Genomic evidence reveals a record-breaking range in Tubulanus lutescens (Nemertea) from shallow coasts to deep-sea seeps

Morphological and low-coverage genomic data reveal that the shallow-water Swedish ribbon worm Tubulanus lutescens is conspecific with nemerteans collected from bathyal methane seeps along the Costa Rican margin, thus spanning two ocean basins and environmental extremes. Of approximately 1350 described nemertean species, only approximately 35 are known from below 500 m, and only six species are genetically confirmed as eurybathic. In 2022, we reported the Costa Rican material but withheld a species description because its cytochrome c oxidase subunit I (COI) closely matched that of T. lutescens. Here, we re-examined serial sections of the holotype plus its COI vouchers and generated mitochondrial genomes and hundreds of single-copy nuclear orthologues from one Costa Rican and one Swedish specimen using low-coverage whole-genome sequencing. Morphological and genome-scale comparisons support a single species spanning shallow to bathyal habitats across both the Atlantic and Pacific. This establishes T. lutescens as the first confirmed eurybathic palaeonemertean, exhibiting the widest geographic and bathymetric range among nemerteans and an exceptional range even among benthic marine invertebrates. Because one population inhabits a deep-sea seep, anthropogenic dispersal is unlikely. This unexpected conspecificity, despite increased genomic resolution that often reveals cryptic diversity, highlights how much remains to be discovered about marine invertebrate distributions.