Cryptic biodiversity of freshwater fish species in Bangladesh

PLOS One | https://doi.org/10.1371/journal.pone.0318982 April 28, 2025 12 / 18 PLOS One Cryptic biodiversity of freshwater fish Erethistes, exhibited a genetic variation of more than 5.9% from the most closely related E. pusilus (Fig 2). This specimen was collected from the Netrokona region. Similarly, Schistura sp. under the family Nemacheilidae (collected from Sakal Bazar, Jamalpur),belonging to the genus Schistura demonstrated a genetic variation of more than 9.2% from its closest related species Schistura corica (Fig 3). Two specimens of genus Cynoglossus, collected from Khulna and Chittagong close to the coastal belt in Bangladesh, identified as Cynoglossus sp. (Cyno- glossidae) were found to be distantly related (COI gene divergence 17.52%) with their conge- ners Cynoglossus (MK572284) (Fig 4).The graphic quality of seven cryptic species is subpar, yet we have included images here for enhanced clarification and comprehension (see Fig 5). Discussion Bangladesh is home to 265 [4] or 293 freshwater species, including coastal fishes, since no physiological alterations transpire during their journey [27]. Rahman et al. [14] collected 694 specimens nationwide, representing 243 species-level Operational Barcode Units (OBUs), although our sampled specimens are fewer than their findings. The absence of anticipated spe- cies in our aquatic environments was one of the problems we faced throughout the sampling procedure. The current sample size is extensive and includes the majority of available species; nonetheless, it is likely incomplete.

Our research identified seven cryptic fish species in the northeastern areas of Bangladesh and has contributed to a more thorough method of clarifying species delimitation based on COI gene. Although Rahman et al. [14] developed the DNA barcoding system, our work largely differs from their work with few overlapping aspects due to the study’s inadequate cov- erage of Bangladesh’s western and southwestern regions, primarily the Sundarbans and a fig of river Ganga and its tributaries. We attempted to cover the Sherpur region and some selected areas of the Mymensingh region as well as the exclusive region of the Old Brahmaputra River, which is included in the Ganga tributaries. Although specimens from the Khulna (Sundarban) area are limited, we have included samples from this location (see Table 1). Despite the several investigation based on molecular data [14,28,29], the presence of overlooked species of fresh- water fishes in this study indicates that the hidden diversity among well-known and widely distributed fish species in Bangladesh might be more prevalent than previously believed. Despite being a low-lying country, Bangladesh’s robust river systems impede gene flow and affect the population structure of Hoplobatarchus tigerinus frogs [30]. Further, both intrinsic species characteristics, such as dispersal capability and habitat specialization, and extrinsic environmental factors, such as physical obstructions and geographic separation, often deter- mine the level of population connectivity and gene flow in the freshwater fishes [31].

The occurrence of cryptic, spatially dictated variety in fish species in Bangladesh, as indicated in this study, is not a recent revelation. Hasan et al. [2] examined the significant cryptic anuran biodiversity in Bangladesh utilizing 16S mitochondrial DNA (mtDNA); however, Raman et al. [14] contended the taxonomic ambiguity and advocated for the use of DNA barcoding (COI sequence) for the taxonomic revision of Bangladeshi freshwater fish species. We employed the COI gene in conjunction with restricted morphological data to identify the cryptic species. Identifying species merely based on physical features may prove challenging, especially when their phenotypes exhibit considerable variation [32]. Moreover, precise taxonomic identification is sometimes unattainable due to the reliance on species identification keys, which are often effective only at specific life stages. Researchers have used DNA as an alternative diagnostic tool for species, regardless of the integrated taxonomic approach [14,33]. Generally closely related vertebrate species consistently demonstrate over