The influence of nutritional state on shoal choice in zebrafish, Danio rerio

We investigated the mechanisms and functions of shoal choice in relation to nutritional state in the zebrafish. Single fish that had been well fed or food deprived for 2 days were presented with a choice between two stimulus shoals. Food-deprived test fish showed a significant preference for well-fed stimulus fish over food-deprived ones whereas well-fed test fish did not exhibit any significant preference. Subsequent experiments showed that food-deprived test fish had a significantly higher foraging success in shoals consisting of well-fed individuals than in ones that comprised food-deprived fish. No difference in the locomotory behaviour of food-deprived and well-fed stimulus fish was found with respect to the proportion of time spent swimming (as opposed to being motionless), the proportion of time spent in the upper part of the test tank and the number of sharp turns. However, body weight, stomach width (measured directly behind the pectoral girdle) and ventro-dorsal height significantly decreased over a 48-h food-deprivation period. The potential use of the latter factors for the recognition of food-deprived individuals is discussed. Copyright 1999 The Association for the Study of Animal Behaviour.     

The influence of hunger on shoal size choice by three-spined sticklebacks, Gasterosteus aculeatus

Single sticklebacks ( Gasterosteus aculeatus ) were presented with a choice between (a) a shoal and an empty compartment, and (b) two different sized shoals of conspecifics. The choice of hungry test fish (food-deprived for 24 h and 72 h, respectively) was compared to that of fish which were fed to satiation once a day. It was hypothesized that due to the increased level of competition for food in large shoals, hungry fish would spend less time with the larger of two shoals than their well-fed conspecifics. The results showed that there was indeed a significant trend for test fish to spend a decreasing amount of time near the compartment with the larger number of fish with increasing duration of food deprivation.     

The Effect of Hunger on Shoal Choice in Golden Shiners (Pisces: Cyprinidae,Notemigonus crysoleucas)

When an animal has a choice of joining one group over another, its decision may depend on its relative vulnerabilities to predation and starvation. For example, a well-fed animal may choose a large group of individuals with body size matching its own because this gives good protection against predators, but a hungry animal may prefer smaller groups made up of smaller individuals because this decreases food competition. To test this idea, a choice between various shoals was given to golden shiners, Notemigonus crysoleucas, that were either well fed or deprived of food for 48 h. In a choice of 10 vs. 3 shoalmates, both well-fed and hungry shiners spent more time near the shoal of 10. In a choice of 20 vs. 3 shoalmates, both well-fed and hungry shiners again preferred the larger shoal, but in one replicate this preference was significantly weaker in the hungry fish. This reduced preference did not appear to be an artefact of increased mobility by hungry fish searching for food. In a choice between shoals of small vs. large conspecifics, small well-fed shiners, small hungry shiners, and large well-fed shiners preferred shoalmates with body size matching their own, but large hungry shiners preferred smaller individuals. These results are consistent with the hypothesis that hungry fish sacrifice safety from predation in their shoaling behaviour (by avoiding larger groups to a certain extent and by risking the oddity effect) so as to decrease food competition.     

Shoaling Decisions in Angelfish: The Roles of Social Status and Familiarity

Abstract Past research has shown that angelfish, Pterophyllum scalare , are capable of discriminating between shoals composed of familiar dominant and subordinate companions, whereas they show no preference for shoals of unfamiliar conspecifics. In this study, the relative importance of familiarity and social status (shoal factors) on the shoaling decision of juvenile angelfish, which also differed in social status (individual factor), was investigated as very little is known about such tradeoffs in fishes. Dominant and subordinate individuals were given the choice to shoal with a group of conspecifics composed of familiar dominants vs. unfamiliar dominants and composed of familiar subordinates vs. unfamiliar subordinates. The findings demonstrate that fish with different social status differed in their shoaling preference. Subordinate test fish showed a preferential association with familiar subordinates over unfamiliar subordinates, but preferred the unfamiliar shoal over the familiar one when both shoals were constituted by dominant individuals. The shoaling behaviour shown by dominant test fish, on the other hand, indicated no significant preference for any of the shoals regardless of their composition. A replicate preference test carried out 2 h 30 min after the first one indicated that the association pattern was relatively consistent. Results suggest that angelfish are able to differentiate between the stimulus shoals and demonstrate that the pervasive influence of familiarity on the shoaling decision may be restrained or overridden by the composition of the familiar shoals and the social status of the test fish.     

