The correlation between subordinate fish eye colour and received attacks: a negative social feedback mechanism for the reduction of aggression during the formation of dominance hierarchies

Eye darkening has been linked to social status in fish. The subordinate's eyes darken, while the eyes of the dominant fish become pale. Although this phenomenon has been described in salmonid fishes and in the African cichlid Nile tilapia Oreochromis niloticus, it is unclear whether eye darkening correlates with a reduction in aggressive behaviour. Thus, we evaluated the link between social status and eye darkening. We evaluated whether the eye colours of subordinate fish correlate with the frequency of received attacks in a neotropical fish, the pearl cichlid Geophagus brasiliensis. We paired pearl cichlids and quantified both the aggressive behaviour and the eye darkening of each fish. As has been described for Nile tilapia and Atlantic salmon, a clear-cut hierarchical relationship formed, where dominance and subordination were associated with pale and dark eye colours, respectively. Initially, eye colour darkening was positively correlated with the frequency of received attacks; however, a negative association occurred following eye darkening, in which the intensity of aggressive interactions decreased. Thus, fish that initially received a high number of attacks signalled subordination more rapidly and intensely (rapid and dramatic eye darkening), thereby inducing a negative social feedback mechanism that led to reduced aggression.     

The impacts of invasive Nile tilapia (Oreochromis niloticus) on the fisheries in the main rivers of Guangdong Province,China

Nile tilapia (Oreochromis niloticus) is one of the most widespread invasive fish species, and this species has successfully established populations in the major rivers of Guangdong Province, China. Field surveys and manipulative experiments were conducted to assess the impacts of Nile tilapia on fisheries. We determined that the increase of Nile tilapia in these rivers not only affects the CPUE (catch-per-unit-per-effort) of the fish community and native fish species but also reduces the income of fishermen. In the manipulative experiments, we observed that the growth of native mud carp decreased in the presence of Nile tilapia. Our results suggest that the invasion of Nile tilapia negatively affected the fishery economy and native fish species, and suitable control measurements should be taken.     

Nile perch (Lates niloticus,L.) and cichlids (Haplochromis spp.) in Lake Victoria: could prey mortality promote invasion of its predator?

The invasion of Nile perch into Lake Victoria is one of the iconic examples of the destructive effect of an introduced species on an ecosystem but no convincing explanation exists of why Nile perch only increased dramatically after a 25 year lag. Here, we consider this problem using a mathematical model that takes into account interactions between Nile perch and its cichlid prey. We examined competing hypotheses to explain Nile perch invasion and show that suppression of juvenile Nile perch by cichlids may cause the system to have two alternative stable states: one with only cichlids and one with coexistence of cichlids and Nile perch. Without cichlid predation on Nile perch, alternative stable states did not occur. Our analysis indicates that cichlid mortality, for example fishing mortality, may have induced the observed shift between the states.     

Diet overlap of non‐native common carp and native Nile tilapia: A potential cause for population reduction of Nile tilapia stock in Lake Ziway,Ethiopia

Lake Ziway harbours indigenous and exotic fish species including Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio). Nile tilapia was the dominant and preferred fish. However, its contribution to total catch has dramatically declined from 89.3% in 1994 to 27% in 2014 while the introduced common carp has increased from 0% before 2012 to 25% in 2014. Common carp potentially compete with Nile tilapia for available resources and could be a cause for the decline. Thus, the study explored the dietary overlap of the two species from April to August 2017. Schoener's overlap index (α) revealed significant dietary overlap between the two species (α = 0.84, between juveniles and α = 0.63, between adults). Juveniles fed mainly on animal origin (zooplankton and insects), while adult Nile tilapia have consumed plant origin (macrophyte and phytoplankton), and adult common carp fed on macrophytes and detritus. The presence of significant dietary overlap between the two species, particularly due to intense competition among juveniles, might cause the reduction of Nile tilapia stock because the native species has shown a competitive disadvantage for food in the presence of common carp. This study provides baseline information to researchers and decision makers working towards the sustainable resource utilisation of the system.     

