before the onset of exogenous feeding. Composition of these maternally derived nutrients depends partly on maternal diet. Diet-egg relationships for fatty acids are well described for some species, but little is known about lipid transfer to eggs. To examine the effects of maternal diet on the egg composition, we fed adult red drum Sciaenops ocellatus six different diets, and measured lipid class and fatty acid composition of eggs they produced. Egg lipid class profiles remained relatively stable with only subtle differences in the concentrations of several lipid classes. Neutral lipid classes (wax ester/steryl ester (WE/SE), triglyceride (TG), sterol) varied more than polar lipid classes, with egg TG content being directly related to TG content of maternal diets. Dietary variations rapidly affected fatty acid composition of all major lipid classes in eggs (TG, WE/SE, phosphatidylcholine), with greater effects on neutral lipids than on the polar lipid. Results suggest a degree of maternal control over the provisioning of lipids as structural components (phospholipids) and energy substrates (neutral lipids), which may ensure proper development of larvae. But, egg fatty acid composition within lipid classes is more variable, and this may have consequences for larval survival and performance. This study also suggests that the pathways of maternal-offspring nutrient transfer are likely different for neutral and polar lipids.

Maternal diet affects utilization of endogenous lipids by red drum Sciaenops ocellatus embryos and early larvae.

Embryonic and early larval development and metabolism of most teleost fishes are fueled entirely by maternally derived nutritional resources (yolk and oil) before the onset of exogenous feeding. The composition of those resources depends, in part, on maternal diet. To examine how diet-induced variations in egg composition affect embryonic and larval utilization of endogenous lipids and fatty acids, we fed red drum (Sciaenops ocellatus) broodstock four different diets to produce distinctive fatty acid compositions in eggs but with no significant difference in total lipid content. We sampled embryos and unfed larvae every 12 h from 12 h post-fertilization (hpf) until starvation (96-120 hpf; n = 5 spawns per diet group). Rates of utilization of the oil globule were significantly different among diet groups, resulting in significant differences in mean oil globule size at the first feeding stage (84 hpf). Utilization rates for 15 fatty acids were significantly different among diet groups and were proportional to their initial concentration. As a result, differences in larval fatty acid compositions among maternal diet groups diminished over time but remained different even at later stages. In addition, larval standard length at 84 hpf was positively correlated with egg total lipid content. This study suggests that the fatty acid composition of yolk and oil affects energy metabolism and tissue composition in red drum larvae. These effects could have consequences for the survival and essential physiological functions of fish larvae during the critical period of transition to exogenous feeding.

Dynamics of diet-egg transfer of fatty acids in the teleost fish, red drum (Sciaenops ocellatus).

Eggs of marine organisms are increasingly being recognized as important components of marine food webs. The degree to which egg fatty acid profiles reflect maternal diet fatty acid profiles, and therefore the value of fatty acids in eggs as trophic biomarkers, depends on the species' reproductive strategy and the extent of modification of ingested fatty acids. We measured the dynamics of transfer of recently ingested fatty acids to spawned eggs in a batch-spawning teleost, red drum (Sciaenops ocellatus). Results of 21 diet-shift experiments, from which the fatty acid profiles of the diets and eggs were compared, showed that 15 of 27 fatty acids measured (one saturated, two monounsaturated and 12 polyunsaturated fatty acids) in eggs were correlated with their levels in the recent diet, and the rate of incorporation into eggs was proportional to the magnitude of the diet shift. Large shifts in diet might occur naturally during spawning migrations or when prey communities vary over time. Results of this study indicate that fatty acids in red drum eggs can be useful for studying adult diet and exploring trophic linkages in marine systems. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.

Ontogenetic change in predicted acoustic pressure sensitivity in larval red drum (Sciaenops ocellatus).

