Shoaling Behaviour Research Articles

Experimental infection with the directly transmitted parasite Gyrodactylus influences shoaling behaviour in sticklebacks

Animals usually benefit from joining groups, but joining a group can also come at a cost when members expose themselves to competition and the risk of contracting a contagious disease. Therefore, individuals are expected to adjust grouping behaviour to the ecological circumstances, their own competitiveness and the composition of the group. Here, we used experimental infections and classic binary choice tests to test whether the monogenean flatworm Gyrodactylus spp. has the potential to influence shoaling behaviour in the three-spined stickleback, Gasterosteus aculeatus , a model organism in behavioural ecology and evolutionary biology. Gyrodactylus spp. is a genus of widespread and rather inconspicuous, small ( Gyrodactylus species infecting sticklebacks have short generation times and those species typically residing on the skin or fins of their hosts are easily spread via body contact. In our experiments uninfected sticklebacks significantly preferred a group of uninfected fish over a group of Gyrodactylus -infected fish, while Gyrodactylus -infected sticklebacks did not discriminate between the two stimulus shoals with regard to their Gyrodactylus infection status. As infected fish were in poorer condition, were less likely to shoal and had a relatively heavy spleen, we suggest a generally reduced health state caused by the infection as a possible indirect mechanism of the altered shoaling preference. Although parasitism has been shown to play an important role in group formation, only a few studies have used experimental infections to directly test its influence on shoaling decisions. Our results show that Gyrodactylus spp. can influence shoaling decisions in three-spined sticklebacks and affirm the suitability of the Gyrodactylus –stickleback system for studying the role of parasitic infections on host group dynamics.

Two-dimensional numerical simulations of shoaling internal solitary waves at the ASIAEX site in the South China Sea

Abstract. The interaction of barotropic tides with Luzon Strait topography generates some of the world's largest internal solitary waves which eventually shoal and dissipate on the western side of the northern South China Sea. Two-dimensional numerical simulations of the shoaling of a single internal solitary wave at the site of the Asian Seas International Acoustic Experiment (ASIAEX) have been undertaken in order to investigate the sensitivity of the shoaling process to the stratification and the underlying bathymetry and to explore the influence of rotation. The bulk of the simulations are inviscid; however, exploratory simulations using a vertical eddy-viscosity confined to a near bottom layer, along with a no-slip boundary condition, suggest that viscous effects may become important in water shallower than about 200 m. A shoaling solitary wave fissions into several waves. At depths of 200–300 m the front of the leading waves become nearly parallel to the bottom and develop a very steep back as has been observed. The leading waves are followed by waves of elevation (pedestals) that are conjugate to the waves of depression ahead and behind them. Horizontal resolutions of at least 50 m are required to simulate these well. Wave breaking was found to occur behind the second or third of the leading solitary waves, never at the back of the leading wave. Comparisons of the shoaling of waves started at depths of 1000 and 3000 m show significant differences and the shoaling waves can be significantly non-adiabatic even at depths greater than 2000 m. When waves reach a depth of 200 m, their amplitudes can be more than 50% larger than the largest possible solitary wave at that depth. The shoaling behaviour is sensitive to the presence of small-scale features in the bathymetry: a 200 m high bump at 700 m depth can result in the generation of many mode-two waves and of higher mode waves. Sensitivity to the stratification is considered by using three stratifications based on summer observations. They primarily differ in the depth of the thermocline. The generation of mode-two waves and the behaviour of the waves in shallow water is sensitive to this depth. Rotation affects the shoaling waves by reducing the amplitude of the leading waves via the radiation of long trailing inertia-gravity waves. The nonlinear-dispersive evolution of these inertia-gravity waves results in the formation of secondary mode-one wave packets.

