Results of the histopathological study of mantle tissues of the commercial geoduck Panopea abbreviata hosting the nemertean Malacobdella arrokeana revealed that the normal histology of mantle tissues of the bivalve was not altered by the attachment structure of the nemertean, even when the maximum individuals per clam reached 191 nemerteans. However, the vacuum force generated by the nemertean sucker seems to elicit a slight mechanical stretching of epithelial cells and a negligible infiltration response affecting the connective tissue between inner and outer mantle epithelia beneath the point of attachment. The 99.4% bivalves examined (n = 657) hosted at least 1 specimen of M. arrokeana. Adult nemerteans were able to survive outside the host for up to 3 mo at 13°C. These results suggest that the relationship between M. arrokeana and P. abbreviata should be considered as a commensal rather than a parasitic relationship.