The effect of turbidity on recognition and generalization of predators and non-predators in aquatic ecosystems

Uncertainty about predation risk: a conceptual review

Uncertainty has long been of interest to economists and psychologists and has more recently gained attention among ecologists. In the ecological world, animals must regularly make decisions related to finding resources and avoiding threats. Here, we describe uncertainty as a perceptual phenomenon of decision-makers, and we focus specifically on the functional ecology of such uncertainty regarding predation risk. Like all uncertainty, uncertainty about predation risk reflects informational limitations. When cues are available, they may be novel (i.e. unknown information), incomplete, unreliable, overly abundant and complex, or conflicting. We review recent studies that have used these informational limitations to induce uncertainty of predation risk. These studies have typically used either over-responses to novelty (i.e. neophobia) or memory attenuation as proxies for measuring uncertainty. Because changes in the environment, particularly unpredictable changes, drive informational limitations, we describe studies assessing unpredictable variance in spatio-temporal predation risk, intensity of predation risk, predator encounter rate, and predator diversity. We also highlight anthropogenic changes within habitats that are likely to have dramatic impacts on information availability and thus uncertainty in antipredator decisions in the modern world.

Differential effects of fluoxetine on the phototactic behavior of 3 amphipod species (Crustacea; Amphipoda)

We document phototactic responses in different amphipod populations of Gammarus minus, Stygobromus tenuis, and Crangonyx shoemakeri, each collected at 2-3 sites within the Washington DC area. We then assessed how baseline phototaxis was altered following either short-term (3-week) or long-term (6-week) exposure to 0.05 µg/L or 0.5 µg/L fluoxetine. Our results classify all species as significantly photonegative, a response that depended solely on the presence, not quality, of light. Short-term fluoxetine exposure caused some animals to become photoneutral, regardless of concentration, while others remained photonegative. Long-term exposure to 0.5 µg/L fluoxetine caused photoneutral behaviors in all surviving populations; exposure to 0.05 µg/L had variable effects. These differential effects were due to a significant effect of population/sampling location on photobehavior. Overall, these results identify species-specific effects of chronic fluoxetine exposure and underscore how the response to light in 7 geographically distinct populations is uniquely tuned to requirements for survival.

Tadpole Responses to Environments With Limited Visibility: What We (Don’t) Know and Perspectives for a Sharper Future

Amphibian larvae typically inhabit relatively shallow freshwater environments, and within these boundaries there is considerable diversity in the structure of the habitats exploited by different species. This diversity in habitat structure is usually taken into account in relation to aspects such as locomotion and feeding, and plays a fundamental role in the classification of tadpoles into ecomorphological guilds. However, its impact in shaping the sensory worlds of different species is rarely addressed, including the optical qualities of each of these types of water bodies and the challenges and limitations that they impose on the repertoire of visual abilities available for a typical vertebrate eye. In this Perspective article, we identify gaps in knowledge on (1) the role of turbidity and light-limited environments in shaping the larval visual system; and (2) the possible behavioral and phenotypic responses of larvae to such environments. We also identify relevant unaddressed study systems paying special attention to phytotelmata, whose small size allows for extensive quantification and manipulation providing a rich and relatively unexplored research model. Furthermore, we generate hypotheses ranging from proximate shifts (i.e., red-shifted spectral sensitivity peaks driven by deviations in chromophore ratios) to ultimate changes in tadpole behavior and phenotype, such as reduced foraging efficiency and the loss of antipredator signaling. Overall, amphibians provide an exciting opportunity to understand adaptations to visually limited environments, and this framework will provide novel experimental considerations and interpretations to kickstart future research based on understanding the evolution and diversity of strategies used to cope with limited visibility.

Enhancing the ecological realism of evolutionary mismatch theory

Following rapid environmental change, why do some animals thrive, while others struggle? We present an expanded, cue-response framework for predicting variation in behavioral responses to novel situations. We show how signal detection theory can be used when individuals have three behavioral options (approach, avoid, or ignore). Based on this theory, we outline predictions about which animals are more likely to make mistakes around novel conditions (i.e., fall for a trap or fail to use an undervalued resource) and the intensity of that mismatch (i.e., severe versus moderate). Explicitly considering three options provides a more holistic perspective and allows us to distinguish between severe and moderate traps, which could guide management strategies in a changing world.

Tone deaf: Association of an auditory stimulus with predation risk by zebrafish Danio rerio does not generalize to another auditory stimulus

Predator recognition by small fishes can be acquired when chemical alarm cues released from damaged skin (by a predator attack) are paired with a novel stimulus, such as the appearance or odor of a predator. Once learned, fish can extend recognition of risk by generalizing to associate risk with additional stimuli that are similar to the conditioned novel stimulus. Here, we trained zebrafish to associate a novel auditory stimulus with predation risk, and then tested to see if they generalize risk to all sound stimuli or whether the conditioned response is limited to the sound frequency of the conditioning stimulus. We found that zebrafish Danio rerio readily associated risk of predation with Tone 1 (285 Hz), as evidenced by reduction in activity, increased time spent near the substratum and increased shelter use, but fish conditioned to fear Tone 1 completely ignored presentation of a second tone of 762 Hz. These data suggest that generalization does not occur as easily for auditory cues as they do for olfactory and visual cues, perhaps due to differences in the properties of sensory biology or the cognitive mechanisms that process information in different sensory modalities.

