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Steering clear of trouble Fish are not famous for being smart, yet they can be added to the list of animals that show the rudiments of logical thinking. Transitive inference, the ability to deduce unknown relationships from knowledge of known relationships, is essential to logical reasoning. This ability is seen as an important step in a child's development, and similar capabilities are found in nonhuman primates, rats and birds. Astatotilapia burtoni , a territorial fish in which reproductive success of males depends on their status in 'fish society', can learn an implied hierarchy among other unfamiliar fish by watching fights between them. Remarkably, fish do this indirectly, as 'bystanders', with no direct reinforcement. This behaviour calls into question previous models of transitive inference and suggests that these fish have evolved distinct mechanisms for making inferences in situations specific to their survival and reproduction. The cover image shows an A. burtoni male. Transitive inference is demonstrated in fish populations by showing that they can learn the implied hierarchy among other unfamiliar fish by watching fights between rivals. Remarkably, fish can do this indirectly, as 'bystanders', without any reinforcement; they also make sophisticated use of contextual information available to them. Transitive inference (TI) involves using known relationships to deduce unknown ones (for example, using A  >  B and B  >  C to infer A  >  C ), and is thus essential to logical reasoning. First described as a developmental milestone in children 1 , TI has since been reported in nonhuman primates 2 , 3 , 4 , rats 5 , 6 and birds 7 , 8 , 9 , 10 . Still, how animals acquire and represent transitive relationships and why such abilities might have evolved remain open problems. Here we show that male fish ( Astatotilapia burtoni ) can successfully make inferences on a hierarchy implied by pairwise fights between rival males. These fish learned the implied hierarchy vicariously (as ‘bystanders’), by watching fights between rivals arranged around them in separate tank units. Our findings show that fish use TI when trained on socially relevant stimuli, and that they can make such inferences by using indirect information alone. Further, these bystanders seem to have both spatial and featural representations related to rival abilities, which they can use to make correct inferences depending on what kind of information is available to them. Beyond extending TI to fish and experimentally demonstrating indirect TI learning in animals, these results indicate that a universal mechanism underlying TI is unlikely. Rather, animals probably use multiple domain-specific representations adapted to different social and ecological pressures that they encounter during the course of their natural lives.

Similarities in the behavior of diverse animal species that form large groups have motivated attempts to establish general principles governing animal group behavior. It has been difficult, however, to make quantitative measurements of the temporal and spatial behavior of extensive animal groups in the wild, such as bird flocks, fish shoals, and locust swarms. By quantifying the formation processes of vast oceanic fish shoals during spawning, we show that (i) a rapid transition from disordered to highly synchronized behavior occurs as population density reaches a critical value; (ii) organized group migration occurs after this transition; and (iii) small sets of leaders significantly influence the actions of much larger groups. Each of these findings confirms general theoretical predictions believed to apply in nature irrespective of animal species.

A clean-cut image If we see someone do a good turn to someone else we are more inclined to help the altruist at some point in the future, rather than someone with a reputation for being selfish. This ‘image scoring’ behaviour is thought to help maintain human cooperation. Redouan Bshary and Alexandra Grutter now provide experimental evidence that it can also stabilize cooperation between non-human animals. The cooperation in this case is displayed by cleaner fish that remove ectoparasites from their partners in a mutual relationship, the ‘client’ fish. The cleaners may cooperate in the task of removing parasites; or a cleaner fish may ‘cheat’ and simply feed on client mucus leaving the other cleaners to do the work. A series of foraging experiments shows that client fish engage in image scoring, and that the cleaners cooperate more in the presence of an image-scoring client. Humans are highly social animals and often help unrelated individuals that may never reciprocate the altruist's favour 1 , 2 , 3 , 4 , 5 . This apparent evolutionary puzzle may be explained by the altruist's gain in social image: image-scoring bystanders, also known as eavesdroppers, notice the altruistic act and therefore are more likely to help the altruist in the future 5 , 6 , 7 . Such complex indirect reciprocity based on altruistic acts may evolve only after simple indirect reciprocity has been established, which requires two steps. First, image scoring evolves when bystanders gain personal benefits from information gathered, for example, by finding cooperative partners 8 , 9 , 10 . Second, altruistic behaviour in the presence of such bystanders may evolve if altruists benefit from access to the bystanders. Here, we provide experimental evidence for both of the requirements in a cleaning mutualism involving the cleaner fish Labroides dimidiatus. These cleaners may cooperate and remove ectoparasites from clients or they may cheat by feeding on client mucus 11 , 12 . As mucus may be preferred over typical client ectoparasites 13 , clients must make cleaners feed against their preference to obtain a cooperative service. We found that eavesdropping clients spent more time next to ‘cooperative’ than ‘unknown cooperative level’ cleaners, which shows that clients engage in image-scoring behaviour. Furthermore, trained cleaners learned to feed more cooperatively when in an ‘image-scoring’ than in a ‘non-image-scoring’ situation.

