A new species of Euborlasia Vaillant, 1890 (Nemertea: Pilidiophora) from Southwestern Atlantic Ocean

The genus Euborlasia comprises relatively large ribbon worms, typically recognized by their distinctive mottled, dark body coloration, often ornamented with thin rings encircling the body, and a pale cephalic tip. In this study, we describe Euborlasia lidiae sp. nov., a new species from Patagonia, Argentina, representing the first record of this genus in the Southwestern Atlantic Ocean. The newly described species is primarily distinguished from its congeners by its unique body coloration. It exhibits an anterior-posterior gradient in background coloration, ranging from whitish to creamy pink, mottled with olive-brown dots that become less dense towards the anterior end, with the tip of the head remaining whitish. Ventrally, the density of the dots gradually decreases towards the posterior end, where only the creamy pink background remains. Additionally, it possesses a proboscis with a single layer of longitudinal musculature, and its cutis is separated from the outer longitudinal muscle layer of the body wall by connective tissue. Phylogenetic analyses based on partial mitochondrial 16S rRNA and cytochrome c oxidase subunit I (COI) gene sequences indicate that E. lidiae sp. nov. forms a clade with three other Euborlasia species and Corsoua takakurai. Furthermore, we provide a revised diagnosis of the genus Euborlasia. We propose that the diagonal musculature located between the outer longitudinal and circular muscle layers of the body wall, along with the nerve plexus situated between the layers of this musculature, represent synapomorphies of the heteronemertean Lineage O.