When cryptogenic species are also cryptic: reframing biogeographic uncertainty in the environmental DNA era

Distinguishing native from non-native species is complicated by cryptogenic taxa, whose native or non-native status remains uncertain, and cryptic species, which are genetically distinct yet morphologically indistinguishable taxa. These challenges can intersect when molecular tools uncover hidden diversity and inadvertently confound efforts to trace species origins. In marine systems, environmental DNA (eDNA) has transformed biodiversity monitoring, but reliance on short, single-locus markers makes it vulnerable to false-positive detections of cryptic sibling species with markers lacking sufficient taxonomic resolution. Extending previous investigations into the occurrence and spread of Chondria tumulosa, we conducted eDNA screening in multiple island regions to evaluate possible range connectivity. A quantitative polymerase chain reaction assay developed from a Northwestern Hawaiian Islands barcode produced a novel eDNA detection at Majuro in the Marshall Islands. However, subsequent multilocus sequencing (cytochrome c oxidase subunit I, ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit, 18S small subunit ribosomal RNA, Universal Plastid Amplicon) of collected material revealed consistent sequence divergence, suggesting independently evolving lineages. Our findings suggest that the Hawaiian C. tumulosa and the Majuro lineage are sibling taxa with a broader Pacific distribution, shaped by historical allopatry and possible human-mediated dispersal. This case highlights the need to confirm eDNA matches using informative multilocus barcoding regions and emphasizes the need to integrate genetic, morphological and ecological data to resolve the identity and origins of cryptogenic taxa.