Evolution of cooperation: two heads are better than one The evolution of cooperation between unrelated individuals is a puzzle for researchers in both the social and natural sciences. One possible obstacle to a greater understanding of the phenomenon is the tendency for theorists and experimentalists to work independently. Bshary et al . avoid that charge by combining game theory modelling, field observations and experimental testing in a study of a hitherto unexplored problem of cooperation, the cleaning mutualism between stable male–female pairs of the cleaner wrasse Labroides dimidiatus and their client fish. Theory predicts that two providers should offer a higher service quality to clients than single providers, as long as they cooperate with each other. Field observations and experiments confirm the model prediction. Key to the success of a pair of cleaner wrasse is the fact that while one cleaner eats ectoparasites off the client the other can carry on with its preferred cheating behaviour, eating the client's mucus, while client satisfaction is still guaranteed. The evolution of cooperation between unrelated individuals is a puzzle that attracts interest both in the social and in the natural sciences. Theoretical concepts may fail to capture essential features of real life cooperation. This paper focuses on the cleaning mutualism between cleaner wrasse and their client fish. A game theoretical model predicts that two providers should offer a higher service quality to clients than single providers, as long as they cooperate among each other. Field observations and an experimental test confirm the model prediction. Service providers may vary service quality depending on whether they work alone or provide the service simultaneously with a partner. The latter case resembles a prisoner’s dilemma 1 , 2 , 3 , 4 , in which one provider may try to reap the benefits of the interaction without providing the service. Here we present a game-theory model based on the marginal value theorem 5 , which predicts that as long as the client determines the duration, and the providers cooperate towards mutual gain, service quality will increase in the pair situation. This prediction is consistent with field observations and with an experiment on cleaning mutualism, in which stable male–female pairs of the cleaner wrasse Labroides dimidiatus repeatedly inspect client fish jointly. Cleaners cooperate by eating ectoparasites 6 off clients but actually prefer to cheat and eat client mucus 7 . Because clients often leave in response to such cheating, the benefits of cheating can be gained by only one cleaner during a pair inspection. In both data sets, the increased service quality during pair inspection was mainly due to the smaller females behaving significantly more cooperatively than their larger male partners. In contrast, during solitary inspections, cleaning behaviour was very similar between the sexes. Our study highlights the importance of incorporating interactions between service providers to make more quantitative predictions about cooperation between species.

Coral reefs resemble islands of productive habitats where fishes aggregate, forage, and spawn. Although it has been suggested that some reef fishes use biogenic chemicals as aggregation cues, specific chemicals have not been identified. Dimethylsulfoniopropionate (DMSP), a secondary metabolite of many marine algal species, is released during foraging by higher-order consumers. DMSP has been studied intensively for its role in oceanic sulfur cycles and global climate regulation, but its ecological importance to marine fishes is unknown. We present evidence that planktivorous reef fishes will aggregate to experimental deployments of DMSP over coral reef habitats in the wild.

The sea urchin cardinalfish, Siphamia tubifer (Perciformes: Apogonidae), is unusual among coral reef fishes for its use of bioluminescence, produced by symbiotic bacteria, while foraging at night. As a foundation for understanding the relationship between the symbiosis and the ecology of the fish, this study examined the diel behavior, host urchin preference, site fidelity, and homing of S. tubifer in June and July of 2012 and 2013 at reefs near Sesoko Island, Okinawa, Japan (26°38′N, 127°52′E). After foraging, S. tubifer aggregated in groups among the spines of the longspine sea urchin, Diadema setosum, and the banded sea urchin, Echinothrix calamaris. A preference for D. setosum was evident (P < 0.001), especially by larger individuals (>25 mm standard length, P < 0.01), and choice experiments demonstrated the ability of S. tubifer to recognize and orient to a host urchin and to conspecifics. Tagging studies revealed that S. tubifer exhibits daily fidelity to a host urchin; 43–50 and 26–37 % of tagged individuals were associated with the same urchin after 3 and 7 days. Tagged fish also returned to their site of origin after displacement; by day two, 23–43 and 27–33 % of tagged individuals returned from displacement distances of 1 and 2 km. These results suggest that S. tubifer uses various environmental cues for homing and site fidelity; similar behaviors and cues might be used by larvae for recruitment to settlement sites and for the acquisition of luminous symbiotic bacteria.

AIM: One of the main gaps in the assessment of biodiversity is the lack of a unified framework for measuring its taxonomic and functional facets and for unveiling the underlying patterns. LOCATION: Europe, 25 large river basins. METHODS: Here, we develop a decomposition of functional β‐diversity, i.e. the dissimilarity in functional composition between communities, into a functional turnover and a functional nestedness‐resultant component. RESULTS: We found that functional β‐diversity was lower than taxonomic β‐diversity. This difference was driven by a lower functional turnover compared with taxonomic turnover while the nestedness‐resultant component was similar for taxonomic and functional β‐diversity. MAIN CONCLUSIONS: Fish faunas with different species tend to share the same functional attributes. The framework presented in this paper will help to analyse biogeographical patterns as well as to measure the impact of human activities on the functional facets of biodiversity.