Neurobehavioral impairments caused by developmental imidacloprid exposure in zebrafish

BackgroundNeonicotinoid insecticides are becoming more widely applied as organophosphate (OP) insecticides are decreasing in use. Because of their relative specificity to insect nicotinic receptors, they are thought to have reduced risk of neurotoxicity in vertebrates. However, there is scant published literature concerning the neurobehavioral effects of developmental exposure of vertebrates to neonicotinoids. MethodsUsing zebrafish, we investigated the neurobehavioral effects of developmental exposure to imidacloprid, a prototypic neonicotinoid pesticide. Nicotine was also administered for comparison. Zebrafish were exposed via immersion in aqueous solutions containing 45μM or 60μM of imidacloprid or nicotine (or vehicle control) from 4h to 5d post fertilization. The functional effects of developmental exposure to both imidacloprid and nicotine were assessed in larvae using an activity assay and during adolescence and adulthood using a battery of neurobehavioral assays, including assessment of sensorimotor response and habituation in a tactile startle test, novel tank swimming, and shoaling behavior. ResultsIn larvae, developmental imidacloprid exposure at both doses significantly decreased swimming activity. The 5D strains of zebrafish were more sensitive to both nicotine and imidacloprid than the AB* strain. In adolescent and adult fish, developmental exposure to imidacloprid significantly decreased novel tank exploration and increased sensorimotor response to startle stimuli. While nicotine did not affect novel tank swimming, it increased sensorimotor response to startle stimuli at the low dose. No effects of either compound were found on shoaling behavior or habituation to a startling stimulus. DiscussionEarly developmental exposure to imidacloprid has both early-life and persisting effects on neurobehavioral function in zebrafish. Its developmental neurotoxicity should be further investigated.

Turbidity influences individual and group level responses to predation in guppies, Poecilia reticulata

Increasing turbidity (either sedimentary or organic) from anthropogenic sources has significant negative impacts on aquatic fauna, both directly and indirectly by disrupting behaviour. In particular, antipredator responses of individuals are reduced, which has been attributed to a reduced perception of risk. Here, we explored the effect of turbidity on shoaling behaviour, which is known to carry important antipredator benefits, predicting that fish in turbid water should show reduced shoal cohesion (increased interindividual distances) and reduced responses to a simulated predatory threat. We explored both the individual and shoal level responses to a predation threat at four different levels of turbidity. At the shoal level, we found that shoals were less cohesive in more turbid water, but that there was no effect of turbidity on shoal level response to the predation threat. At an individual level, guppies in turbid water were more likely to freeze (rather than dart then freeze), and those that darted moved more slowly and over a shorter distance than those in clear water. Fish in turbid water also took longer to recover from a predation threat than fish in clear water. We suggest that because fish in turbid water behaved in a manner more similar to that expected from lone fish than to those in a shoal, the loss of visual contact between individuals in turbid water explains the change in behaviour, rather than a reduced perception of individual risk as is widely supposed. We suggest that turbidity could lead to a reduced collective response to predators and a loss of the protective benefits of shoaling.

A Delaunay Triangle Network Based Model of Fish Shoaling Behavior for Water Quality Monitor

Fish actively form shoals, which is a behavior highly sensitive to experimental changes in environmental conditions. Here we evaluated the potential for using the shoaling behavior of red crucian carp as an early-warning biosensor system for assessing water quality. To reliably characterize shoaling behavior, we propose a novel method for determining the size of the shoal based on a Delaunay triangle network. We examined the effect of group size (two to 10 fish) in the shoaling paradigm and evaluated whether measurements of shoaling behavior could be used to assess water quality using test fish exposed to chemicals. The test chemicals were sodium hydroxide (NaOH), hydrochloric acid (HCl) and glyphosate, which are commonly used in agriculture or industry. There was a significant effect of group size on the shoaling behavior of unexposed fish. Furthermore, NaOH (20 mg/L), HCl (20 mg/L) and glyphosate at three concentrations (0.1 mg/L, 0.05 mg/L and 0.025 mg /L) significantly decreased shoaling behavior relative to controls. The average alarm time in response to a change in water quality was about 21 min. We conclude that the shoaling behavior of red crucian carp is a useful tool for monitoring water quality.

