The boreal lysianassid P. nobilis is associated with very well sorted, fine sand of low intertidal and shallow subtidal habitats. It is a winter to early spring breeder, with recruitment from June-Aug. in New England. Females produce up to 50 eggs and have 1 brood/breeding period. Generation time is 2 yr. Larger adults occur only during colder months at more exposed intertidal habitats, while all life stages occur at sheltered intertidal habitats throughout the year. Females are larger than males and are more abundant at the landward periphery of intertidal populations. Subtidal populations show smaller adults and juveniles present most of the year. The species is a scavenger-omnivore exhibiting an aggregated pattern of distribution. Swimming activity is highest in the dark during falling tides. Respiration increases significantly at salinities < 20.permill. The length of pereopod 7 was the sole sexually dimorphic character observed. The heretofore enigmatic, elongate gnathopod 2 is not used in precopula, but for removing organic debris from the thoracic cavity.