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The crucian carp Carassius carassius (Linnaeus, 1758), is native to many European freshwaters. Despite its wide distribution, the crucian carp is declining in both the number and sizes of populations across much of its range. Here we studied 30 individuals of a putative pure population from Helsinki, Finland. Despite clear external morphological features of C. carassius, an individual was of a higher ploidy level than the others. We therefore applied a set of molecular genetic (S7 nuclear and cytochrome b mitochondrial genes) and cytogenetic tools (sequential fluorescent 4', 6-diamidino-2-phenylindole [DAPI], Chromomycin A3 [CMA3], C-banding and in situ hybridization [FISH] with both 5S and 28S ribosomal DNA probes) to determine its origin. While all examined characteristics of a diploid representative male (CCAHe2Fi) clearly corresponded to those of C. carassius, a triploid individual (CCAHe1Fi) was more complex. Phylogenetic analysis revealed that the nuclear genome of CCAHe1Fi contained three haploid sets: two C. gibelio and one C. carassius. However the mitochondrial DNA was that of C. gibelio, demonstrating its hybrid origin. The FISH revealed three strong (more intensive) 5S rDNA loci, confirming the triploid status, and an additional 24 weak (less intensive) signals were observed in the chromosome complement of CCAHe1Fi. On the other hand, only two strong and 16 weak 5S rDNA signals were visible on the chromosomes of the CCAHe2Fi male. 28S rDNA FISH revealed four strong signals in both CCAHe1Fi and CCAHe2Fi individuals. CMA3 staining revealed four to six CMA3-positive bands of CCAHe1Fi, while that of diploids contained only two to four. The fact that a polyploid hybrid Carassius female with a strong invasive potential may share morphological characters typical for endangered C. carassius highlights a need to combine genetic investigations of Carassius cryptic diversity with conservation measures of C. carassius in Europe.

Gut microbiota is increasingly regarded as an integral component of the host, due to important roles in the modulation of the immune system, the proliferation of the intestinal epithelium and the regulation of the dietary energy intake. Understanding the factors that influence the composition of these microbial communities is essential to health management, and the application to aquatic animals still requires basic investigation. In this study, we compared the bacterial communities harboured in the intestines and in the rearing water of grass carp (Ctenopharyngodon idellus), crucian carp (Carassius cuvieri), and bighead carp (Hypophthalmichthys nobilis), by using 454-pyrosequencing with barcoded primers targeting the V4 to V5 regions of the bacterial 16S rRNA gene. The specimens of the three species were cohabiting in the same pond. Between 6,218 and 10,220 effective sequences were read from each sample, resulting in a total of 110,398 sequences for 13 samples from gut microbiota and pond water. In general, the microbial communities of the three carps were dominated by Fusobacteria, Firmicutes, Proteobacteria and Bacteroidetes, but the abundance of each phylum was significantly different between species. At the genus level, the overwhelming group was Cetobacterium (97.29 ± 0.46 %) in crucian carp, while its abundance averaged c. 40 and 60 % of the sequences read in the other two species. There was higher microbial diversity in the gut of filter-feeding bighead carp than the gut of the two other species, with grazing feeding habits. The composition of intestine microbiota of grass carp and crucian carp shared higher similarity when compared with bighead carp. The principal coordinates analysis (PCoA) with the weighted UniFrac distance and the heatmap analysis suggested that gut microbiota was not a simple reflection of the microbial community in the local habitat but resulted from species-specific selective pressures, possibly dependent on behavioural, immune and metabolic characteristics.

The widely distributed ray-finned fish genus Carassius is very well known due to its unique biological characteristics such as polyploidy, clonality, and/or interspecies hybridization. These biological characteristics have enabled Carassius species to be successfully widespread over relatively short period of evolutionary time. Therefore, this fish model deserves to be the center of attention in the research field. Some studies have already described the Carassius karyotype, but results are inconsistent in the number of morphological categories for individual chromosomes. We investigated three focal species: Carassius auratus, C. carassius and C. gibelio with the aim to describe their standardized diploid karyotypes, and to study their evolutionary relationships using cytogenetic tools. We measured length (q+plength) of each chromosome and calculated centromeric index (i value). We found: (i) The relationship between q+plength and i value showed higher similarity of C. auratus and C. carassius. (ii) The variability of i value within each chromosome expressed by means of the first quartile (Q1) up to the third quartile (Q3) showed higher similarity of C. carassius and C. gibelio. (iii) The fluorescent in situ hybridization (FISH) analysis revealed higher similarity of C. auratus and C. gibelio. (iv) Standardized karyotype formula described using median value (Q2) showed differentiation among all investigated species: C. auratus had 24 metacentric (m), 40 submetacentric (sm), 2 subtelocentric (st), 2 acrocentric (a) and 32 telocentric (T) chromosomes (24m+40sm+2st+2a+32T); C. carassius: 16m+34sm+8st+42T; and C. gibelio: 16m+22sm+10st+2a+50T. (v) We developed R scripts applicable for the description of standardized karyotype for any other species. The diverse results indicated unprecedented complex genomic and chromosomal architecture in the genus Carassius probably influenced by its unique biological characteristics which make the study of evolutionary relationships more difficult than it has been originally postulated.