The effect of hunger on the exploratory behaviour of shoals of mosquitofish Gambusia holbrooki

The question of how hunger affects locomotory behaviour, in particular how it affects the kinematics of movement and an animal’s interaction with the physical structures in its environment is of broad relevance in behavioural ecology. We experimentally manipulated the hunger levels of individual mosquitofish (Gambusia holbrooki) and recorded their swimming behaviour in shoals of 4 fish. We found that hungry individuals in shoals moved at greater speeds and had higher turning speeds than satiated individuals in shoals, as well as a greater variance in speed and turning speeds. We also found that hungry individuals explored more of the arena and used more of its internal space, away from the square arena’s walls and displayed less wall-following behaviour than satiated individuals. A functional explanation for this change in swimming behaviour and interaction with environmental heterogeneity is discussed in the context of social foraging, as is the consequence of these results for models of search patterns and collective movement.

Nonapeptide influences on social behaviour: effects of vasotocin and isotocin on shoaling and interaction in zebrafish

Nonapeptides are important regulators of social behaviour across vertebrate taxa. While their role in simple grouping behaviour has been explored in estrildid finches, other taxa are understudied, prompting us to investigate nonapeptide influences on shoaling behaviour in zebrafish. Subjects received injections of isotocin, an isotocin antagonist, vasotocin, a vasotocin antagonist, or saline, followed by a test of grouping behaviour. Vasotocin decreased social interaction with the shoal. Unexpectedly, the vasotocin antagonist also reduced social interaction with the shoal, as well as general shoaling behaviour. Isotocin and its antagonist had minimal effects on grouping behaviours. These results suggest social interaction and shoaling are discrete aspects of sociality differentially influenced by vasotocin, although we cannot discount possible anxiogenic effects of vasotocin. Contrasting these results with studies in other systems demonstrates that each nonapeptide’s role in social behaviour varies across taxa, and cautions against a simplistic characterisation of nonapeptides as prosocial regulators of behaviour.

Individually assessed boldness predicts Perca fluviatilis behaviour in shoals, but is not associated with the capture order or angling method.

Selectivity of recreational angling on fish behaviour was studied by examining whether capture order or lure type (natural v. artificial bait) in ice-fishing could explain behavioural variation among perch Perca fluviatilis individuals. It was also tested if individually assessed personality predicts fish behaviour in groups, in the presence of natural predators. Perca fluviatilis showed individually repeatable behaviour both in individual and in group tests. Capture order, capture method, condition factor or past growth rate did not explain variation in individual behaviour. Individually determined boldness as well as fish size, however, were positively associated with first entrance to the predator zone (i.e. initial risk taking) in group behaviour tests. Individually determined boldness also explained long-term activity and total time spent in the vicinity of predators in the group. These findings suggest that individual and laboratory-based boldness tests predict boldness of P. fluviatilis in also ecologically relevant conditions, i.e. in shoals and in the presence of natural predators. The present results, however, also indicate that the above-mentioned two angling methods may not be selective for certain behavioural types in comparison to each other.

A bio-inspired hierarchical clustering algorithm with backtracking strategy

Biological entities, such as birds with their flocking behavior, ants with their social colonies, fish with their shoaling behavior and honey bees with their complex nest construction, represent a great source of inspiration in the optimization and data mining domains. Following this line of thought, we propose the Communicating Ants for Clustering with Backtracking strategy (CACB) algorithm, which is based on a dynamic and an adaptive aggregation threshold and a backtracking strategy where artificial ants are allowed to turn back in their previous aggregation decisions. The CACB algorithm is a hierarchical clustering algorithm that generates compact dendrograms since it allows the aggregation of more than two clusters at a time. Its high performance is experimentally shown through several real benchmark data sets and a content-based image retrieval system.

