On Anguillosyllis cf. hessleri Maciolek, 2020 – A species complex from the Clarion-Clipperton zone, abyssal central Pacific

The benthic annelid fauna of polymetallic nodule fields in the eastern Clarion-Clipperton Zone (CCZ) has recently been the subject of taxonomic investigations aiming to document the biodiversity of this region. While investigating the annelids, a particularly high diversity was discovered within the genus Anguillosyllis Day, 1963 using a combination of morphological and molecular approaches. Initially, 38 specimens of Anguillosyllis were assigned to the known CCZ species Anguillosyllis hessleri Maciolek, 2020 based on morphological examination, including examination of the type specimen. However, our genetic data revealed that these specimens represent four distinct lineages (putative species) based on COI and 16S markers. Further morphological investigation using SEM revealed a complex structure of the prostomium, which could potentially differentiate these lineages. However, as no molecular or SEM data are available from the type specimen of A. hessleri , we cannot ascertain which, if any, of these lineages represent the known species described by Maciolek (2020). Thus, currently, we can only assign CCZ specimens to Anguillosyllis cf. hessleri and further distinguish the lineages by assigning each a unique code in the form of “NHM_####”. The SEM observations are the first reported for Anguillosyllis , revealing new taxonomic characters that may be used to disentangle the A. hessleri species complex and be of use in taxonomy of Anguillosyllis in general. • High genetic diversity of Anguillosyllis Day, 1963. • Evidence for CCZ species A. hessleri Maciolek (2020) to represent a species complex. • Novel observations and Anguillosyllis characters obtained by SEM.