Juvenile convict cichlids shoaling decisions in relation to shoal size and age

Many animals form groups and socialize in response to evolutionary pressures such as predation, food availability, and mate acquisition. Evidence of social choice based on various phenotypic characters (Group Phenotypic Composition [GPC]) has been observed in several animal species. In addition to the physical characteristics of the social group, it is also interesting to consider how decisions of who to socialize with might be expected to change for an individual over time. Younger individuals with limited life experience may discriminate differently between social groups than older conspecifics who have had the opportunity to learn and who may be faced with different ecological or environmental pressures. Here, we used a traditional two‐choice design to explore the shoaling behavior of juvenile convict cichlids and determine whether the number of fish and/or the size/life stage of the individuals within a shoal influenced social choices. We found that juvenile convict cichlids spent more time shoaling with similarly sized juvenile individuals and also preferred to shoal with larger shoals, but not when shoals were comprised of adult fish. The size of the individuals in a shoal was a more influential factor than the size of the shoal itself. Size of individual juveniles was correlated with tendency to visit shoals, but was not correlated with overall time spent shoaling, regardless of shoal composition. As juveniles, convict cichlids can make discriminatory choices that are influenced by specific aspects of shoal composition.     

Recent social environment affects colour-assortative shoaling in juvenile angelfish (Pterophyllum scalare)

Theory predicts that fish should show colour-assortative shoaling in order to avoid the oddity effect whereby individuals that differ in some feature from the group majority appear to incur increased risk of predation. It has also been shown that early experience plays an important role in affecting social preferences in some fish species. In this study, the importance of colour phenotype in promoting assortative shoaling and the role played by the recent social environment on its expression were investigated in juvenile angelfish, Pterophyllum scalare. Individuals of the uniformly black and golden colour morphs were housed in a group with conspecifics of like and unlike body colour to themselves, as well as in mixed-colour groups for 4 weeks. Subsequently, they were subjected to a binary choice to shoal with a group of conspecifics composed of unfamiliar fish of either a like or unlike colour phenotype to themselves. The response of the individuals to the colour attributes of the shoals was related to their recent social environment. Fish in like- and mixed-colour previous treatments showed a preferential association with like colour conspecifics. In contrast, the shoaling behaviour exhibited by fish previously maintained with a group of unlike-coloured conspecifics (cross-housed treatment) indicated no significant preference for any of the shoals. The results suggest that angelfish use body colouration as an intraspecific shoaling cue and that learning, in the form of recent familiarisation with a specific colour phenotype of conspecifics, can affect colour-assortative shoaling preferences in this species. This learning component of the choice need not be restricted to early developmental stages.     

Modelling density-dependent fish shoal distributions in the laboratory and field

Density-dependent variables have long been established as an important area of ecological research, but the effects of the local density of conspecifics on grouping behaviour are less well-studied. We compared the influence of the density of conspecifics on the shoal size distribution of killifish, Fundulus diaphanus , in the laboratory and the field. In both environments we observed an increase in shoal size and shoal number with the density of individuals present. The increase in shoal size was markedly steeper in the field than in the laboratory, but direct comparison of the two was complicated by the fact that the absolute numbers of fish present at the field site were considerably higher than those used in the laboratory trials. We developed an individual-based model that was first used as a null model of shoal formation (defined by proximity to others) in fish with no shoaling tendency over the same range of densities used in the laboratory. Group size increased much more rapidly with increasing density in the laboratory than predicted by the null model. When we incorporated shoaling behaviour into our model, the laboratory results could be reproduced with high accuracy. However, when extrapolated to match conditions in the field, the model predicted smaller, more numerous shoals than were actually observed. We suggest this is due to heterogeneity of the field environment because fish were found to be highly aggregated in certain areas of our field site. The predictive power of laboratory studies for the field is discussed with regards to using individual-based modelling as a tool for deriving such predictions.     

