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     

The influence of differential swimming speeds on composition of multi‐species fish shoals

The importance of species‐specific swim speeds for the formation of multi‐species fish shoals in a Canadian freshwater system was investigated. Swim speed was positively correlated with total length and differed between species but was unrelated to shoal size.     

Positioning behaviour in fish shoals: a cost–benefit analysis

Shoal position can have a strong influence on individual fitness. Individuals in front positions of shoals were observed to have higher feeding rates than individuals elsewhere. Manipulation of nutritional state showed that hungry individuals had a stronger preference for front positions and that the duration of food-deprivation was positively correlated with the degree of the position preference. On the other hand, front positions (like other peripheral positions) probably incur costs in terms of increased predation risks. Experiments with Schreckstoff showed that frightened individuals seek the central part of the shoal. This suggests that individuals rotate their shoal positions according to the tradeoff between energy intake and predation risk.     

Social familiarity and shoal formation in juvenile fishes

The potential influence of social familiarity in shoal‐choice decisions was investigated in two sympatric species of north temperate fishes, juvenile banded killifish Fundulus diaphanus and juvenile bluegill sunfish Lepomis macrochirus. Groups of socially familiar and unfamiliar conspecifics were formed in the laboratory using wild‐caught fishes. Juvenile F. diaphanus demonstrated a strong preference for familiar conspecific shoalmates, whereas juvenile L. macrochirus exhibited no preference for either unfamiliar or familiar conspecific shoalmates. The differential influence of familiarity on shoalmate choice in juveniles of these two species could be due to their different ecologies, local population densities and life histories.     

A grid-net technique for the analysis of fish positions within free-ranging shoals

The design and operation of a grid-net are described, which allows the capture of entire shoals of wild fish and preserves information about the two-dimensional structure of shoals and the spatial positions of individual fish within the shoal. This simple technique facilitates investigation of several aspects of individual differences in fish shoaling behaviour in the wild for the first time.     

Turbidity amplifies the non‐lethal effects of predation and affects the foraging success of characid fish shoals

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Toxicity versus avoidance response of golden shiner, Notemigonus crysoleucas, to five metals

Avoidance thresholds and 96-h LC50 values were determined for golden shiners, Noiemigonus crysoleucas , for five individual elements: chromium, Cr; copper, Cu; cadmium, Cd; arsenic, As; selenium, Se. The avoidance concentrations were 73, 26 and 28 μgl^-1 for Cr-VI, Cu and As-III, respectively. Cadmium and Se were not avoided at experimental concentrations up to 68 and 3489 μg1^-1, respectively. Acute flow-through 96-h LC50 values were Cr-VI 55·0, Cu 84·6, and As-III 12·5 mg 1^-1, which were more than two orders of magnitude above avoidance concentrations. The acute flow-through 96-h LC50 values for Cd and Se were 2·8 and 11·2 mg 1^-1, respectively. These concentrations are 31 and 2·2 times the highest concentration employed in the avoidance tests, neither of which were avoided by the test organisms. Thus, simple toxicity tests do not identify the environmental hazard of some elements, and the most toxic elements may not elicit a behavioural response. When used in concert with tests of organism function, more realistic indicators of environmental hazard or safety may be determined.     

Shoaling preferences of two common killifish (Fundulus heteroclitus and F. diaphanus) in the laboratory and in the field: A new analysis of heterospecific shoaling

Heterospecific grouping behavior of mummichogs (Fundulus heteroclitus) and banded killifish (Fundulus diaphanus) was analyzed in the laboratory and in a freshwater tidal marsh in Cremona, Maryland. Several parameters of wild, intact shoals were measured, including species composition, body length, parasite load, gender, and any physical abnormalities. Fish collected were used for laboratory analysis of shoaling preferences. When size was equal, banded killifish and mummichogs preferred conspecific shoals to heterospecific shoals, consisting of mummichogs, banded killifish, and sheepshead minnows (Cyprinodon variegates). Shoal collection from the field resulted in mixed species shoals with individuals predominantly unaffected by parasites or other physical abnormalities. Size appeared to be a sorting mechanism. A temporal shift in lengths was evident. Initial shoals caught contained significantly smaller fish compared to the final shoals caught. Results are compared with previous studies on heterospecific shoaling in killifish and new characteristics of heterospecific shoals inhabiting freshwater tidal marshes are discussed.     

