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.     

Body length variation within multi-species fish shoals: the effects of shoal size and number of species

Theory predicts that selection should favour phenotypic homogeneity in fish shoals, and field studies have indeed confirmed that variation in body length within fish shoals is significantly lower than expected from a random distribution of fish among shoals. We investigated the extent to which variation in fish body length within shoals is determined by the shoal mean of body length, the number of species in a shoal, and the overall shoal size. We collected 34 fish shoals, ranging in size from 6 to 776 individuals, from the littoral zone of a Canadian lake. Shoals consisted of up to four different species, with multi-species shoals being larger and more frequent than single-species ones. The strongest determinant of body length variation within shoals was the shoal mean of body length, followed by the number of fish species in a shoal; i.e. multi-species shoals were less size-assorted than single-species ones. A more detailed analysis showed that the higher body length variation observed in multi-species shoals was due to increased body length variation both within and between component species. Shoal size had no significant effect on body length variation within shoals. Potential explanations of the positive relationship between body length variation and the number of species in a shoal are suggested. The implications of the above results for the evolution of multi-species shoals are discussed. Received: 6 May 1997 / Accepted: 14 October 1997     

Kin assortment in juvenile shoals in wild guppy populations

Grouping provides many potential benefits to individuals in terms of foraging and anti-predator protection. However, it has been suggested that individuals could gain additional benefits in terms of indirect fitness by grouping with kin. Surprisingly, the genetic composition of wild fish shoals and the importance of kin-associated shoaling remain poorly understood. The Trinidadian guppy (Poecilia reticulata) has life history traits that might promote kin structure of shoals such as internal fertilisation and small brood size in contrast to many other fish species. Even though previous studies did not find any indication of kin structure in shoals of adult guppies, it is possible that related juveniles remain together in shoals, partly because of lower mobility and because the advantages of kin association may change with age. Using 10 microsatellite markers, we conducted a genetic analysis on 40 shoals from four populations. Pair-wise relatedness was inferred using a modified version of the software package COLONY and permutation tests were conducted to test the hypothesis that kin occur together in juvenile shoals more often than expected by chance. The frequency of sib dyads among juveniles within shoals was significantly larger than that between shoals in two high predation populations but not in two low predation populations. This finding contributes to the understanding of factors underlying shoal composition and highlights the potential of recent methodological advances for detecting such relationships.     

The Role of Body Surface Area in Quantity Discrimination in Angelfish (Pterophyllum scalare)

Although some fish species have been shown to be able to discriminate between two groups (shoals) of conspecifics differing in the number of members, most studies have not controlled for continuous variables that covary with number. Previously, using angelfish (Pterophyllum scalare) we started the systematic analysis of the potential influence of such continuous variables, and found that they play different roles in shoal discrimination depending on whether large (≥4 fish) or small (<4 fish) shoals were contrasted. Here, we examine the potential role of the overall body surface area of stimulus fish in shoal preference, a prominent variable not yet examined in angelfish. We report that both when numerically large (5 versus 10 fish) and when small (2 versus 3 fish) shoals were contrasted, angelfish were unable to discriminate the numerically different shoals as long as the surface area of the contrasted shoals was equated. Thus, we conclude that body surface may be an important continuous variable in shoal discrimination. This conclusion was further supported by the analysis of preference when shoals of the same numerical size but different body surface area were contrasted. We found subjects to spend significantly more time close to the shoals with the greater overall surface area. Last, we conducted an experiment in which we simultaneously controlled a set of continuous variables, including overall surface area, and found angelfish to use the number of shoal members as a cue only in large shoal contrasts but not in small shoal contrasts. This result suggests the potential existence of different processing systems for large and small numbers in fish.     

Fish shoal composition: mechanisms and constraints

Observations were made on three fish species (banded killifish (Fundulus diaphanus), golden shiner (Notemigonus crysoleucas) and white sucker (Catostomus commersoni)) in a temperate lake (New Brunswick, Canada) in order to investigate the relationship between shoal choice behaviour of individual fishes and shoal composition. Encounters between shoals were observed to take place every 1.1 min per shoal and an encounter lasted 3.7 s on average. The duration of shoal encounters was influenced by shoal size but not by differences between shoals in either body length or species. Conversely, the outcome of shoal encounters (i.e. ences between shoals in either body length or species. Conversely, the outcome of shoal encounters (i.e. whether or not an individual changes shoal) was influenced by body length and species differences but not by shoal size. Together, these results suggest that encounter duration itself is unlikely to have an important influence on encounter outcome. The collection of ten entire fish shoals showed that they were assorted by species and body length. A simulation model demonstrated that individual shoal choice behaviour alone could account for the generation and maintenance of the observed levels of size assortedness of shoals without invoking the existence of other sorting mechanisms such as differential swimming speeds. However, the generation of species assortedness was not predicted by the model. Furthermore, our data suggest that fish density acts as a constraint on shoal choice, influencing both shoal size and composition. This work has implications for studies on information transfer and reciprocal altruism within populations.     