The importance of structural complexity in coral reefs has come to the fore with the global degradation of reef condition; however, the limited scale and replication of many studies have restricted our understanding of the role of complexity in the ecosystem. We qualitatively and quantitatively (where sufficient standardised data were available) assess the literature regarding the role of structural complexity in coral reef ecosystems. A rapidly increasing number of publications have studied the role of complexity in reef ecosystems over the past four decades, with a concomitant increase in the diversity of methods used to quantify structure. Quantitative analyses of existing data indicate a strong negative relationship between structural complexity and algal cover, which may reflect the important role complexity plays in enhancing herbivory by reef fishes. The cover of total live coral and branching coral was positively correlated with structural complexity. These habitat attributes may be creating much of the structure, resulting in a collinear relationship; however, there is also evidence of enhanced coral recovery from disturbances where structural complexity is high. Urchin densities were negatively correlated with structural complexity; a relationship that may be driven by urchins eroding reef structure or by their gregarious behaviour when in open space. There was a strong positive relationship between structural complexity and fish density and biomass, likely mediated through density-dependent competition and refuge from predation. More variable responses were found when assessing individual fish families, with all families examined displaying a positive relationship to structural complexity, but only half of these relationships were significant. Although only corroborated with qualitative data, structural complexity also seems to have a positive effect on two ecosystem services: tourism and shoreline protection. Clearly, structural complexity is an integral component of coral reef ecosystems, and it should be incorporated into monitoring programs and management objectives.

The decline of predators in a variety of ecosystems has transformed community structure through mesopredator release and trophic cascades. Elasmobranch fishes, one of the earth’s most ubiquitous and diverse clade of predatory species, provide a model group for defining marine predator roles. We consider whether the ecological predatory role of sharks is adequately defined by terrestrial-derived notions of apex- and mesopredation. Indeterminate growth and ontogenetic diet shifts may mean species-level classification of predatory roles is inadequate. We propose that examining the trophic level and body size of species might be the most pragmatic and informative way to define the ecological roles of predators.

Population diversity boosts fishery resilience The role of species diversity in ecosystem stability is well appreciated, but population diversity within a species is also important and often overlooked. An analysis of over 50 years of data on sockeye salmon returns to the rivers of Bristol Bay, Alaska, shows just how important this portfolio effect — so-called by analogy with risk-spreading in financial markets — can be. The sockeye salmon fishery is one of the most valuable in the United States, with more than 60% of it coming from this region. The fact that it is made up of several hundred discrete populations makes the observed population variability about half what would be expected in a single homogenous population, and numerical modelling predicts that a homogenous population would be subject to ten times more fisheries closures. In terms of fisheries management, this work suggests that reducing the homogenizing effects of hatcheries on genetic diversity, protecting weak stocks from over-harvesting in mixed stock fisheries, and maintaining intact habitat networks should be prioritized. The value of having a diversity of species within an ecosystem is well appreciated: species-rich communities are thought to produce more stable ecosystem services. But population diversity within a species is important too. Here, the effects of diversity in population and life history in a heavily exploited Alaskan salmon species are quantified. The results show that population diversity increases the resilience of this ecosystem, and hence the value of salmon fisheries. One of the most pervasive themes in ecology is that biological diversity stabilizes ecosystem processes and the services they provide to society 1 , 2 , 3 , 4 , a concept that has become a common argument for biodiversity conservation 5 . Species-rich communities are thought to produce more temporally stable ecosystem services because of the complementary or independent dynamics among species that perform similar ecosystem functions 6 . Such variance dampening within communities is referred to as a portfolio effect 7 and is analogous to the effects of asset diversity on the stability of financial portfolios 8 . In ecology, these arguments have focused on the effects of species diversity on ecosystem stability but have not considered the importance of biologically relevant diversity within individual species 9 . Current rates of population extirpation are probably at least three orders of magnitude higher than species extinction rates 10 , so there is a pressing need to clarify how population and life history diversity affect the performance of individual species in providing important ecosystem services. Here we use five decades of data from Oncorhynchus nerka (sockeye salmon) in Bristol Bay, Alaska, to provide the first quantification of portfolio effects that derive from population and life history diversity in an important and heavily exploited species. Variability in annual Bristol Bay salmon returns is 2.2 times lower than it would be if the system consisted of a single homogenous population rather than the several hundred discrete populations it currently consists of. Furthermore, if it were a single homogeneous population, such increased variability would lead to ten times more frequent fisheries closures. Portfolio effects are also evident in watershed food webs, where they stabilize and extend predator access to salmon resources. Our results demonstrate the critical importance of maintaining population diversity for stabilizing ecosystem services and securing the economies and livelihoods that depend on them. The reliability of ecosystem services will erode faster than indicated by species loss alone.