The establishment of introduced fishes can be inhibited by the biotic resistance from species in the receiving environment, including strong consumptive resistance from specific piscivorous fishes. In response to predation pressure, prey fish population responses include predator-induced morphological changes, where an extreme example is seen in the crucian carp Carassius carassius, which forms deep-bodied morphs in predator presence that reduces individual predation risk. As its congener, gibel carp Carassius gibelio is a highly invasive fish across in its non-native range in Eurasia. Here we test whether their introduced populations also respond to the presence of piscivorous fishes by altering their body shape and trophic ecology by testing differences across 16 non-native lentic populations in Turkey that provided groups of piscivorous fish presence versus absence. In piscivore presence, gibel carp had a higher ratio of body length-to-depth than in piscivore absence, but with their body condition factor being higher in absence. Stable isotope mixing models predicted that gibel carp had diets that were more animal based (gastropods and zooplankton) in piscivore absence, but plant based in piscivore presence. Moreover, diet predictions of piscivore diet suggested gibel carp were consistently consumed less than other prey fishes. These results suggest that these alien gibel carp were responding to piscivory as per crucian carp, reducing their predation risk at the individual level by forming deep-bodied morphs. We suggest these morphological responses then decrease the strength of the biotic resistance against their invasion at the population level.

The crucian carp ( Carassius carassius ) is a cyprinid fish with its natural distribution in Europe and the western part of Asia. Due to its hardiness and unique ability to survive winter anoxia, it has been translocated to small lakes and ponds, and in Northern Europe since medieval times has been used as a food source. Crucian carp was the only fish in the pond that survived anoxia. Small lakes and ponds with winter anoxia result in dense populations of stunted and slender fish. In lakes with other fish species present, the crucians’ numbers and densities are low and they grow to large sizes. In the presence of piscivores such as pike, crucians are deep bodied. The presence of pike-eating crucians, or the pike odors, induce a change in the body form of crucians. The change in body form makes it more difficult for pikes to swallow crucians and the handling time increases. Closely related invasive Carassius species have become a serious threat to crucian carp populations in Central-Eastern Europe and South-Eastern England through competition for space and food resources and hybridization. The crucian carp’s close relationship to goldfish ( Carassius auratus ), the most studied species concerning sex pheromones, has made it possible to demonstrate that sex pheromones are also present in a wild Carassius species and in their natural environment. The results indicate that two species use the same sex hormonal pheromone system. The crucian carp has become an important model for laboratory studies of olfaction and taste.

Without oxygen, most vertebrates die within minutes as they cannot meet cellular energy demands with anaerobic metabolism. However, fish of the genus Carassius (crucian carp and goldfish) have evolved a specialized metabolic system that allows them to survive prolonged periods without oxygen by producing ethanol as their metabolic end-product. Here we show that this has been made possible by the evolution of a pyruvate decarboxylase, analogous to that in brewer’s yeast and the first described in vertebrates, in addition to a specialized alcohol dehydrogenase. Whole-genome duplication events have provided additional gene copies of the pyruvate dehydrogenase multienzyme complex that have evolved into a pyruvate decarboxylase, while other copies retained the essential function of the parent enzymes. We reveal the key molecular substitution in duplicated pyruvate dehydrogenase genes that underpins one of the most extreme hypoxic survival strategies among vertebrates and that is highly deleterious in humans.

Background An important aspect of studying evolution is to understand how new species are formed and their uniqueness is maintained. Hybridization can lead to the formation of new species through reorganization of the adaptive system and significant changes in phenotype. Interestingly, eight stable strains of 2nNCRC derived from interspecies hybridization have been established in our laboratory. To examine the phylogeographical pattern of the widely distributed genus Carassius across Eurasia and investigate the possible homoploid hybrid origin of the Carassius auratus complex lineage in light of past climatic events, the mitochondrial genome (mtDNA) and one nuclear DNA were used to reconstruct the phylogenetic relationship between the C. auratus complex and 2nNCRC and to assess how demographic history, dispersal and barriers to gene flow have led to the current distribution of the C. auratus complex. Results As expected, 2nNCRC had a very close relationship with the C. auratus complex and similar morphological characteristics to those of the C. auratus complex, which is genetically distinct from the other three species of Carassius . The estimation of divergence time and ancestral state demonstrated that the C. auratus complex possibly originated from the Yangtze River basin in China. There were seven sublineages of the C. auratus complex across Eurasia and at least four mtDNA lineages endemic to particular geographical regions in China. The primary colonization route from China to Mongolia and the Far East (Russia) occurred during the Late Pliocene, and the diversification of other sublineages of the C. auratus complex specifically coincided with the interglacial stage during the Early and Mid-Pleistocene in China. Conclusion Our results support the origin of the C. auratus complex in China, and its wide distribution across Eurasia was mainly due to natural Pleistocene dispersal and recent anthropogenic translocation. The sympatric distribution of the ancestral area for both parents of 2nNCRC and the C. auratus complex, as well as the significant changes in the structure of pharyngeal teeth and morphological characteristics between 2nNCRC and its parents, imply that homoploid hybrid speciation (HHS) for C. auratus could likely have occurred in nature. The diversification pattern indicated an independent evolutionary history of the C. auratus complex, which was not separated from the most recent common ancestor of C. carassius or C. cuvieri . Considering that the paleoclimate oscillation and the development of an eastward-flowing drainage system during the Pliocene and Pleistocene in China provided an opportunity for hybridization between divergent lineages, the formation of 2nNCRC in our laboratory could be a good candidate for explaining the HHS of C. auratus in nature.