The impact of turbidity on foraging and risk taking in the invasive Nile tilapia (Oreochromis niloticus) and a threatened native cichlid (Oreochromis amphimelas)

Abstract

Anthropogenic activity can increase water turbidity, changing fish behaviour by reducing visibility. The spread of invasive species is also facilitated by human activity, further increasing the pressure on native species. In two experiments, we measured the foraging efficiency, risk perception and inter-individual consistency of risk-taking (personality variation in boldness) of an invasive species, the Nile tilapia (Oreochromis niloticus), and a threatened tilapia, the Manyara tilapia (Oreochromis amphimelas), in clear and turbid water. In experiment one, O. niloticus was faster to initiate feeding, encountered more food items and consumed more than O. amphimelas. The latency to start foraging by O. niloticus decreased in turbid water. Turbidity did not affect the latency to start foraging in O. amphimelas but the number of food items they encountered was highest at the intermediate turbidity. There was however no significant effect of turbidity in either species on the total food consumed. In contrast to this foraging context, in experiment two with a refuge and no food available, risk taking behaviour was similar in both species and they both responded with similarly reduced risk taking in turbid water. Evidence of personality variation was weak, being observed only in O. amphimelas when first leaving the shelter in turbid water. Overall, species differences were greater in the foraging context but turbidity was more important in the risk-taking context. O. amphimelas is more sensitive to turbidity during foraging, and O. niloticus is likely to have a competitive advantage in foraging situations, especially in degraded turbid habitats.

Significance statement

Under human-induced environmental change, native species are often exposed to multiple stressors. Here, we tested the responses of two cichlid fish to increasing turbidity, the Nile tilapia (Oreochromis niloticus), which is invasive throughout the tropics, and the Manyara tilapia (Oreochromis amphimelas), a threatened species, indigenous to Tanzania. We found that turbidity was beneficial to the foraging of O. niloticus, which in both clear and turbid water consumed and encountered more food than O. amphimelas. In contrast, without food present, both species displayed similar responses of increased risk perception in turbid water with little evidence of personality variation between individuals in either species. Our results suggest that invasive species tolerant of degraded habitats may outcompete less well-adapted native species for food.

Environmental stress gradients regulate the relative importance of predator density- and trait-mediated indirect effects in oyster reef communities

Predators affect community structure by influencing prey density and traits, but the importance of these effects often is difficult to predict. We measured the strength of blue crab predator effects on mud crab prey consumption of juvenile oysters across a flow gradient that inflicts both physical and sensory stress to determine how the relative importance of top predator density-mediated indirect effects (DMIEs) and trait-mediated indirect effects (TMIEs) change within systems. Overall, TMIEs dominated in relatively benign flow conditions where blue crab predator cues increased oyster survivorship by reducing mud crab-oyster consumption. Blue crab DMIEs became more important in high sensory stress conditions, which impaired mud crab perception of blue crab chemical cues. At high physical stress, the environment benefitted oyster survival by physically constraining mud crabs. Thus, factors that structure communities may be predicted based on an understanding of how physical and sensory performances change across environmental stress gradients.

Act now before its too late: Copper exposure drives chemo-ecology of predator-prey dynamics of freshwater common spiny loach, Lepidocephalichthys thermalis (Valenciennes, 1846)

Due to the extensive use of copper (Cu) in various commercial products, its existence in aquatic bodies (freshwater and marine) is not unusual. Cu is well known for its effect on the olfactory physiology of fish. However, there are limited studies on the effect of Cu on important ecological functions in fish (predator-prey dynamics) that are primarily influenced by olfaction. In a series of experiments, we studied the effect of Cu exposure on the chemoreceptive behavior of the prey fish, Lepidocephalichthys thermalis. Prey fishes were exposed to an environmentally relevant concentration (5 μg/L) of Cu for 3 h and the anti-predator responses against native (Channa gachua) and alien predatory fish (tilapia) were quantified using an ethological assay. Cu exposed prey fishes did not recognize the native predator and had a lower survival rate than control (unexposed) fishes in predation trials. Cu exposed prey fishes have failed to learn associatively to detect a non-native predator resulting in higher mortality in prey population in direct encounters with tilapia. However, such a lack of predator recognition was found to be short-term and the treated prey fishes recovered anti-predator responses within 72 h. In addition, Cu inactivated the alarm cue which acts as a signal for the presence of predators and ensures associative learning and therefore it was considered to be an 'info-disruptor' in the present study. These outcomes together demonstrate that even at low concentration, Cu influences ecological decisions and survival against predators. Owing to the ubiquitous occurrence of Cu in water bodies, the present investigation will contribute to the knowledge of how environmental stressors alter the crucial ecological decisions of prey individuals in aquatic ecosystems. In addition, we suggest that freshwater reservoirs containing high levels of Cu could be unsuitable for the long-term survival of prey fishes and freshwater biodiversity.