The decline of the native fishes of lakes Victoria and Kyoga (East Africa) and the impact of introduced species,especially the Nile perch,Lates niloticus,and the Nile tilapia,Oreochromis niloticus

Synopsis There has been a decline, and in some cases an almost total disappearance, of many of the native fish species of lakes Victoria and Kyoga in East Africa since the development of the fisheries of these lakes was initiated at the beginning of this century. The Nile perch, Lates niloticus, a large, voracious predator which was introduced into these lakes about the middle of the century along with several tilapiine species, is thought to have caused the reduction in the stocks of several species. But overfishing and competition between different species also appear to have contributed to this decline. By the time the Nile perch had become well established, stocks of the native tilapiine species had already been reduced by overfishing. The Labeo victorianus fishery had also deteriorated following intensive gillnetting of gravid individuals on breeding migrations. L. niloticus is, however, capable of preying on the species which haven been overfished and could have prevented their stocks from recovering from overfishing. L. niloticus is also directly responsible for the decline in populations of haplochromine cichlids which were abundant in these lakes before the Nile perch became established. Even without predation by Nile perch, it has been shown that the haplochromine cichlids could not have withstood heavy commercial exploitation if a trawl fishery had been established throughout Lake Victoria. Their utilisation for human food has also posed some problems. The abundance of the native tilapiine species may also have been reduced through competition with introduced species which have similar ecological requirements. At present, the Nile perch and one of the introduced tilapiine species, Oreochromis niloticus, form the basis of the fisheries of lakes Victoria and Kyoga.Invited editorial     

Genetic structure and gene flow in an endangered native tilapia fish (Oreochromis esculentus) compared to invasive Nile tilapia (Oreochromis niloticus) in Yala swamp, East Africa

The introduction of invasive Nile tilapia (Oreochromis niloticus), and the rapacious predator Nile perch (Lates niloticus), into Lake Victoria resulted in a decline in population sizes, genetic diversity and even extirpation of native species which were previously the mainstay of local fisheries. However, remnant populations of native fish species, including tilapia, still persist in satellite lakes around Lake Victoria where they may coexist with O. niloticus. In this study we assessed population genetic structure, diversity, and integrity of the native critically endangered Singidia tilapia (O. esculentus) in its refugial populations in the Yala swamp, Kenya, and contrasted this diversity with populations of the invasive tilapia O. niloticus in satellite lakes (Kanyaboli, Namboyo and Sare) and Lake Victoria. Based on mtDNA control region sequences and eight nuclear microsatellite loci, we did not detect any mtDNA introgression between the native and the invasive species in Lakes Kanyaboli and Namboyo, but did find low levels of nuclear admixture, primarily from O. niloticus to O. esculentus. Some genetic signal of O. esculentus in O. niloticus was found in Lake Sare, where O. esculentus is not found, suggesting it has recently been extirpated by the O. niloticus invasion. In both species, populations in the satellite lakes are significantly genetically isolated from each other, with private mtDNA haplotypes and microsatellite alleles. For O. niloticus, genetic diversity in satellite lakes was similar to that found in Lake Victoria. Our data imply a low frequency of immigration exchange between the two populations of O. esculentus and we suggest that the populations of this endangered species and important fisheries resource should be conserved separately in Lakes Kanyaboli and Namboyo and with high priority.     

Monogeneans in introduced and native cichlids in México: evidence for transfer

We examined 2 cichlid fish species native to México, Cichlasoma callolepis and C. fenestratum, and 2 introduced African cichlids, Oreochromis aureus and O. niloticus, from 3 localities in southeastern México for monogeneans. Six monogenean species infected the African cichlids: Cichlidogyrus haplochromii, C. dossoui, C. longicornis longicornis, C. sclerosus, C. tilapiae, and Enterogyrus malmbergi. We found all these parasite species, except C. haplochromii and C. dossoui, on the native C. fenestratum and C. callolepis. Prevalences of Cichlidogyrus spp. were 3-10% and abundances ranged from 0.03 +/- 0.2 to 0.1 +/- 0.3 for native cichlids. We only recovered a single E. malmbergi from 1 C. callolepis. We found Sciadicleithrum bravohollisae, a monogenean of native Cichlasoma spp., on the gills of the introduced O. aureus from Lake Catemaco (prevalence 3%, abundance 0.03 +/- 0.2). Although prevalence and abundance in atypical hosts were fairly low, the present findings provide evidence of monogenean transfer from African to American cichlids and vice versa. This is the first record of exotic monogeneans in the genus Cichlidogyrus and Enterogyrus infecting native American cichlid fish. It is also the first record from southeastern México of a native American monogenean infecting introduced African cichlids.     

Time-place learning in individually reared angelfish, but not in pearl cichlid

Time-place learning based on food association was investigated in the cichlids angelfish (Pterophyllum scalare) and pearl cichlid (Geophagus brasiliensis) reared in isolation, therefore eliminating social influence on foraging. During a 30-day period, food was placed in one side of the aquarium (containing three compartments) in the morning and in the opposite side in the afternoon. Learning was inferred by the number of correct side choices of all fish in each day of test (15th and 30th). During the test day fish were not fed. The angelfish learned to switch sides at the correct day period in order to get food, suggesting this species has time-place learning ability when individually reared. On the other hand, the same was not observed for pearl cichlid.     

