Non-native fish invasions are among the greatest threats to the sustainability of freshwater ecosystems worldwide. Tilapia and catfish are regularly cultured in South China which is similar to their climate in native areas and may also support their invasive potential. We systematically collected fish from eight main rivers of South China, from 2016 to 2018, to investigate and analyse species’ composition and the distribution of non-native fishes. The data reveal that non-native fishes are widespread and abundant in the sampled rivers: of the 98,887 fish collected, 11,832 individuals representing 20 species were not native, which were distributed in the 96% sampled sites. Of the non-native fish species, 17 are used in aquaculture and 19 are native to the tropics; 13 are omnivores while the other seven are predators. Based on dissimilarity of the non-native fish species distributions across the eight rivers, the different rivers could be divided into four assemblages. Geographical isolation and temperature were identified as affecting the distribution patterns of non-native fishes, thereby influencing fish species composition, species number, dominant species, and distribution variations in the South China rivers. Species composition of the non-native fishes in these rivers are related to their introduction vector, compatibility with their native habitat, and feeding strategies. Their distribution was mainly influenced by geographical location and temperature. To mitigate the impacts of non-native fish, a series of stricter management practices, systematic monitoring, and more research are needed.

Background: Mitochondrial (mt) genome sequences are widely used for species identification and to study the phylogenetic relationships among Gastropoda. However, to date, limited data are available as taxon sampling is narrow. In this study we sequenced the complete mt genomes of the freshwater gastropods Radix swinhoei (Lymnaeidae) and Planorbarius corneus (Planorbidae). Based on these sequences, we investigated the gene rearrangement in these two species and the relationships with respect to the ancestral gene order and assessed their phylogenetic relationships. Methods: The complete mt genomes of R. swinhoei and P. corneus were sequenced using Illumina-based paired-end sequencing and annotated by comparing the sequence information with that of related gastropod species. Putative models of mitochondrial gene rearrangements were predicted for both R. swinhoei and P. corneus , using Reishia clavigera mtDNA structure as the ancestral gene order. The phylogenetic relationships were inferred using thirteen protein sequences based on Maximum likelihood and Bayesian inference analyses. Results: The complete circular mt genome sequences of R. swinhoei and P. corneus were 14,241 bp and 13,687 bp in length, respectively. Comparison of the gene order demonstrated complex rearrangement events in Gastropoda, both for tRNA genes and protein-coding genes. The phylogenetic analyses showed that the family Lymnaeidae was more closely related to the family Planorbidae, consistent with previous classification. Nevertheless, due to the position recovered for R. swinhoei , the family Lymnaeidae was not monophyletic. Conclusion: This study provides the complete mt genomes of two freshwater snails, which will aid the development of useful molecular markers for epidemiological, ecological and phylogenetic studies. Additionally, the predicted models for mt gene rearrangement might provide novel insights into mt genome evolution in gastropods.

This study was conducted to investigate the effects of replacing fish meal with either whole-fat or defatted krill powder on the growth, body color, immunity, and related gene expression of red–white koi carp. A total of 630 red–white koi carp with an initial body mass of 13.5 ± 0.05 g were randomly divided into seven groups with three replicates per group and 30 fish per replicate. The control group was fed a basic diet (C0). The other six diets were supplemented with different levels of whole krill meal or defatted krill meal as replacements (10% whole fat, 20% whole fat, 30% whole fat, 10% defatted, 20% defatted, and 30% defatted) in the experimental groups, named W10, W20, W30, D10, D20, and D30, respectively, for a total duration of 60 days. The growth, body color, immunity and gene expression indexes were measured in the koi after completion. The results indicate the following. (1) Compared with C0, the experimental groups of koi showed a significant increase in the specific growth rate (SGR) (p < 0.05), while the hepatosomatic index (HSI) and viscerosomatic ratio (VSI) decreased. Additionally, there was a significant increase in the relative expression level of insulin-like growth factor (IGF-1) in both the liver and muscle (p < 0.05). (2) The experimental groups of koi carp exhibited a significant increase in the carotenoid content in the scales and skin, as well as an elevated relative expression level of the tyrosinase (TYR) gene in the muscle (p < 0.05). (3) The lysozyme (LZM) and superoxide dismutase (SOD) activities were significantly increased in the experimental groups compared to C0 (p < 0.05). Additionally, the SOD activity was significantly higher in the defatted groups than in the whole-fat groups (p < 0.05). Furthermore, the liver alkaline phosphatase (AKP) activity was significantly lower in the D20 and D30 groups compared to the other five groups. (4) The expression level of LPL was significantly lower in the liver and muscle of the whole-fat group compared to the defatted group (p < 0.05). Tissue section observation revealed that the hepatocytes in the W20 and W30 groups exhibited a reduced size and an increased lipid droplet count, while the vacuolar degeneration of the hepatocytes increased in the D30 group. In conclusion, replacing fish meal with whole-fat or defatted krill powder or defatted Antarctic krill meal significantly improves the growth performance, body color, and immunity of red–white koi carp. However, excessive addition of krill meal can easily cause liver damage. The recommended replacement level for whole krill powder is 20% to 30%, while defatted krill powder should be replaced at a level of 10% to 20%.