A new species of sclerobiont polychaete (Polychaeta, Nereididae) is described from Mazatlán, Sinaloa, Mexico.
Pseudonereis deleoni sp. nov. is unique among the 10 worldwide valid species in the genus by having a set of remarkable features: shape and distribution of paragnaths, dorsal cirrus length and position, presence of lobes on neuropodia, development of ventral ligule, and type and distribution of chaetae.
Pseudonereis deleoni sp. nov. inhabits in metallic buoys (channel markers), temperature between 20.8 and 31.5°C, salinity of 25.6-34.6 ups and dissolved oxygen 3.1-6.6 mg/L. A taxonomic key for species of
Pseudonereis from the world is provided.