Kin discrimination in sticklebacks is mediated by social learning rather than innate recognition

Theory predicts several advantages for animals to recognize kin. These include inbreeding avoidance and an increase in inclusive fitness. In shoaling species, kin recognition may lead to an increased amount of altruism among shoal members. Adult, non‐reproductive three‐spined sticklebacks, Gasterosteus aculeatus, prefer to shoal with kin. This preference was shown for familiar as well as for unfamiliar individuals. However, whether it is based on learned cues of familiar individuals or on innate mechanisms like self‐referent phenotype matching or ‘true’ kin recognition through recognition alleles remains unknown. In our experiments, juvenile fish were given the choice between shoals that differed in relatedness and familiarity. The number of testfish who joined each group indicated that sticklebacks prefer to shoal with familiar kin when the alternative shoal was composed of unfamiliar non‐kin. When one shoal consisted of familiar kin while the second consisted of familiar non‐kin testfish did not show any preference. Kin recognition in sticklebacks is thus most likely mediated by social learning.     

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.     

Are shoals of minnow Phoxinus phoxinus formed by close kin?

A molecular analysis examining the level of relatedness in shoaling minnows Phoxinus phoxinus was conducted. The results revealed that individuals from within the same shoal were not more closely related to each other than to individuals from other shoals. This led to the conclusion that Schreckstoff may be less likely to have evolved in the context of kin selection.     

Is there always an influence of shoal size on predator hunting success?

Theoretical and empirical studies predict that there should be a decrease in hunting success of predators with increasing prey group size. Most of these studies investigated situations in which predator and prey were in full view of each other before, during and after an attack. In this study, single rock bass Ambloplites rupestris were given an opportunity to launch surprise attacks at shoals of creek chub Semotilus atromaculatus that ranged in size from two to 13 fish. There was no significant influence of either shoal size or attack distance on predator success rate and no significant relationship between attack distance and shoal size. Furthermore, it was found that the leading fish of a shoal was attacked significantly more often than fish in other shoal positions, indicating that predation risk was not shared equally among shoal members. Also, leading fish in larger shoals (eight to 13 fish) were not more likely to survive a predator attack than ones in small shoals (two to seven fish).The consequences of these results are discussed in the general context of antipredator benefits of grouping.     

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.     

Parasitism and Shoal Size in Juvenile Sticklebacks: Conflicting Selection Pressures from Different Ectoparasites?

In gregarious animals, group size correlates negatively with infection levels by some kinds of parasites and positively with infection by others. Conflicting selection pressures can be exerted simultaneously on a host species by different parasite species. Among stationary, mixed-species shoals of juvenile threespine sticklebacks, Gasterosteus aculeatus , and blackspotted sticklebacks, Gasterosteus wheatlandi , shoal size correlated differently with levels of infection by two species of ectoparasites. Stickleback shoal size correlated positively with infection levels by the copepod Thersitina gasterostei , which is transmitted among fish by short-lived planktonic larvae. In contrast, infection levels by the highly mobile crustacean parasite Argulus funduli did not decrease as shoal size increased, as predicted from an earlier laboratory experiment. The species composition of the different stickleback shoals also had an influence on some aspects of infection by these two parasite species. The contrasting mode of transmission of the two parasites results in one parasite species having a higher transmission rate among fish within large shoals, whereas the success of the other parasite species is independent of fish shoal size. The two ectoparasites may thus exert different selection pressures on stickleback shoal sizes.     

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.     

Kin cohesiveness and possible inbreeding in the mouthbrooding tilapia Sarotherodon melanotheron (Pisces Cichlidae)

Four microsatellite markers were used to study genetic variation among individuals of the mouthbrooding tilapia Sarotherodon melanotheron (Rüppel 1852) caught in separate but adjacent shoals. A comparison was also made with fish from six other localities. Populations originating from riverine environments appear to be panmictic, while samples from open waters such as lagoons showed highly significant heterozygote deficiencies. For instance, at the 33-allele locus SMEL4, 32 homozygous individuals were observed among the 82 individuals from the same lagoon location instead of only five homozygotes expected if random mating occurred. A further assessment of the genetic similarity of individuals within each shoal, validated by robust permutation techniques requiring no precise knowledge of gene frequencies, showed that related individuals tend to aggregate, and suggested that mating occurs preferentially within small groups of kin. Cichlids are often presented as a group of fish where microallopatric speciation takes place. The possible link between kin aggregation, inbreeding and shoaling behaviour we propose here may have important consequences for our understanding of the mechanisms involved in this fast speciation process.     

The effect of shoal size on patterns of body colour segregation in mollies

Individual white and black mollies Poecilia latipinna spent significantly more time near the larger group when given the choice between two shoals of similar colouration to themselves. When given the choice between a large and a small shoal of dissimilar colouration to themselves, black test fish spent significantly more time with the larger shoal while white test fish showed no preference for either group. Both white and black mollies chose the smaller of two shoals when given the choice between a large dissimilarly coloured shoal and a small similarly coloured shoal. The results indicate that mollies actively discriminate between shoals on the basis of both body colour and shoal size. However, body colour segregation appears to have a stronger influence on shoal choice.     

