<i>Clarias batrachus</i> and Its Cestode Tenants – A Hub for Many Species or Spectacles to Clear Illusion

The diversity of unsegmented freshwater cestodes (Order Caryophyllidea) remains less explored. Globally, 121 valid species across 42 genera are known, whereas India alone has reported over 100 species. Many of these are endemic and new, such as Mystocestus anindoi from Mystus catfishes, and some hold significance in host biogeography and evolution (e.g., Paracaryophyllaeus lepidocephali, the only Indomalayan representative showing vicariance). However, a large proportion of Indian “new taxa” have been described from Clarias batrachus (Linnaeus, 1758), a widely available catfish species. Many of these taxa were later invalidated for being indistinguishable with existing species or non-compliance with the International Code of Zoological Nomenclature. This study investigates whether many cestodes reported from C. batrachus represent distinct species or merely exhibit fixative-induced morphological variations. Specimens from C. batrachus were freshly collected, fixed using different fixatives, and analysed with molecular data (hologenophores) to assess their taxonomic validity. Literature data were evaluated to assess the validity of previously described taxa from C. batrachus. Significant morphological variations were observed across different fixatives. However, molecular data confirmed their conspecificity. Although more than 100 new taxa were reported from C. batrachus, only 7 considered as valid. Helminth taxonomy remains in its early stages, with unresolved issues concerning species complexes, cryptic species, generic classification, biogeography, and life cycles. This study underscores the importance of selecting appropriate fixatives and fixation methods for accurate species identification. The most important steps are to follow previously published data and comply with ICZN rules.