Detecting acoustic pressure can improve a fish's survival and fitness through increased sensitivity to environmental sounds. Pressure detection results from interactions between the swim bladder and otoliths. In larval fishes, those interactions change rapidly as growth and development alter bladder dimensions and otolith-bladder distance. We used computed tomography imagery of lab-reared larval red drum (Sciaenops ocellatus) in a finite-element model to assess ontogenetic changes in acoustic pressure sensitivity in response to a plane wave at frequencies within the frequency range of hearing by fishes. We compared the acceleration at points on the sagitta, asteriscus and lapillus when the bladder was air filled with results from models using a water-filled bladder. For larvae of 8.5-18 mm in standard length, the air-filled bladder amplified simulated otolith motion by a factor of 54-3485 times that of a water-filled bladder at 100 Hz. Otolith-bladder distance increased with standard length, which decreased modeled amplification. The concomitant rapid increase in bladder volume partially compensated for the effect of increasing otolith-bladder distance. Calculated resonant frequency of the bladders was between 8750 and 4250 Hz, and resonant frequency decreased with increasing bladder volume. There was a relatively flat frequency dependence of these effects in the audible frequency range, but we found a small increase in amplification with increasing excitation frequency. Using idealized geometry, we found that the larval vertebrae and ribs have negligible influence on bladder motion. Our results help clarify the auditory consequences of ontogenetic changes in bladder morphology and otolith-bladder relationships during larval stages.

Metabolic programming mediated by an essential fatty acid alters body composition and survival skills of a marine fish.

Metabolic programming occurs when variations in nutrition during a specific developmental window result in long-term metabolic effects. It has been studied almost exclusively in humans and other mammals but never in an ecological context. Here, we report metabolic programming and its functional consequences in a marine fish, red drum. We demonstrate that maternal provisioning of eggs with an essential fatty acid, docosahexaenoic acid (DHA), varies with DHA content of the maternal diet. When offspring are reared on a DHA-replete diet, whole-body DHA content of offspring depends upon the amount of DHA that was in the egg. We further demonstrate that whole-body DHA content is correlated with traits related to offspring fitness (escape responses, routine swimming, growth, and survival). DHA content of red drum eggs produced in nature is in the range where the effects of metabolic programming are most pronounced. Our findings indicate that during a brief developmental window, DHA plays a role in establishing the metabolic capacity for its own uptake or storage, with protracted and possibly permanent effects on ecologically important survival skills of individuals and important implications for dynamics of populations and food webs.

Egg boons: central components of marine fatty acid food webs.

Food web relationships are traditionally defined in terms of the flow of key elements, such as carbon, nitrogen, and phosphorus, and their role in limiting production. There is growing recognition that availability of important biomolecules, such as fatty acids, may exert controls on secondary production that are not easily explained by traditional element-oriented models. Essential fatty acids (EFAs) are required by most organisms for proper physiological function but are manufactured almost entirely by primary producers. Therefore, the flow of EFAs, especially docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (ARA), through aquatic food webs is critical for ecosystem functioning. A meta-analysis of data on the EFA content of marine organisms reveals that individual eggs of marine animals have exceptionally high concentrations of EFAs, and that superabundances of eggs released in temporally and spatially discrete patches create rich, but temporary, nutritional resources for egg predators, called "egg boons." Mortality rates of fish eggs are disproportionately higher than animals of similar size, and those eggs are consumed by predators, both larger and smaller than the adults that produce the eggs. Thus, egg boons are a major trophic pathway through which EFAs are repackaged and redistributed, and they are among the few pathways that run counter to the main direction of trophic flow. Egg boons can transport EFAs across ecosystems through advection of patches of eggs and spawning migrations of adults. Recognizing the significance of egg boons to aquatic food webs reveals linkages and feedbacks between organisms and environments that have important implications for understanding how food webs vary in time and space. Examples are given of top-down, bottom-up, and lateral control mechanisms that could significantly alter food webs through their effects on eggs. Our results suggest that trophodynamic food web models should include EFAs generally, and egg production and egg EFA content in particular.

Ocean acidification increases fatty acids levels of larval fish.