The effect of sex and shoal size on shoaling behaviour in Danio rerio

Male and female zebra fish Danio rerio were given choices of shoals that differed in sex and size. Male zebra fish preferred to associate with female shoals over male shoals, but had no preference when given a choice between a mixed‐sex shoal and either a male or female shoal. Female zebra fish showed no significant preference when given a choice between male and females shoals, nor between mixed‐sex shoals and either male or female shoals. When given choices between shoals of differing size, females preferred to associate with the larger shoal, whether or not they were composed of males or females. Males, however, had no preference for larger shoals over smaller shoals, whether or not they were composed of males or females. These results showed that male zebra fish were capable of distinguishing between males and females solely on the basis of visual cues. Furthermore, these results demonstrated a significant difference between the shoaling choices of male and female zebra fish, which may indicate a difference in the function of shoaling for the two sexes.     

Shoaling in juvenile guppies: the effects of body size and shoal size

While factors affecting shoal mate choice have been examined extensively in adult guppies (Poecilia reticulata), few studies have focused on the shoaling behavior of juveniles. In this study, juvenile guppies were tested for their ability to shoal as well as their response to shoal mates of different body size and to shoals with different numbers of individuals. In dichotomous choice tests, 10-day-old guppies (mean body length=8.83 mm), 30-day-old guppies (13.17 mm) and 50-day-old guppies (18.6mm) were given the opportunity to swim near shoals of five fish or an empty chamber. In most cases, the juvenile fish demonstrated shoaling behavior, swimming near a group of fish rather than an empty chamber, regardless of the age of the stimulus shoal. When presented with two shoals, one of similar age and body size and one of dissimilar age and body size, only the 50-day-old guppies showed a significant preference for the age-matched shoal. Similarly, when choosing between a large shoal and a small shoal, only the 50-day-old guppies spent significantly more time near the larger shoal. Thus, while juveniles at each age shoaled, only 50-day-old fish demonstrated the shoal mate discrimination seen in adult fish.     

Shoaling behaviour of sticklebacks infected with the microsporidian parasite, <Emphasis Type="Italic">Glugea anomala</Emphasis>

We compared the shoaling behaviour of three-spined sticklebacks, Gasterosteus aculeatus, infected with the microsporidian, Glugea anomala, to that of non-infected conspecifics. Infected fish lost significantly more weight than non-infected fish during a period of food deprivation, suggesting a metabolic cost to parasitism. In binary shoal choice tests, non-infected test fish showed an association preference for a shoal of non-infected over a shoal of infected conspecifics; infected test fish displayed no preference. Infected fish, however, showed a higher overall tendency to shoal than non-parasitised fish. Furthermore, infected fish occupied front positions within a mixed school. We consider the behavioural differences between infected and uninfected fish in the context of their potential benefits to the fish hosts and the parasites.     

The effect of hunger and cestode parasitism on the shoaling decisions of small freshwater fish

Individual minnows Phoxinus phoxinus and three-spined sticklebacks Gasterosteus aculeatus were provided with a mutually exclusive choice between joining a shoal of conspecifics and foraging alone in a maze. The shoaling decisions and foraging behaviour of individual fish were studied when the fish were satiated and after 24- and 48-h periods of food deprivation. Hunger level was found to have a significant effect on shoaling behaviour. When satiated, fish of both species spent a greater proportion of time within one body length of the shoal and spent less time out of visual contact with the shoal than after periods of food deprivation. The effect of the cestode parasite Schistocephalus solidus on the shoaling behaviour of stickleback hosts was complex. When satiated, infected fish spent less time than uninfected fish within one body length of the shoal, preferring to remain outside the shoal, yet within visual contact, although when food deprived there was no difference in the proportion of time spent by infected and uninfected fish close to the shoal. The possible ecological significance of this change in behaviour is discussed with reference to the manipulation hypothesis of host-parasite interactions.     

Shoal Choice in the Banded Killifish (Fundulus diaphanus,Teleostei, Cyprinodontidae): Effects of Predation Risk,Fish Size,Species Composition and Size of Shoals

Banded killifish (Fundulus diaphanus) were presented individually with a choice of shoaling with either of two stimulus shoals which differed in shoal size, species composition, and fish body size, before and after a simulated avian predator attack. When threatened, test fish preferred to shoal with the larger of two conspecific shoals, but only if members of both stimulus shoals were of the same size class as the test fish. Otherwise, they preferred to shoal with similarly sized fish irrespective of shoal size; threat of predation increased the magnitude of this preference. Furthermore, test fish preferred a shoal of similarly sized shiners (Notemigonus crysoleucas) over larger killifish, when shoal sizes were identical. This indicates that body size plays a key role in shoal choice, overriding the effects of shoal size and species preference. Notwithstanding the above, shoal choice was affected by predator threat only when differences between shoal size or body size of stimulus fish were large.     

