Microbiology and Biotechnology Research: An Overview Vol. 12

Fish-parasitic isopods present an unusually demanding taxonomic problem because environmentally responsive morphology, sexual and ontogenetic transformations, abbreviated historical descriptions and life cycles that disconnect parasitic and diagnostic stages can all obscure species boundaries. This critical narrative review evaluates how molecular taxonomy and DNA barcoding have changed species identification, life-stage matching and recognition of cryptic diversity, with emphasis on Cymothoidae and Gnathiidae. Literature published from 2000 to 10 July 2026 was searched in multidisciplinary, biological and agricultural scholarly sources, supplemented by taxonomic and sequence-reference resources and by backward and forward citation searching; foundational earlier studies were retained where necessary. The reviewed evidence indicates that mitochondrial cytochrome c oxidase subunit I (COI) has become the principal identification marker for cymothoids, while 16S ribosomal RNA and, less frequently, nuclear markers such as 18S ribosomal RNA and internal transcribed spacer 2 provide complementary resolution. Molecular evidence has exposed substantial overlooked diversity in morphologically conservative groups, most convincingly within Anilocra, and has corrected several morphology-based identifications. In gnathiids, DNA data have been especially transformative because adult males, females and blood-feeding juveniles differ so markedly that rearing or molecular matching is often required to connect life stages. Yet the literature remains constrained by single-locus designs, small and geographically restricted samples, incomplete voucher-to-sequence linkage, sparse representation of valid species in reference libraries and occasional discordance among database labels. Consequently, barcode divergence alone is insufficient evidence for a new species, and nearest-neighbour matches can reproduce rather than resolve taxonomic error. The strongest current practice is integrative taxonomy centred on expert morphology, deposited vouchers, explicit host and locality data, multiple specimens, at least one complementary locus where feasible and critical comparison with type material or reliably identified topotypic material. A transition from "barcode confirmation" to voucher-centred, hypothesis-driven molecular taxonomy is required if molecular data are to reveal rather than merely relabel cryptic diversity.