This study provides the first case reported of Paraprionospio treadwelli (Hartman, 1951) in the Gulf of Mexico. Based on 242 individuals collected between 20.8 and 176 m depth during three oceanographic expeditions, we describe in detail the morphology of the identified specimens, including the description of the pygidium, so far unknown in this species, and provide SEM photographs to support their identification. Paraprionospio treadwelli was originally found in Chesapeake Bay, Northwestern Atlantic, and we now extend its distribution southwards to the Western Gulf of Mexico. Remarks on the environmental conditions where this spionid species was found and the observed abundance seasonal pattern are also provided.