The influence of alarm substance and shoal size on the behaviour of zebra danios, Brachydanio rerio (Cyprinidae)

Timidity, vigilance and response to alarm substance were examined in shoals consisting of one to 12 zebra danios, Brachydanio rerio . Measures of timidity were not clearly related to shoal size. Fish in larger shoals spent more time in central areas of the tank away from cover than fish in smaller shoals. However, fish in small shoals appeared to be as aggressive as those in larger shoals. Vigilance and foraging rates did not appear to be related to shoal size. Per capita foraging rates and shoal size were not correlated. After being frightened by alarm substance, danios in larger shoals did not return to foraging sooner than those in smaller groups. Zebra danios in all shoal sizes responded behaviourally to alarm substance. It appears that the presence of conspecifics is unnecessary for alarm behaviours to occur and that the nature of the behaviours are independent of shoal size.     

The Influence of Food Competition and Prédation Risk on Size-assortative Shoaling in Juvenile Chub (Leuciscus cephalus)

Size-assortative shoaling is a widespread phenomenon potentially linked to competition for resources among shoal members and anti-predator benefits gained from living in shoals. This paper investigates the influence of both factors on size-assortative shoaling in chub (Leuciscus cephalus). In a first experiment food competition was studied by manipulating the proportions of small and large chub in shoals of 20 fish. Large fish were generally more competitive than small ones. However, the competitive effect of large fish was strongly dependent on their number in the shoal. This has important consequences for the theory of competition because it makes it difficult to predict stable combinations of different phenotypes when foragers divide between food patches. Despite the strong effects of food competition no significant trend for size-assortativeness was observed in free-swimming shoals when undisturbed. This changed after the shoal had been frightened by introducing Schreckstoff (an alarm substance) to the test tank. Large and small fish still remained in the same shoal but showed a strong preference to be close to neighbours of their own size class. Large fish were observed mainly in the centre of the shoal and small fish were found more in the periphery. It is unclear whether these position differences are due to aggressive behaviour of large fish towards small fish which excludes them from the potentially safe centre of the shoal. These results indicate that prédation plays a more important role than food competition for size-assortative shoaling in chub.     

The genetic structure and microdistribution of shoals of Phoxinus phoxinus, the European minnow

Efficient foraging and a reduction in predation risk have been proposed as reasons for shoal formation. Some behaviours in cyprinid shoals are at first sight altruistic (e.g. predator inspection behaviour, reactions to alarm substance), such that kin selection may have been involved in their evolution. If shoaling behaviour does evolve through kin selection, then genetic differentiation is expected to be greater between shoals than within shoals. Such a hypothesis was tested here by examining shoal integrity and the relatedness of individuals within and between shoals in the European minnow, Phoxinus phoxinus , using nuclear and mitochondrial DNA markers. The breeding structure of 13 minnow shoals collected from Dorset and North Wales, U.K., was examined using allozymes. Genetic affinity within and between shoals was tested using mitochondrial DNA and multi-locus DNA fingerprinting. Shoals consisted of a random assortment of allozyme genotypes, shoal members did not share the same maternal mtDNA lineages and DNA fingerprint profiles were as varied within shoals as between them. The data indicate that it is unlikely that kin selection occurs in P. phoxinus and there is no apparent relationship between shoaling behaviour and genotype distribution in this species.     

Cross-species familiarity in shoaling fishes

Preferential association with familiar shoal mates confers a number of potentially important benefits to individuals, including improved anti-predator effects and the reduction of aggression in competitive interactions. Until now, however, familiarity has been demonstrated purely between conspecifics. Here, we present evidence that familiarity preferences can override natural preferences for conspecifics. Individual focal fishes (chub, Leuciscus cephalus) were given a choice of two stimulus shoals of the same size composed of conspecifics or of heterospecifics (minnows, Phoxinus phoxinus) in a flow tank. A series of four treatments was carried out to investigate the effects of familiarity, induced by a 15 day association between the focal fish and the stimulus fishes, on the choices made by the focal fish. Focal fishes showed a significant preference for conspecifics over heterospecifics when both stimulus shoals were composed of non-familiar individuals. Focal fishes also showed a significant preference for stimulus shoals composed of familiar fishes over stimulus shoals composed of non-familiar fishes when both shoals were conspecific and when both shoals were heterospecific. Finally, the preference of focal fishes for conspecifics disappeared when the alternative, a shoal of heterospecifics, was composed of familiar individuals. The importance of this work is discussed in the context of species interactions in free-ranging shoals.     