Towards of a firmer explanation of large shoal formation, maintenance and collective reactions in marine fish

Avoiding predation is generally seen as the most common explanation for why animals aggregate. However, it remains questionable whether the existing theory provides a complete explanation of the functions of large shoals formation in marine fishes. Here, we consider how well the mechanisms commonly proposed to explain enhanced safety of group living prey explain fish shoals reaching very large sizes. By conceptually re-examining these mechanisms for large marine shoals, we find little support from either empirical studies or classical models. We address first the importance of reassessing the functional theory with predator-dependent models and the need to consider factors other than predation to explain massive fish shoals. Second, we argue that taking into account the interplay between ultimate benefits and proximate perspectives is a key step in understanding large fish shoals in marine ecosystems. Third, we present the growing body of evidence from field studies that identify shoal internal structure as an important feature for how large shoals can form, maintain and react as a coordinated unit to external stimuli. In particular, we consider a mechanistic basis of local rules of interaction for group formation and collective dynamic properties that can account for groups reaching very large sizes. Recent research in collective animal behaviour has shifted focus from the importance of global properties (group size) to local properties (local density and information transfer). In contrast to studies of fish shoals in the laboratory, the difficulty in measuring behaviour in large shoals in marine systems remains a major constraint to further work. Advances in acoustical observation have shown the greatest potential to provide data that can link proximate mechanisms in, and ultimate functions of, large marine fish shoals.

Combined effects of flow condition and parasitism on shoaling behaviour of female guppies Poecilia reticulata.

Group living in fish can provide benefits of protection from predators and some parasites, more efficient foraging for food, increased mating opportunities and enhanced energetic benefit when swimming. For riverine species, shoaling behaviour can be influenced by various environmental stressors, yet little is known how flow rate might influence the shoaling of diseased fish shoals. In view of the increasingly unpredictable flow rates in streams and rivers, this study aimed to assess the combined effect of flow condition and parasitism on the shoaling behaviour of a model fish species. Shoal size, shoal cohesion and time spent shoaling of female guppies Poecilia reticulata were compared when infected with the directly transmitted ectoparasite Gyrodactylus turnbulli under flow and static conditions. Flow condition was an important factor in influencing shoaling behaviour of guppies with the fish forming larger shoals in the absence of flow. When a shoal member was infected with G. turnbulli, shoal cohesion was reduced, but the magnitude of this effect was dependent on flow condition. In both flow and static conditions, bigger fish formed larger shoals than smaller counterparts. Future changes to stream hydrology with more frequent flooding and drought events will affect the shoaling tendency of fish. During high-flow events, diseased fish may not be able to keep up with shoal mates and therefore have a higher risk of predation. Additionally, these findings may be important for aquaria and farmed species where an increase in flow rate may reduce aggregation in fish.

A new method for measuring group behaviours of fish shoals from recorded videos taken in near aquaculture conditions

Monitoring fish shoal behaviour is a growing concern for scientists studying fish stress and welfare. This study presents an algorithm developed to calculate, from videos taken from above aquaria, two indexes characterizing fish shoal behaviour. These two indexes quantify the dispersion and the swimming activity of the fish shoal in the aquaria. The reliability of these indexes was tested on fish shoal simulations following the rules of Reynolds' model on flocks, herds and schools. Since coordinates of each simulated fish in the shoal was known, these simulations provided true values of dispersion and swimming speed of each fish in the shoal, which were compared to values calculated using the presented algorithm. Further, the two indexes were tested on videos of rainbow trout in aquaria. Behavioural variations of the shoal were estimated before and after food distribution in one test, and before and after a four hours confinement stress in a second test. Data resulting from simulations indicate that the two indexes are sensitive to the simulated changes in the cohesion or the swimming speed of the group. Thus, indexes faithfully translated true values in simulations, with a minimum of 94% of the total variation in true values explained by indexes. Furthermore, the two indexes were sensitive to shoal behaviour modifications observed in the two case studies. Indeed, as expected, a strong group dispersion decrease associated with an important swimming activity increase could be detected just after food distribution using our method. Similarly, our indexes were sensitive to a group behaviour change observed after the four hours confinement stress. Finally, our method was compared to Israeli's, and was found to be more sensitive and more accurate in our conditions. This method provides therefore a sensitive, non-invasive, simple and widely applicable tool to quantify behavioural changes associated with various challenges in aquacultural conditions. • Two indexes quantifying dispersion and activity of a fish group are presented. • The reliability of these indexes was tested on fish shoal simulations. • The indexes were also tested in real conditions using videos of trout in aquaria. • We found them to faithfully translate simulated and real behaviour modifications.