Higher parasite richness, abundance and impact in native versus introduced cichlid fishes

Empirical studies suggest that most exotic species have fewer parasite species in their introduced range relative to their native range. However, it is less clear how, ecologically, the loss of parasite species translates into a measurable advantage for invaders relative to native species in the new community. We compared parasitism at three levels (species richness, abundance and impact) for a pair of native and introduced cichlid fishes which compete for resources in the Panama Canal watershed. The introduced Nile tilapia, Oreochromis niloticus, was infected by a single parasite species from its native range, but shared eight native parasite species with the native Vieja maculicauda. Despite acquiring new parasites in its introduced range, O. niloticus had both lower parasite species richness and lower parasite abundance compared with its native competitor. There was also a significant negative association between parasite load (abundance per individual fish) and host condition for the native fish, but no such association for the invader. The effects of parasites on the native fish varied across sites and types of parasites, suggesting that release from parasites may benefit the invader, but that the magnitude of release may depend upon interactions between the host, parasites and the environment.     

Unusually fine-scale genetic structuring found in rapidly speciating Malawi cichlid fishes

Mechanisms behind the explosive radiation of over 500 cichlid fish species from a single founding population in Lake Malawi during the last 700 000 years are poorly understood. Recent studies have suggested that the degree of population subdivision among the habitat patches within the lake may be responsible, but the evidence has been circumstantial: lack of a dispersal stage in haplochromine cichlids; genetic and colour variation among populations separated by large-scale geographical barriers; and fluctuating lake levels. One reason for the rapidity of speciation in these fishes may be that population subdivision is on a much finer scale than previously thought. Here we quantify the level of population subdivision and estimate migration at a scale of 700-1400 m, in order to investigate whether cichlid populations are sufficiently isolated from each other for allopatric divergence and perhaps speciation to take place. Using six microsatellite loci, we demonstrate the existence of highly significant genetic differentiation between subpopulations on adjacent headlands in each of four rock-dwelling haplochromine cichlid species. Our results suggest that these fish populations are divided into thousands of subunits among which genetic divergence is currently occurring, and that this may provide unprecedented opportunities for allopatric speciation. <br>     

尼罗罗非鱼早期发育形态及其在珠江水系的空间分布

【背景】罗非鱼作为联合国粮农组织（FAO）向全世界推广养殖的优良品种之一,有多个品系,其养殖范围已遍布85个国家和地区。1956年我国从越南引进莫桑比克罗非鱼,经过养殖及推广,2006年我国罗非鱼产量达到100万t。但是,该外来物种在给我国带来良好经济价值的同时,对土著种类及水环境造成了极大的影响。【方法】对近几年珠江水系渔获物的调查数据进行整理,并观察尼罗罗非鱼早期发育形态,统计尼罗罗非鱼苗对其他鱼苗的最大捕食量及捕食规格,以分析罗非鱼生物学特性及其在珠江水系的入侵现状。【结果】尼罗罗非鱼早期发育快,卵黄营养非常丰富,比珠江水系土著种类更有竞争性;罗非鱼苗呈现很强的攻击性与捕食性;尼罗罗非鱼已经扩散到珠江水系各主要河流,并在部分江段形成优势种群。【结论与意义】尼罗罗非鱼种群快速扩张,对土著种已构成严重威胁,有必要将其列为珠江水系高危入侵种。     

Positive Darwinian Selection Drives the Evolution of the Morphology-Related Gene, EPCAM, in Particularly Species-Rich Lineages of African Cichlid Fishes

The study of genetic evolution within the context of adaptive radiations offers insights to genes and selection pressures that result in rapid morphological change. Cichlid fishes are very species-rich and variable in coloration, behavior, and morphology, and so provide a classical model system for studying the genetics of adaptive radiation. In this study, we researched the evolution of the epithelial cell adhesion molecule (EPCAM), a candidate gene for the adaptive evolution of morphology broadly, and skin development specifically, in fishes. We compared EPCAM gene sequences from a rapidly speciating African cichlid lineage (the haplochromines), a species-poor African lineage (Nile tilapia Oreochromis niloticus), and a very young adaptive radiation in the Neotropics (sympatric crater lake Midas cichlids, Amphilophus sp.). Our results, based on a hierarchy of evolutionary analyses of nucleotide substitution, demonstrate that there are different selection pressures on the EPCAM gene among the cichlid lineages. Several waves of positive natural selection were identified not only on the terminal branches, but also on ancestral branches. Interestingly, significant positive or directional selection was found in the haplochromine cichlids only but not the comparatively species-poor tilapia lineage. We hypothesize that the strong signal of selection in the ancestral African cichlid lineage coincided with the transition from riverine to lacustrine habitat. The two neotropical species for which we collected new sequence data were invariant in the EPCAM locus. Our results suggest that functional changes promoted by positive Darwinian selection are widespread in the EPCAM gene during African cichlid evolution.     