Size-segregative behaviour in minnow shoals

Shoals composed of equal numbers of two size-classes of European minnows were observed undisturbed, feeding and after threat from a pike in a large arena tank.
The time/frequency budget and analysed sequences of behaviour of the two size-classes were very similar. Irrespective of size, for standard behaviour measures, fish in the shoal behaved similarly under the same external influences, including predator threat.
In contrast, however, the distribution of the two size-classes provided evidence of size segregation within the shoal. This was brought about by individual minnows making shoaling responses preferentially to their own size-class. After exposure to the predator, shoaling responses changed and differed between small and large minnows.
The outcomes of contests at foraging patches were governed primarily by fish size and information asymmetry rather than by occupation of a feeding site.
The experiment shows that asymmetrical pay-offs in foraging and in response to predator threat are the probable reasons for size-segregation behaviours. This conclusion supports the views of earlier workers that mechanical sorting by swimming speed is not an important factor in size segregation in shoals.     

Influence of black spot disease on shoaling behaviour in female western mosquitofish, <Emphasis Type="Italic">Gambusia affinis</Emphasis> (Poeciliidae,Teleostei)

Parasites can fundamentally alter the cost–benefit ratio of living in a group, e.g. if infected individuals increase the predation risk of shoal mates. Here, the effect of an infection with a trematode, Uvulifer sp. (Diplostomatidae) on the shoaling behaviour of female western mosquitofish, Gambusia affinis, was investigated. The parasite examined causes a direct phenotypical change of the host by forming black spots on its body surface. When given a choice between a stimulus shoal and no shoal, we found shoaling tendencies to be significantly reduced in infected focal fish. In another experiment, we tested for association preferences relative to the infection status of the stimulus fish. Given the choice between an infected and a healthy stimulus fish, both infected and healthy focal fish preferred to associate with non-infected stimulus fish. Our results suggest that (1) the cost–benefit ratio of shoaling might be different for infected and non-infected individuals. Infected fish may be more affected by competition for food within a shoal. (2) Associating with infected conspecifics appears to be costly for female mosquitofish, maybe due to increased predation risk.     

Balancing the dilution and oddity effects: decisions depend on body size

Background

Grouping behaviour, common across the animal kingdom, is known to reduce an individual''s risk of predation; particularly through dilution of individual risk and predator confusion (predator inability to single out an individual for attack). Theory predicts greater risk of predation to individuals more conspicuous to predators by difference in appearance from the group (the ‘oddity’ effect). Thus, animals should choose group mates close in appearance to themselves (eg. similar size), whilst also choosing a large group.

Methodology and Principal Findings

We used the Trinidadian guppy (Poecilia reticulata), a well known model species of group-living freshwater fish, in a series of binary choice trials investigating the outcome of conflict between preferences for large and phenotypically matched groups along a predation risk gradient. We found body-size dependent differences in the resultant social decisions. Large fish preferred shoaling with size-matched individuals, while small fish demonstrated no preference. There was a trend towards reduced preferences for the matched shoal under increased predation risk. Small fish were more active than large fish, moving between shoals more frequently. Activity levels increased as predation risk decreased. We found no effect of unmatched shoal size on preferences or activity.

Conclusions and Significance

Our results suggest that predation risk and individual body size act together to influence shoaling decisions. Oddity was more important for large than small fish, reducing in importance at higher predation risks. Dilution was potentially of limited importance at these shoal sizes. Activity levels may relate to how much sampling of each shoal was needed by the test fish during decision making. Predation pressure may select for better decision makers to survive to larger size, or that older, larger fish have learned to make shoaling decisions more efficiently, and this, combined with their size relative to shoal-mates, and attractiveness as prey items influences shoaling decisions.