Cryptic species across deep-sea habitats: a case study on two sympatric cryptic complexes of Gymnonereidinae (Annelida: Nereididae)

Reports of cryptic species have increased in recent decades, driven by advances in molecular studies and accessibility of DNA sequencing, resulting in integrative taxonomy as the standard for species description. This is especially evident in environments where sampling is difficult, such as the deep-sea. Specimens belonging to the subfamily Gymnonereidinae were collected from various deep-water habitats in Brazil. Initial morphological analysis divided them into two morphotypes belonging to the genera Micronereides and Ceratocephale. However, species delimitation using the COI genetic marker revealed the presence of seven distinct species, suggesting the existence of cryptic species complexes within each morphotype, where species are morphologically indistinguishable. Three species recovered in the analyses belong to the genus Micronereides and are morphologically indistinguishable from the type species Micronereides capensis, previously reported in Brazilian deep waters. The lack of molecular data on the type M. capensis hampers the description of these new species. We, therefore, suggest the existence of the Micronereides capensis cryptic complex. Similarly, the Ceratocephale cryptica cryptic complex encompasses the remaining four species identified in this study. Among them, only Ceratocephale cryptica sp. nov. is formally described here. This study provides strong support for the validity of the genus Micronereides, which was questioned in previous studies. Cryptic speciation processes are further discussed, comparing evolutionary divergence rates of the different genetic markers used for phylogenetic inference. These results highlight the challenges of deep-sea species delimitation, calling for revised biodiversity assessments and more complete genetic databases, especially including type species sequences, and at the same time reinforcing the critical role of integrative taxonomy in revealing cryptic diversity. ZooBank: urn:lsid:zoobank.org:pub:8FFA8C17-DFF5-44D3-B343-ABD8E45C62CD.