Rising levels of anthropogenic carbon dioxide in the atmosphere are acidifying the oceans and producing diverse and important effects on marine ecosystems, including the production of fatty acids (FAs) by primary producers and their transfer through food webs. FAs, particularly essential FAs, are necessary for normal structure and function in animals and influence composition and trophic structure of marine food webs. To test the effect of ocean acidification (OA) on the FA composition of fish, we conducted a replicated experiment in which larvae of the marine fish red drum (Sciaenops ocellatus) were reared under a climate change scenario of elevated CO2 levels (2100 µatm) and under current control levels (400 µatm). We found significantly higher whole-body levels of FAs, including nine of the 11 essential FAs, and altered relative proportions of FAs in the larvae reared under higher levels of CO2. Consequences of this effect of OA could include alterations in performance and survival of fish larvae and transfer of FAs through food webs.

Locomotion and the Cost of Hunting in Large, Stealthy Marine Carnivores.

Foraging by large (>25 kg), mammalian carnivores often entails cryptic tactics to surreptitiously locate and overcome highly mobile prey. Many forms of intermittent locomotion from stroke-and-glide maneuvers by marine mammals to sneak-and-pounce behaviors by terrestrial canids, ursids, and felids are involved. While affording proximity to vigilant prey, these tactics are also associated with unique energetic costs and benefits to the predator. We examined the energetic consequences of intermittent locomotion in mammalian carnivores and assessed the role of these behaviors in overall foraging efficiency. Behaviorally-linked, three-axis accelerometers were calibrated to provide instantaneous locomotor behaviors and associated energetic costs for wild adult Weddell seals (Leptonychotes weddellii) diving beneath the Antarctic ice. The results were compared with previously published values for other marine and terrestrial carnivores. We found that intermittent locomotion in the form of extended glides, burst-and-glide swimming, and rollercoaster maneuvers while hunting silverfish (Pleuragramma antarcticum) resulted in a marked energetic savings for the diving seals relative to continuously stroking. The cost of a foraging dive by the seals decreased by 9.2-59.6%, depending on the proportion of time gliding. These energetic savings translated into exceptionally low transport costs during hunting (COTHUNT) for diving mammals. COTHUNT for Weddell seals was nearly six times lower than predicted for large terrestrial carnivores, and demonstrates the importance of turning off the propulsive machinery to facilitate cost-efficient foraging in highly active, air-breathing marine predators.

Exercise at depth alters bradycardia and incidence of cardiac anomalies in deep-diving marine mammals.

Unlike their terrestrial ancestors, marine mammals routinely confront extreme physiological and physical challenges while breath-holding and pursuing prey at depth. To determine how cetaceans and pinnipeds accomplish deep-sea chases, we deployed animal-borne instruments that recorded high-resolution electrocardiograms, behaviour and flipper accelerations of bottlenose dolphins (Tursiops truncatus) and Weddell seals (Leptonychotes weddellii) diving from the surface to >200 m. Here we report that both exercise and depth alter the bradycardia associated with the dive response, with the greatest impacts at depths inducing lung collapse. Unexpectedly, cardiac arrhythmias occurred in >73% of deep, aerobic dives, which we attribute to the interplay between sympathetic and parasympathetic drivers for exercise and diving, respectively. Such marked cardiac variability alters the common view of a stereotypic 'dive reflex' in diving mammals. It also suggests the persistence of ancestral terrestrial traits in cardiac function that may help explain the unique sensitivity of some deep-diving marine mammals to anthropogenic disturbances.

Batch spawning facilitates transfer of an essential nutrient from diet to eggs in a marine fish.

Fatty acid composition of eggs affects development, growth and ecological performance of fish embryos and larvae, with potential consequences for recruitment success. Essential fatty acids in eggs derive from the maternal diet, and the time between ingestion and deposition in eggs is ecologically important but unknown. We examined the dynamics of diet-egg transfer of arachidonic acid (ARA) in the batch-spawning fish, red drum (Sciaenops ocellatus), by measuring ARA concentrations in eggs after a single diet shift and during a period of irregular variations in diet. ARA concentrations in eggs changed within 2-16 days of a diet shift. The rate of change was proportional to the magnitude of the shift, with no evidence of equilibration. These results are not consistent with eggs being assembled entirely from accumulated body stores. The immediate source of ARA in eggs appears to be the recent diet. We propose that batch spawning produces rapid diet-egg transfer of ARA because it removes large amounts of fatty acids from the body and prevents equilibration. The immediacy of the diet-egg connection suggests that spawning migration combined with short-interval batch spawning may have evolved to take advantage of nutrients critical for offspring survival that are available at the spawning site.