The introduction of invasive species increases interspecific competition with native species, especially if the invasive fish have a similar ecological role in the ecosystem. The vacant niche hypothesis postulates that an invasive species may be a stronger competitor if it has, additionally to a native species niche, access to a food unavailable to native species. However, there are very few model examples of nearly identical invasive and native species differing in trophic niche utilization. The once common crucian carp (Carassius carassius) has become endangered or extirpated in many regions of Europe mainly due to the invasion of gibel carp (C. gibelio). To estimate the trophic niche divergence between gibel and crucian carp living in syntopy, a non-lethal method of stable isotope analysis (SIA) of fish scales (δ13C and δ15N) was employed. Samples were collected from four sites in the Czech Republic to determine the overlap and sizes of the trophic niches of these two species. The results showed that at two sites, gibel carp had significantly lower δ15N than crucian carp, indicating its lower trophic position. The gibel carp also significantly higher δ13C at two sites indicating higher utilization of littoral sources compared to crucian carp. In addition, isotopic niches partially overlapped at the four study sites, with the most divergent trophic niches found in the macrophyte-rich site. Finally, the gibel carp had higher probability to occur within the crucian carp niche space than vice versa. Our results provide support for the vacant niche hypothesis, indicating that invasive gibel carp gain a competitive advantage over the native crucian carp via feeding on plant material that is underexploited by native crucian carp. Furthermore, data suggest that more “natural” environmental conditions, such as a rich littoral zone, may help to decrease isotopic niche overlap between the two species. Lower trophic position and higher reliance on unexploited food sources seem to contribute to the competitive superiority of the invasive gibel carp over the native crucian carp.

A strain of Clostridium butyricum (Cb) was isolated and characterized as an immunostimulant for gibel carp. Healthy gibel carp were fed and immersed with or without Cb (group Cb + V and group V), and then were intraperitoneally injected with Carassius auratus herpesvirus (CaHV) respectively. The fish began to die at 4 days post injection (dpi). The overall survival rate of group Cb + V (45%) is more than that of group V (11%), which indicated the significant protection effect of Cb. The RT-qPCR results showed that virus major capsid protein gene (MCP) copy numbers of group Cb + V was significantly lower (P < 0.05) than that of group V at 3 dpi. Meanwhile, the expression levels of the four host innate immunity-related genes (IL11, IRF7, PKR, and Mx) were significantly higher (P < 0.01) in group Cb + V than in group V at this time point. IRF7 and PKR also had significantly higher expression levels in group Cb + V than in group V at 0 dpi, which revealed enhanced immune responses in Cb-treated fish. Treatment with Cb prompted stronger host immune responses at the early stage of virus infection, and then resulted in the survival of gibel carp. To our knowledge, it is the first report about the immune protection of fish from herpesvirus by probiotics.

The analysis of present-day crucians’ settlements in water systems of Eastern Ukraine designated the predominance of the digeneous Goldfish, C. auratus, in the region, the number of which made 78.7 %, from the total number of the examined representatives of the genus. The second group consists of gynogenetic Prussian carps, C. gibelio (14.3 %); it is represented by the clone biotype and recombinant individuals. Crucian carp, C. carassius (3.6 %), turned out to be rare and its number did not exceed the number of the caught hybrids C. auratus × C. carassius (3.4 %). The retrospective analysis of literature data and museum collections gave an opportunity to describe the changes in species composition of the genus which took place during the last 150 years. Within this period the crucian carp, which used to be the single and most common representative of genus Carassius (Jarocki, 1822) in the region, became nearly an extinct species. In the meanwhile the representatives of the group of species of Prussian carps, C. auratus + C. gibelio, which appeared in the region in the late 1960s, rapidly increased their number and became the most numerous fish of the Eastern Ukraine. The discovered tendency is not unique for the researched region; in general it reflects the European tendency for the crucian species. The reasons for that are rivers’ regulation and destruction of bottomland ecosystems. The secondary factors for the elimination of C. carassius are the competitive relations of individuals representing both species and easy hybridization, during which the more numerous species C. auratus absorbs the rare C. carassius.