International Journal of Aquaculture, 2025, Vol.15, No.3, 116-134 http://www.aquapublisher.com/index.php/ija 127 variation. This is consistent with the results of the global analysis mentioned above, indicating that although there is a genetic structure between the upper and lower Mekong marble goby, it has not yet reached the level of species differentiation. Figure 3 Phylogenetic tree of marble goby (Adapted from Syaifudin et al., 2021) To verify this, we constructed a neighbor-joining tree and haplotype network for the COI barcode sequences. The results showed that the COI haplotype of the upstream population was different from that of the downstream population, but the two were connected by several intermediate types and did not completely break into independent networks. This means that genetic exchange may not be completely interrupted geographically, or there was a connection in history. For example, it is possible that during certain flood periods, fish from upstream have the opportunity to migrate downstream, and vice versa, thus maintaining a certain degree of genetic continuity. This speculation is also consistent with the overall pattern of fish in the Mekong River - although the fish fauna in different sections of the Mekong River are different, there are still a large number of shared species (Fu et al., 2021). Despite this, we still found some clues with species-level significance in the data. In the samples from Tonle Sap Lake in Cambodia, the COI sequences of two individuals were quite different from those of other marble goby, with a genetic distance of more than 2.5%. At first, we suspected that it might be a sequencing or identification error, but after repeated confirmation, the two sequences were not contaminated, and the morphology of the corresponding individuals was also highly similar to that of marble goby. Further comparison of the two sequences with the database revealed that they had a high similarity with the sequences of another Channa fish in Southeast Asia, suggesting that this might not be the cloud-spotted sharp-spined tang, but a previously unrecognized closely related species. Combined with morphological observations, the number of dorsal fin spines and the arrangement of body side stripes of these two individuals were slightly different from those of typical marble goby. We speculate that they may represent a new species that has not been formally described (Adamson et al., 2019). Due to limited samples, we have not yet named and published it, but this suggests that species diversity may be hidden in the marble goby community in the Mekong region (Wang et al., 2021).
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