Alternative responses to predation in two headwater stream minnows is reflected in their contrasting diel activity patterns.

Animals exhibit diel periodicity in their activity in part to meet energy requirements whilst evading predation. A competing hypothesis suggests that partitioning of diel activities is less important because animals capitalise on opportunity. To test these hypotheses we examined the diel activity patterns for two cyprinid minnows, chubbyhead barb Barbus anoplus and the Eastern Cape redfin minnow Pseudobarbus afer that both occur within headwater streams in the Eastern Cape, South Africa. Chubbyhead barbs exhibited consistent nocturnal activity based on both field and laboratory observations. Due to the absence of fish predators within its habitat, its nocturnal behaviour suggests a response to the cost associated with diurnal activity, such as predation risk by diving and wading birds. In contrast, redfin minnows showed high diurnal activity and a shoaling behaviour in the wild, whereas, in the laboratory, they showed high refuge use during the diel cycle. Despite their preference for refuge in the laboratory, they were diurnally active, a behaviour that was consistent with observations in the wild. The diurnal activity of this species suggests a response to the cost associated with nocturnal activity. Such a cost could be inferred from the presence of the longfin eel, a native predator that was active at night, whereas the daytime shoaling behaviour suggests an anti-predator mechanism to diurnal visual predators. The implications of these findings relate to the impacts associated with the potential invasions by non-native piscivores that occur in the mainstem sections. Diurnal activity patterns for redfin minnows, that are IUCN-listed as endangered, may, in part, explain their susceptibility to high predation by visual non-native piscivores, such as bass and trout. In contrast, the nocturnal habits of chubbyhead barbs suggest a probable pre-adaptation to visual predation. The likelihood of invasion by nocturnally-active sharptooth catfish Clarias gariepinus, however, may compromise this prior advantage.

Strong personalities, not social niches, drive individual differences in social behaviours in sticklebacks

Understanding the mechanisms responsible for consistent individual differences in behaviour is a recent challenge for behavioural ecology. Although theory is rapidly developing in this area, there are few empirical tests. There are at least two hypotheses to explain why individuals behave differently from one another in a dynamic social environment. The social niche specialization hypothesis proposes that repeated social interactions generate consistent individual differences in social behaviour. The behavioural type hypothesis proposes that an individual's social behaviour reflects its behavioural type. We tested these two hypotheses by manipulating the opportunity for repeated social interactions in groups of three spine stickleback, Gasterosteus aculeatus, and by measuring the behavioural types of the same individuals in three contexts: when in a novel environment, when presented with an opportunity to associate with conspecifics and when confronted by an intruder. We found no evidence that repeated social interactions increased between-individual variation in social foraging behaviour. Instead, individuals' social foraging behaviour was related to their behavioural type, specifically their shoaling behaviour. In addition, the behavioural types of the members of a group strongly influenced a group's average foraging behaviour. Together, these results do not support the hypothesis that social dynamics within groups generates individual differences in behaviour. Instead, they suggest the reverse: individual differences in behaviour drive group-level dynamics.

