Cryptic biodiversity of freshwater fish species in Bangladesh

PLOS One | https://doi.org/10.1371/journal.pone.0318982 April 28, 2025 14 / 18 PLOS One Cryptic biodiversity of freshwater fish 2% divergence in the mitochondrial cytochrome b gene [34] and COX1 gene for conspecific genetic divergence in fish [35]. Fouquet et al. [36] demonstrated that a 3% cutoff value for 16S mtDNA yielded a greater number of potential neotropical frog species. In this work, intra- specific genetic divergence exceeds the minimum intraspecific distance of 3% (the COI gene), which is higher than the threshold value of 2% for fish species identification via DNA barcod- ing [32,37]. Based on the results of this biodiversity study, it is tempting to make conjectures about the extent of unexplored freshwater fish diversity that still exists in Bangladesh. There might still be several undiscovered cryptic species in Bangladesh. Our investigation involving the analysis of the COI gene has revealed the presence of seven likely cryptic species. These species are genetically distinct from their closely related counterparts. Ailia sp. is often wrongly identified as its closely related species, Ailia coila. Our preliminary morphological (Table 2) and genetic findings indicate that Ailia sp. is cryptic and necessitates further investigation, utilizing multiple datasets, to establish its classification as a new species. Other genera, such as Awaous, Lepidocephalichthys, Ophichthys, Erethistes, Schistura, and Cynoglossus, contain cryptic species

Fig 4. Maximum Likelihood (ML) tree of freshwater species of Bangladesh based on nucleotide sequences of COI gene (partial portion of S1 Fig). https://doi.org/10.1371/journal.pone.0318982.g004