Sex biases in kin shoaling and dispersal in a cichlid fish

Animal dispersal is associated with diverse costs and benefits that vary among individuals based on phenotype and ecological conditions. For example, females may disperse when males benefit more from defending territories in familiar environments. Similarly, size differences in dispersal propensity may occur when dispersal costs are size-dependent. When individuals do disperse, they may adopt behavioral strategies that minimize dispersal costs. Dispersing fish, for example, may travel within shoals to reduce predation risks. Further, kin shoaling may augment inclusive fitness by reducing predation of relatives. However, studies are lacking on the role of kin shoaling in dispersal. We explored how sex and size influence dispersal and kin shoaling in the cichlid Neolamprologus caudopunctatus. We microsatellite genotyped over 900 individuals from two populations separated by a potential dispersal barrier, and documented patterns of population structure, migration and within-shoal relatedness. Genetic differentiation across the barrier was greater for smaller than larger fish, suggesting larger fish had dispersed longer distances. Females exhibited weaker genetic differentiation and 11 times higher migration rates than males, indicating longer-distance female-biased dispersal. Small females frequently shoaled with siblings, possibly offsetting dispersal costs associated with higher predation risks. In contrast, small males appeared to avoid kin shoaling, possibly to avoid local resource competition. In summary, long-distance dispersal in N. caudopunctatus appears to be female-biased, and kin-based shoaling by small females may represent a behavioral adaptation that reduces dispersal costs. Our study appears to be the first to provide evidence that sex differences in dispersal influence sex differences in kin shoaling.     

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.     

Effects of fasting on collective movement and fission–fusion dynamics in both homogeneous and heterogeneous shoals of a group-living cyprinid fish species

The present study aimed to reveal the effect of fasting (21 days) on collective movement and interaction dynamics in both homogeneous (eight members fed a commercial diet or deprived of food) and heterogeneous (four fed + four starved members) shoals of juvenile qingbo (Spinibarbus sinensis). The authors of this study measured the shoaling behaviour in both a commonly used rectangular open arena with no spatial complexity and a radial arm maze. When measured in the open arena, the starved shoals had a faster swimming speed and acceleration rate and a longer interindividual distance than the fed shoals, possibly because of the elevated foraging motivation. Nonetheless, the values of the heterogeneous groups were similar to those of the fed groups. Furthermore, in contrast to the fish in homogeneous shoals, the starved fish in heterogeneous shoals showed a slower acceleration rate and speed than fed members in heterogeneous shoals. These results, combined with the relationships of variables at the among- and within-shoal levels, suggested that starved fish limited their motion in heterogeneous shoals to maintain group cohesion but that the fed fish contributed more to maintaining shoal structure, possibly because of the higher energy expenditure required for movement changes. When monitored in a radial arm maze, starved shoals showed more fission–fusion episodes without sacrificing group cohesion, as they adaptively adjusted the frequency and duration of each majority choice. The among-shoal variation revealed that the heterogeneous groups showed less variation in the open arena but more variation in the radius maze than did the homogeneous groups. This difference might arise because dominant members have opposite effects on shoal behaviour and consensus decisions. In conclusion, the present study showed opposite effects of feeding states on collective behaviour between homogeneous and heterogeneous shoals, possibly because of the complicated interactions among members with different energy storage levels and foraging motivations. Furthermore, the heterogeneous groups showed a difference between shoal behaviour in the open area and exploration in the radial arm maze. Future studies manipulating the personality composition of starved and fed members of heterogeneous groups might yield interesting results.     

Investigating the Effect of Familiarity on Kin Recognition of Three‐Spined Stickleback (Gasterosteus aculeatus)

The ability to discriminate between related and unrelated individuals has been demonstrated in many species. The mechanisms behind this ability might be manifold and depend on the ecological context in which the species lives. In brood‐caring species, both familiarity and phenotype matching are known to be used in kin recognition. However, results of studies disentangling these two phenomena have proved contradictory. We aimed to broaden our knowledge about the mechanisms of kin recognition using shoaling preferences of three‐spined stickleback (Gasterosteus aculeatus) as a model behavior. In our first experiment, focal fish had the choice to shoal either with kin or unfamiliar non‐kin. In half of the trials, kin groups were composed of familiar individuals, while they were unfamiliar in the other half. Focal fish significantly preferred kin as shoaling partner, a result which was not reinforced by familiarity. In our second experiment, focal fish were given the choice between a shoal of familiar kin and a shoal of unfamiliar kin. Here, focal fish did not show any significant preference. These results indicate that familiarity does not impact stickleback's ability to recognize kin. Furthermore, they show that familiarity does not overrule recognition based on phenotype matching or innate recognition, underlining the importance of these mechanisms. Finally, our results lead to the assumption that individual recognition might play a minor role also in non‐kin‐based preferences for familiars.