Reprint of “Effects of the SSRI citalopram on behaviours connected to stress and reproduction in Endler guppy, Poecilia wingei”

Psychoactive drugs, such as selective serotonin reuptake inhibitors (SSRI) have been identified in high levels in effluents from Swedish sewage treatment plants (STP) at concentrations high enough to give pharmacological effects in fish. In humans SSRIs are used in the treatment of depression and they have anxiolytic effects. In the present study we exposed Endler guppy (Poecilia wingei) of both sexes to citalopram that showed the highest concentrations of SSRIs in STP effluents and studied reproductive and non-reproductive behaviour. Male courting behaviours were not affected compared to control fish after 14–28 days exposure to 1μgL−1. In two experiments exposing both sexes to 0.2, 2.3 or 15μgL−1 for 21 days, fish exposed to the two highest doses showed anxiolytic effects when placed in a novel environment (novel tank diving test, NT). Males were only affected by exposure to 15μgL−1. They had significantly longer latency to explore the upper half of the aquarium, more visits and longer time spent in the upper half, and showed less bottom freezing behaviour, all markers of anxiolytic behaviour. In females exposure to 2.3 or 15μgL−1 significantly increased freezing behaviour, while no effects on other behaviour variables were observed. No effects on shoaling behaviour could be discerned. These results show that citalopram have anxiolytic effects on guppy fish and thus affect ecologically relevant behaviours of importance to survival of fish.

Effects of the SSRI citalopram on behaviours connected to stress and reproduction in Endler guppy, Poecilia wingei

Psychoactive drugs, such as selective serotonin reuptake inhibitors (SSRI) have been identified in high levels in effluents from Swedish sewage treatment plants (STP) at concentrations high enough to give pharmacological effects in fish. In humans SSRIs are used in the treatment of depression and they have anxiolytic effects. In the present study we exposed Endler guppy (Poecilia wingei) of both sexes to citalopram that showed the highest concentrations of SSRIs in STP effluents and studied reproductive and non-reproductive behaviour. Male courting behaviours were not affected compared to control fish after 14–28 days exposure to 1μgL−1. In two experiments exposing both sexes to 0.2, 2.3 or 15μgL−1 for 21 days, fish exposed to the two highest doses showed anxiolytic effects when placed in a novel environment (novel tank diving test, NT). Males were only affected by exposure to 15μgL−1. They had significantly longer latency to explore the upper half of the aquarium, more visits and longer time spent in the upper half, and showed less bottom freezing behaviour, all markers of anxiolytic behaviour. In females exposure to 2.3 or 15μgL−1 significantly increased freezing behaviour, while no effects on other behaviour variables were observed. No effects on shoaling behaviour could be discerned. These results show that citalopram have anxiolytic effects on guppy fish and thus affect ecologically relevant behaviours of importance to survival of fish.

The influence of population mixing on newborn shoaling behaviour in the guppy (Poecilia reticulata)

The introduction of non-native populations into new habitats where they can mix with the original inhabitants poses a major thread to the genetic distinctiveness of the native population. Interbreeding between introduced and native individuals can disrupt local adaptations and lead to outbreeding depression in resulting hybrid offspring. In the guppy (Poecilia reticulata), shoaling behaviour plays an important part in avoiding predators and is displayed from birth, but differences in shoaling tendency exist between populations. The populations used for our experiment reflect the ones involved in Haskins’ introduction more than 60 years ago, when Guanapo guppies where introduced into the Turure, thereby bringing genetically very distinct populations into secondary contact. This introduction led to an invasive event and subsequent disappearance of the original Turure genotype. Here we show that the mixing of genepools by interbreeding between populations affects adaptive behaviour such as shoaling and creates more variation in the resulting offspring. Furthermore, parental origin has a subtle effect on offspring shoaling behaviour. We found that the shoaling behaviour of hybrid offspring significantly differed from pure-bred offspring of either parental population, but resembled those found in Turure newborns that are likely to be of mixed ancestry as well.

