A histology-free description of Lineus marcosii (Pilidiophora: Lineidae), a cryptic species resembling Lineus lacteus

Cryptic biodiversity is one of the main challenges to overcome for an accurate characterization of Earth’s biodiversity. The nemertean species Lineus lacteus has been recorded from Sweden to the Black Sea since its description in the first half of the 19th century. However, molecular data suggest that it actually represents a cryptic species complex formed by at least two species. Here, we reanalyze the molecular information for both lineages and provide new sequences from the Cantabrian Sea. As a result, we describe the Mediterranean and Cantabrian lineage as a new species, Lineus marcosii sp. nov., based on newly collected material. Because no morphological differences were observed between the two species, diagnostic characters were searched for in all available sequences of the cytochrome c oxidase subunit I gene. Moreover, new knowledge on its trophic ecology, parasites, and behavior is provided based on opportunistic observations. There is no known overlap in the geographic distributions of L. lacteus and L. marcosii sp. nov. Thus, these two species are considered an allopatric cryptic species complex, and their entire distributions are inferred from the available molecularly confirmed records.