Environmental habituation and sexual composition affect juveniles' shoaling activity in a cichlid fish (Pelvicachromis taeniatus)

Group living is widespread in animals, and many fishes form shoals. Examining within-group interactions in fishes may contribute to the general understanding of dynamic social structures in animals. The sex ratio of a group has been shown to influence grouping decisions of fishes and can be expected to affect behaviour at group level. Behavioural experiments usually involve relatively short acclimatisation times, although the establishment of environmental habituation in fishes is understudied. This study tests whether the sex ratio and long-term habituation to experimental conditions influence general shoal performance (activity parameters, density) and responses of shoals to an acoustic-mechanical disturbance cue in juveniles of the cichlid fish Pelvicachromis taeniatus via individual tracking. The disturbance consisted of a defined hit against the experimental tank, which caused sudden noise and water movement. We found that a higher proportion of females increases shoal activity (swimming speed and distance covered), suggesting that female P. taeniatus are more active than males. Furthermore, shoal activity declined when shoals habituated to the experimental settings and with the time that the shoals were grouped together, which may reflect intensified group member familiarity. Moreover, behavioural changes after disturbance were weaker when individuals were kept with their group longer and more familiar to the experimental conditions. For prey species, lower activity might be beneficial under natural conditions due to lower conspicuousness of the group. We did not find any significant effects of the investigated factors on shoal density (mean interindividual distance) and speed synchronisation. The results indicate that sexual composition, familiarity between shoal members and habituation to the experimental environment affect shoal performance in a cichlid fish.     

多物种集合的种-多度关系模型研究进展

种 -多度关系是群落结构研究最基本的方面之一。Preston最早描述的物种“常见性和罕见性的分布”是群落中种 -多度关系的一般特性。自从 Motomura开创性地提出一个几何序列模型用于描述这种特性后 ,生态学家已建立了许多拟合种 -多度关系的经验模型和理论模型 ,主要包括对数序列分布、对数正态分布等统计模型 ,几何序列模型、分割线段模型等生态位模型以及反映群落动态和环境制约作用的动态模型 ,还有其中一些模型的扩展模型。近年来 ,种 -多度关系研究领域已大大拓宽 ,如“群落”的内涵已延伸为“多物种集合”,物种“多度”的测度推广到“广义多度”。尽管种 -多度关系研究已取得很大进展 ,仍有一些问题尚待解决 ,诸如多度指标选择、种 -多度分布时空变化规律及其机制以及种 -多度关系模型的选择、检验、解释与应用。     

Phylogeography of the banded killifish (Fundulus diaphanus): glacial races and secondary contact

Spatial patterns of morphological and genetic polymorphisms between the two banded killifish sub-species ( Fundulus diaphanus diaphanus and F. d. menona ) provide evidence that these sub-species represent distinct evolutionary lineages corresponding to glacial races of Atlantic and Mississippian origins. Individuals with intermediate phenotypes in Lake Ontario and the upper St Lawrence River probably result from secondary contacts between these races. The hybrid zone unexpectedly extends to Lake Erie and involves the introgression of the F. d. diaphanus mitochondrial genome into fish with morphology like that of F. d. menona .     

Quorum decision-making in foraging fish shoals

Quorum responses provide a means for group-living animals to integrate and filter disparate social information to produce accurate and coherent group decisions. A quorum response may be defined as a steep increase in the probability of group members performing a given behaviour once a threshold minimum number of their group mates already performing that behaviour is exceeded. In a previous study we reported the use of a quorum response in group decision-making of threespine sticklebacks (Gasterosteus aculeatus) under a simulated predation threat. Here we examine the use of quorum responses by shoals of sticklebacks in first locating and then leaving a foraging patch. We show that a quorum rule explains movement decisions by threespine sticklebacks toward and then away from a food patch. Following both to and from a food patch occurred when a threshold number of initiators was exceeded, with the threshold being determined by the group size.     