Widespread geographical distribution of mitochondrial haplotypes in rock-dwelling cichlid fishes from Lake Tanganyika

The spectacularly diverse cichlid fish species flocks of the East African Rift Lakes have elicited much debate on the potential evolutionary mechanisms responsible for the origin of these adaptive radiations. An historical perspective on population structure may offer insights into the processes driving population differentiation and possibly speciation. Here, we examine mitochondrial DNA (mtDNA) sequence variation in two endemic species of rock-dwelling cichlids, Simochromis babaulti and S. diagramma , from Lake Tanganyika. Phylogeographic analyses were used to infer what factors might have been important in the genetic structuring of Simochromis populations. Patterns of mtDNA differentiation in Simochromis were compared to those of other rock-dwelling cichlids to distinguish between competing hypotheses concerning the processes underlying their evolution. In striking contrast to previous findings, populations of Simochromis , even those separated by up to 300 km, were found to share mitochondrial DNA haplotypes. There is no correspondence between mtDNA genealogies and the geographical distribution of populations. Only S. babaulti , but not S. diagramma was found to have a significant association between genetic and geographical distance. These phylogeographic patterns suggest that the evolutionary effects of abiotic and biotic factors shaping population genetic structure may differ substantially even among closely related species of rock-dwelling cichlids. Physical events and barriers to gene flow that are believed to have had a major impact on the geographical distribution and intralacustrine speciation of Tropheus do not seem to have equally strongly affected its close relative Simochromis . These findings emphasize that no single mechanism can be responsible for the formation of population structure, speciation, and the adaptive radiation of all cichlid fishes.     

The ‘club’ cell and behavioural and physiological responses to chemical alarm cues in the Nile tilapia

The alarm response to skin extract has been well documented in fish. In response to skin extract, there is a decline in both locomotion activity and aggressive interactions. Our observation herein of these responses in the cichlid Nile tilapia, Oreochromis niloticus, confirmed the existence of the alarm response in this species. However, so far there has been a paucity of information on the autonomic correlates of this response. In this study, the ventilatory change in response to the chemical alarm cue was evaluated. This parameter was measured 4 min before and 4 min after exposure to 1 mL of either conspecific skin extract or distilled water (extract vehicle). Skin extract induced an increase in the ventilation rate, which suggested an anticipatory adjustment to potentially harmful stimuli. The chemical cue (alarm substance) also interfered with the prioritisation of responses to different environmental stimuli (stimuli filtering); this was suggested by the observation that the Nile tilapia declined to fight after exposure to a cue that indicates a risk of predation. Furthermore, histological analysis of the Nile tilapia skin revealed the presence of putative alarm substance-producing (club) cells.     

Circular DNA intermediate in the duplication of Nile tilapia vasa genes

vasa is a highly conserved RNA helicase involved in animal germ cell development. Among vertebrate species, it is typically present as a single copy per genome. Here we report the isolation and sequencing of BAC clones for Nile tilapia vasa genes. Contrary to a previous report that Nile tilapia have a single copy of the vasa gene, we find evidence for at least three vasa gene loci. The vasa gene locus was duplicated from the original site and integrated into two distant novel sites. For one of these insertions we find evidence that the duplication was mediated by a circular DNA intermediate. This mechanism of gene duplication may explain the origin of isolated gene duplicates during the evolution of fish genomes. These data provide a foundation for studying the role of multiple vasa genes in the development of tilapia gonads, and will contribute to investigations of the molecular mechanisms of sex determination and evolution in cichlid fishes.     