Post-prandial changes in protein synthesis in red drum (Sciaenops ocellatus) larvae.

Protein synthesis is one of the major energy-consuming processes in all living organisms. Post-prandial changes in protein synthesis have been studied in a range of animal taxa but have been little studied in fish larvae. Using the flooding-dose method, we measured post-prandial changes in whole-body rates of protein synthesis in regularly fed red drum Sciaenops ocellatus (Linnaeus) larvae for 24-28 h following their daily meal. Fractional rates of protein synthesis increased from a baseline (pre-feeding) rate of 16% day(-1) to a post-prandial peak of 48% day(-1) ca. 8 h after feeding before declining to 12% day(-1) after 24-28 h. The overall mean daily rate of protein synthesis was calculated as 27% day(-1). Although suggested as energetically impossible in larval poikilotherms, our results show that rates in excess of 30% day(-1) can be attained by larval fishes for a few hours but are not sustained. The average daily energetic cost of protein synthesis was estimated as 34% of daily total oxygen consumption, ranging from 19% immediately before feeding to 61% during the post-prandial peak in protein synthesis. This suggests that during the post-prandial peak, protein synthesis will require a large proportion of the hourly energy production, which, given the limited metabolic scope in fish larvae, may limit the energy that could otherwise be allocated to other energy-costly functions, such as foraging and escape responses.

Process-based approach reveals directional effects of environmental factors on movement between habitats.

1. Understanding the effects of environmental factors on animal distributions is a central issue in ecology. However, movement rules inferred from distribution patterns do not reveal the processes through which animal distribution is realized. 2. We investigated individual movement rules using a process-based approach. In experiments, coastal fish larvae (red drum, Sciaenops ocellatus) were matched with an intraspecific competitor of different sizes, and time series of habitat transition of individuals were fitted with a continuous-time Markov chain model to evaluate the effects of the presence of a competitor, behavioural interactions and habitat quality on the likelihoods of habitat transition. 3. The process-based approach revealed that these factors did not simply act as a 'slope' between habitats that makes it easier to go in one direction and more difficult to return. Rather, these factors modify the movement rules differently depending on the directions of the movement. 4. Individuals were less likely to enter a better habitat in the presence of a larger conspecific, more likely to shift to a poorer habitat when they received aggressive behaviour and more likely to stay in a better habitat in the presence of food. However, no effect was found on the transition intensity for moving in the opposite direction. 5. The process-based approach to evaluating movement rules of animals allowed us to see the contrasting directional effects of different factors on the underlying movement rules used by animals, as opposed to pattern-based fitting of observed distributions. Consideration of these rules would improve the existing habitat-choice models.

Maladaptive behavior reinforces a recruitment bottleneck in newly settled fishes.

Settlement from the plankton ends the major dispersive stage of life for many marine organisms and exposes them to intense predation pressure in juvenile habitats. This predation mortality represents a life-history bottleneck that can determine recruitment success. At the level of individual predator-prey interactions, prey survival depends upon behavior, specifically how behavior affects prey conspicuousness and evasive ability. We conducted an experiment to identify behavioral traits and performance levels that are important determinants of which individuals survive or die soon after settlement. We measured a suite of behavioral traits on late stage, pre-settlement Ward's damsel (Pomacentrus wardi) collected using light traps. These behavioral traits included two measures of routine swimming (indicators of conspicuousness) and eight measures of escape performance to a visual startle stimulus. Fish were then released onto individual patch reefs, where divers measured an additional behavioral trait (boldness). We censused each patch reef until approximately 50% of the fish were missing (~24 h), which we assumed to be a result of predation. We used classification tree analysis to discriminate survivors from fish presumed dead based on poor behavioral performance. The classification tree revealed that individuals displaying the maladaptive combination of low escape response speed, low boldness on the reef, and high routine swimming speed were highly susceptible to predation (92.4% with this combination died within 24 h). This accounted for 55.2% of all fish that died. Several combinations of behavioral traits predicted likely survival over 24 h, but there was greater uncertainty about that prediction than there was for fish that were predicted to die. Thus maladaptive behavioral traits were easier to identify than adaptive traits.