Prey Responses to Exotic Predators: Effects of Old Risks and New Cues

Exotic predators can have major negative impacts on prey. Importantly, prey vary considerably in their behavioral responses to exotic predators. Factors proposed to explain variation in prey response to exotic predators include the similarity of new predators to familiar, native predators, the prevalence and diversity of predators in a prey's past, and variation in a prey's innate ability to discriminate between predators and safety. While these factors have been put forth verbally in the literature, no theory exists that combines these hypotheses in a common conceptual framework using a unified behavioral model. Here, we formalize existing verbal arguments by modeling variation in prey responses to new predators in a state-dependent detection theory framework. We find that while some conventional wisdom is upheld, novel predictions emerge. As expected, prey respond poorly to exotic predators that do not closely resemble familiar predators. Furthermore, a history with more abundant or diverse native predators can lessen effects of some exotic predators on prey; however, under some conditions, the opposite prediction emerges. Also, prey that evolved in situations where they easily discriminate between safe and dangerous situations can be more susceptible to novel predators.

Chronic exposure to dietary selenomethionine dysregulates the genes involved in serotonergic neurotransmission and alters social and antipredator behaviours in zebrafish (Danio rerio)

Selenium (Se) is a metalloid of potential interest from both a toxicological and nutritional perspective, having a range of safe intake. The adverse neuro-behavioural effects of Se have been investigated in both humans and fishes, but little is known about its effects on social behaviours or the serotonergic signaling pathway in the brain. In the present study, we investigated the effects of chorionic dietary exposure to Se (as selenomethionine) at different concentrations (control, 2.1, 11.6 or 31.5 μg/g dry wt.) on antipredator avoidance, shoaling behaviour, and social group preferences in adult zebrafish (Danio rerio). In addition, we also measured the expression of important genes in the serotonergic pathway that influence social behaviours. After 60 days of exposure, the highest dose (31.5 μg/g dry wt.) caused the highest level of baseline fear behaviour, with fish swimming lower in the water column and in tighter shoals compared to fish in the other treatments. With high levels of baseline fear, these fish did not significantly intensify fear behaviours in response to predation risk in the form of exposure to chemical alarm cues. When individual fish were given an opportunity to shoal with groups of differing sizes (3 vs. 4 individuals), fish exposed to the high dose spent less time with groups in general, and only control fish showed a significant preference for the larger group. In the zebrafish brain, we found significant upregulation in the mRNA expression of serotonin receptors (htr1aa and htr1b), a transporter (slc6a4a), and tryptophan hydroxylase-2 (tph2), whereas there was a downregulation of the monoamine oxidase (mao) gene. The results of this study suggest that disruption of serotonergic neurotransmission might have been responsible for Se-induced impairment of antipredator and social behaviour in zebrafish.

Nutritional physiology and ecology of wildlife in a changing world

Over the last century, humans have modified landscapes, generated pollution and provided opportunities for exotic species to invade areas where they did not evolve. In addition, humans now interact with animals in a growing number of ways (e.g. ecotourism). As a result, the quality (i.e. nutrient composition) and quantity (i.e. food abundance) of dietary items consumed by wildlife have, in many cases, changed. We present representative examples of the extent to which vertebrate foraging behaviour, food availability (quantity and quality) and digestive physiology have been modified due to human-induced environmental changes and human activities. We find that these effects can be quite extensive, especially as a result of pollution and human-provisioned food sources (despite good intentions). We also discuss the role of nutrition in conservation practices, from the perspective of both in situ and ex situ conservation. Though we find that the changes in the nutritional ecology and physiology of wildlife due to human alterations are typically negative and largely involve impacts on foraging behaviour and food availability, the extent to which these will affect the fitness of organisms and result in evolutionary changes is not clearly understood, and requires further investigation.

Social learning of predators in the dark: understanding the role of visual, chemical and mechanical information

The ability of prey to observe and learn to recognize potential predators from the behaviour of nearby individuals can dramatically increase survival and, not surprisingly, is widespread across animal taxa. A range of sensory modalities are available for this learning, with visual and chemical cues being well-established modes of transmission in aquatic systems. The use of other sensory cues in mediating social learning in fishes, including mechano-sensory cues, remains unexplored. Here, we examine the role of different sensory cues in social learning of predator recognition, using juvenile damselfish (Amphiprion percula). Specifically, we show that a predator-naive observer can socially learn to recognize a novel predator when paired with a predator-experienced conspecific in total darkness. Furthermore, this study demonstrates that when threatened, individuals release chemical cues (known as disturbance cues) into the water. These cues induce an anti-predator response in nearby individuals; however, they do not facilitate learnt recognition of the predator. As such, another sensory modality, probably mechano-sensory in origin, is responsible for information transfer in the dark. This study highlights the diversity of sensory cues used by coral reef fishes in a social learning context.