Population-structure and genetic diversity in a haplochromine cichlid fish [corrected] of a satellite lake of Lake Victoria

The approximately 500 species of the cichlid fish species flock of Lake Victoria, East Africa, have evolved in a record-setting 100,000 years and represent one of the largest adaptive radiations. We examined the population structure of the endangered cichlid species Xystichromis phytophagus from Lake Kanyaboli, a satellite lake to Lake Victoria in the Kenyan Yala wetlands. Two sets of molecular markers were analysed--sequences of the mitochondrial control region as well as six microsatellite loci--and revealed surprisingly high levels of genetic variability in this species. Mitochondrial DNA sequences failed to detect population structuring among the three sample populations. A model-based population assignment test based on microsatellite data revealed that the three populations most probably aggregate into a larger panmictic population. However, values of population pairwise FST indicated moderate levels of genetic differentiation for one population. Eleven distinct mitochondrial haplotypes were found among 205 specimens of X. phytophagus, a relatively high number compared to the total number of 54 haplotypes that were recovered from hundreds of specimens of the entire cichlid species flock of Lake Victoria. Most of the X. phytophagus mitochondrial DNA haplotypes were absent from the main Lake Victoria, corroborating the putative importance of satellite lakes as refugia for haplochromine cichlids that went extinct from the main lake in the last decades and possibly during the Late Pleistocene desiccation of Lake Victoria.     

The impact of the introduction of Nile perch, Lates niloticus (L.) on the fisheries of Lake Victoria

The piscivorous Nile perch was introduced into Lake Victoria some 30 years ago, since when it has completely transformed the fishing industry and the species composition of the fish fauna of the lake. The original multispecies fishery, based mostly on cichlids (haplochromines, tilapias), cyprinids ( Barbus, Labeo, Rastrineobola ) and siluroids ( Bagrus, Clarias, Synodontis, Schilbe ), has changed dramatically to one based on three species: the introduced Nile perch, the cyprinids, Rastrineobola argenrea (Pellegrin), and the introduced Nile tilapia, Oreochromis niloticus (Linnaeus).
Within 25 years of its introduction the Nile perch became ubiquitous and now occurs in virtually every habitat with the exception of swamps and affluent rivers. It has preyed on all other species with profound effects, especially on the stocks of haplochromines. These originally comprised 80% of the total fish biomass in Lake Victoria, but have now decreased to less than 1% offish catches from the Kenyan waters of the lake. The fishermen of Lake Victoria have adjusted to this ecological crisis by using large-meshed nets to catch Nile perch, which has become the most important commercial species. For the first time in the history of Lake Victoria, fish fillets are now being exported to several overseas countries: the fillets are all from Nile perch.     

Reproductive Biology of the Introduced Mayan cichlid, Cichlasoma urophthalmus, Within an Estuarine Mangrove Habitat of Southern Florida

We investigated the reproductive biology of the Mayan cichlid, Cichlasoma urophthalmus, for twenty consecutive months in a southern Florida mangrove creek and compared it to that of available published accounts where this species is native. Mayan cichlid reproduction occurred during a period of the year when physical conditions facilitated the best growth and survival of the young, and was influenced more by changes in water level and salinity than by temperature. Most nests were observed along mangrove shorelines in April. The majority of reproduction and parental care of the young followed in May, and was completed by June when rising water levels flooded shallow habitats and dispersed young-of-year fish. Mayan cichlids in Florida became reproductively mature at a greater size than in native regions, but produced roughly the same number of eggs at a given body mass. The similarity of environmental conditions between southern Florida and areas where Mayan cichlids naturally occur imparts a very favorable reproductive potential for this species in southern Florida, and facilitates their continued presence within the ichthyofauna of the region.     

Ecological niche modeling of the invasive potential of Nile tilapia Oreochromis niloticus in African river systems: concerns and implications for the conservation of indigenous congenerics

This study applied ecological niche models to determine the potential invasive range of Nile tilapia, Oreochromis niloticus, with a particular focus on river systems in southern Africa where it is now established and spreading. Computational tools such as niche models are useful in predicting the potential range of invasive species, but there are limitations to their application. In particular, models trained on native records may fail to predict the full extent of an invasion. This failure is often attributed to changes in either the niche of the invading species or the variables used to develop the models. In this study, we therefore evaluated the differences in the predictive power of models trained with different environmental variables, the effect of species range (native vs. introduced) on model performance and assessed whether or not there is evidence suggestive of a niche shift in Nile tilapia following its introduction. Niche models were constructed using Maxent and the degree of niche similarity was assessed using Schoener`s index. Null models were used to test for significance. Model performance and niche conservatism varied significantly with variable selection and species range. This indicates that the environmental conditions available to Nile tilapia in its native and introduced ranges are not congruent. Nile tilapia exhibited broad invasive potential over most of southern Africa that overlaps the natural range of endemic congenerics. Of particular concern are areas which are free of exotic species but are now vulnerable due to the promotion of fish introductions mainly for aquaculture and sport fishing.