Growth and protein metabolism in red drum (Sciaenops ocellatus) larvae exposed to environmental levels of atrazine and malathion.

Contaminant exposure can affect development, growth, and behaviour of fish larvae, but its effect on rates of protein synthesis and protein degradation are not known. The aim of the present study was to examine the effects of a single pulsed dose aqueous exposure to environmentally realistic levels of two contaminants, atrazine (0, 40 and 80 microgl(-1)) and malathion (0, 1 and 10 microgl(-1)), on growth and protein synthesis in red drum (Sciaenops ocellatus) larvae. Growth was assessed in terms of increase in length, weight, and protein content over an 8-day period following exposure. Rates of protein synthesis were measured by the flooding dose technique 2, 4, and 8 days after initial exposure to each contaminant by bathing larvae in seawater containing L-[2,6-(3)H] phenylalanine. Exposure to atrazine had no effect on larval growth in length, but caused marginally significant declines in growth in weight (P=0.05) and protein content (P=0.06). However, protein synthesis rates were significantly higher for atrazine-exposed larvae on days 4 (P=0.04) and 8 (P=0.01), suggesting an increase in rates of protein degradation. On day 8, the efficiency with which synthesised proteins contributed to growth was significantly lower (P=0.04) in atrazine-exposed larvae. In contrast, malathion had no significant effects on growth in length or protein content, but there were significant decreases in growth in weight over 8 days. The only other significant effect of malathion was an increase in protein synthesis on day 2 for treated larvae relative to controls. Previous work [Alvarez, M.C., Fuiman, L.A., 2005. Environmental levels of atrazine and its degradation products impair survival skills and growth of red drum larvae. Aquat. Toxicol. 74, 229-241] reported hyperactivity and increased metabolic rate in larval red drum exposed to atrazine, indicating a clear energetic cost. Our results further emphasise the energetic cost of atrazine exposure through elevated rates of protein synthesis and degradation resulting in reduced protein retention efficiency and lower growth rates. Overall, we conclude that exposure to atrazine from surface water run-off can increase the energy requirements and the larval phase duration in red drum larvae, possibly resulting in reduced survival and recruitment in cohorts exposed to atrazine.

Modeling larval fish behavior: scaling the sublethal effects of methylmercury to population-relevant endpoints.

Expressing the sublethal effects of contaminants measured on individual fish as cohort and population responses would greatly help in their interpretation. Our approach combines laboratory studies with coupled statistical and individual-based models to simulate the effects of methylmercury (MeHg) on Atlantic croaker larval survival and growth. We used results of video-taped laboratory experiments on the effects of MeHg on larval behavioral responses to artificial predatory stimuli. Laboratory results were analyzed with a regression tree to obtain the probability of control and MeHg-exposed larvae escaping a real predatory fish attack. Measured changes in swimming speeds and regression tree-predicted escape abilities induced by MeHg exposure were then inputted into an individual-based larval fish cohort model. The individual-based model predicted larval-stage growth and survival under baseline (control) conditions, and low- and high-dose MeHg exposure under two alternative predator composition scenarios (medusa-dominated and predatory fish-dominated). Under MeHg exposure, stage survival was 7-19% of baseline (control) survival, and the roughly 33-day stage duration was extended by about 1-4 days. MeHg effects on larval growth dominated the response under the medusa-dominated predator composition, while predation played a more important role under the fish-dominated predator composition. Simulation results suggest that MeHg exposures near extreme maximum values observed in field studies can have a significant impact on larval cohort dynamics, and that the characteristics of the predator-prey interactions can greatly influence the underlying causes of the predicted responses.