Terpinen-4-ol and carvacrol affect multi-species biofilm composition

The aim of this study was to investigate the cytotoxic activity and inhibitory effect of terpinen-4-ol (T4ol) and carvacrol against single- and multi-species biofilms. The toxicity of each compound was tested on oral keratinocytes and evaluated by XTT assay. Inhibition and eradication of single-species biofilms were analyzed by crystal violet assay and the effect on multi-species biofilm composition was evaluated by qPCR. T4ol and carvacrol did not affect the epithelial cell viability, in contrast to chlorhexidine, which showed a high cytotoxic effect. Inhibition and eradication of single-species biofilms treated with T4ol and carvacrol were observed. The same inhibitory effect was observed for multi-species biofilms, especially on periodontal pathogens. In conclusion, specific concentrations of T4ol and carvacrol without toxicity towards the epithelial cells reduced the numbers of periodontal pathogens in single- and multi-species biofilms.     

Behavioural thermoregulation in two freshwater fish species

In the presence of a vertical thermal gradient, juvenile three‐spined sticklebacks Gasterosteus aculeatus and minnows Phoxinus phoxinus positioned themselves higher in the water column compared with adult conspecifics. This result was consistent regardless of whether age cohorts were tested separately or together. Furthermore, juveniles but not adult fishes positioned themselves higher in water column in the presence of a thermal gradient compared with those in the absence of a thermal gradient. Juvenile G. aculeatus and adult fish of both species did opt to position themselves higher in the water column in the hours immediately following a feeding event relative to their positions in the same gradient when they had not fed.     

Refuge use in sticklebacks as a function of body length and group size

A change in anti–predator strategy from hiding to grouping outside a refuge was observed in large three–spined sticklebacks. No such change in strategy was seen in small fish. The body–length dependence of this strategy change is discussed in the context of metabolic constraints.     

Pasture intake and milk production of dairy cows rotationally grazing on multi-species swards

Increasing plant species diversity has been proposed as a means for enhancing annual pasture productivity and decreasing seasonal variability of pasture production facing more frequent drought scenarios due to climate change. Few studies have examined how botanical complexity of sown swards affects cow performance. A 2-year experiment was conducted to determine how sward botanical complexity, from a monoculture of ryegrass to multi-species swards (MSS) (grasses-legumes-forb), affect pasture chemical composition and nutritive value, pasture dry matter (DM) intake, milk production and milk solids production of grazing dairy cows. Five sward species: perennial ryegrass (L as Lolium), white clover and red clover (both referred to as T as Trifolium because they were always sown together), chicory (C as Cichorium) and tall fescue (F as Festuca) were assigned to four grazing treatments by combining one (L), three (LT), four (LTC) or five (LTCF) species. Hereafter, the LT swards are called mixed swards as a single combination of ryegrass and clovers, whereas LTC and LTCF swards are called MSS as a combination of at least four species from three botanical families. The experimental area (8.7 ha) was divided into four block replicates with a mineral nitrogen fertilisation of 75 kg N/ha per year for each treatment. In total, 13 grazing rotations were carried out by applying the same grazing calendar and the same pasture allowance of 19 kg DM/cow per day above 4 cm for all treatments. Clover represented 20% of DM for mixed and MSS swards; chicory represented 30% of DM for MSS and tall fescue represented 10% of DM for LTCF swards. Higher milk production (+1.1 kg/day) and milk solids production (+0.08 kg/day) were observed for mixed swards than for ryegrass swards. Pasture nutritive value and pasture DM intake were unaffected by the inclusion of clover. Pasture DM, organic matter and NDF concentrations were lower for MSS than for mixed swards. Higher milk production (+0.8 kg/day), milk solids production (+0.04 kg/day) and pasture DM intake (+1.5 kg DM/day) were observed for MSS than for mixed swards. These positive effects of MSS were observed for all seasons, but particularly during summer where chicory proportion was the highest. In conclusion, advantages of grazing MSS on cow performance were due to the cumulative effect of improved pasture nutritive value and increased pasture DM intake that raised milk production and milk solids production.