Daily shoaling patterns in the zebrafish Danio rerio

Abstract Shoaling intensity in zebrafish Danio rerio is believed to vary throughout subjective day and night hours. This experiment examines long term variations in shoaling behavior. Adult zebrafish Danio rerio were maintained under a 12:12 LD cycle (with dim red light serving as reduced visibility during subjective dark hours), and their shoaling behavior was monitored every hour for a three-day period of time. Our results show that zebrafish perform shoaling behavior throughout subjective day and under reduced visibility conditions, although mean shoaling times during the light phase were significantly higher than mean shoaling times during the dark phase. However, on the 3rd day of the experiment, mean shoaling times during the subjective night had increased and mean shoaling times during the subjective day had decreased. This shift in intensity was not seen on the first two days of the study, and may represent the influence of experience on the behavior of the test fish. We believe this study shows that shoaling behavior changes with light/dark cycles and that fish shoal even during reduced visibility conditions.

Biological motion stimuli are attractive to medaka fish.

In many social fish species, visual cues play an important role in inducing shoaling behaviour. The present study is the first to examine whether and how “biological motion” depicting a moving creature by means of only a small number of isolated points induces shoaling behaviour in fish. Medaka (Oryzias latipes) were used because they are known to have high visual acuity and exhibit a strong tendency to form shoals. In experiment 1, we found that the presentation of medaka biological motion resulted in heightened shoaling behaviour when compared with that of non-biological motion (depicted by a small number of points placed at fixed distances that moved at a constant speed in a constant direction). In experiment 2, it was indicated that medaka biological motion was more effective at inducing shoaling behaviour when compared with human biological motion. In experiment 3, it was demonstrated that shoaling behaviour was largely dependent on the smoothness of the biological motion. In experiment 4, we revealed that shoaling behaviour was maximised in normal speed group and decreased in faster- and slower-than-normal speed groups. In experiment 5, it was shown that shoaling behaviour was slightly reduced when a reversed movie was presented. These results suggest that motion information extracted from conspecifics was sufficient to induce shoaling behaviour in medaka and that deviation from normal and familiar motion impeded shoaling behaviour. The naturalness of motion may be responsible for the induction of shoaling behaviour.Electronic supplementary materialThe online version of this article (doi:10.1007/s10071-013-0687-y) contains supplementary material, which is available to authorized users.

Acute Copper Toxicity and Acclimation to Copper Using the Behavioral Endpoint of Shoaling, in the Least Killifish (Heterandria formosa)

While copper is known to be neurotoxic, effects on behavior and especially on group behavior have received much less attention. Yet such behavioral effects can have important ecological consequences. This study determined whether shoaling behavior could be used as an indicator of acute copper toxicity and as an endpoint in studying acclimation in the least killifish (Heterandria formosa). For acute toxicity, least killifish were exposed to 0, 25, 50, or 100 μg/L of Cu for a 2-h duration and then tested for changes in shoaling behavior. The occurrence of copper acclimation was tested by pre-exposing fish to either 0 or 15 μg/L of Cu for a 7-day duration, then exposing them to 25, 50, or 100 μg/L of Cu to assess whether the two pre-exposure groups differed in their behavioral responses to high Cu levels. Behavioral responses were tested in a subdivided aquarium, with one side containing a group of conspecific fish and the other side the focal fish whose behavior was recorded for 3 min. Shoaling was based on the proximity of the focal fish to the divider separating it from the group of conspecific fish. Acute copper exposure resulted in significant decreases in both the time to first shoaling and in the total amount of time spent shoaling. Experiments assessing the occurrence of copper acclimation using shoaling as the toxicity endpoint did not detect a difference between the pre-exposure groups. Overall, this study demonstrated that acute copper exposure can alter shoaling behavior. However, least killifish shoaling behavior appears to be relatively insensitive as an endpoint to assess copper acclimation.