The spatial distribution and coexistence of monogenean dactylogyrids was assessed on the gills of 63 specimens of wild spotted rose snapper, Lutjanus guttatus (Steindachner), caught in the Mazatlan Bay, Sinaloa, Mexico. Five species are reported: Euryhaliotrema perezponcei García-Vargas, Fajer-Ávila et Lamothe-Argumedo, 2008, Euryhaliotrematoides sp., Haliotrematoides spinatus Kritsky et Mendoza-Franco, 2009, H. plectridium Kritsky et Mendoza-Franco, 2009, and H. guttati García-Vargas, Fajer-Ávila et Lamothe-Argumedo, 2008. All except E. perezponcei and H. guttati represent new geographical records for the Pacific coast. The most prevalent dactylogyrid species was E. perezponcei (100%), H. plectridium and H. spinatus had >80% prevalence, and H. guttati and Euryhaliotrematoides sp. had the lowest prevalence. The mean abundance of H. plectridium and E. perezponcei was close to 60 parasites/fish, whereas Euryhaliotrematoides sp. and H. guttati had the lowest abundance. The dactylogyrid species exhibited a tendency for attachment to gill arch 2: 25% attachment occurring on gill arch 1, 30% on 2, 27% on 3 and 18% on 4, and showed a significant preference for the central sector of the gill (42%). Haliotrematoides plectridium had a preference for attachment to gill arches 2 and 3 and the central sector. Haliotrematoides spinatus tended to settle on the gill arches 2 and 3 and had a preference for the central sector. Euryhaliotrema perezponcei tended to settle on the gill arches 1 and 2 and the anterior gill sector. Euryhaliotrematoides sp. and H. guttati did not show a preference for any gill arch or sector. The intraspecific aggregation was stronger than the interspecific aggregation, indicating that all the dactylogyrid species on spotted rose snapper were aggregated, and there was no evidence of competition among the species.
Based on light and scanning electron microscopical studies, three new gonad-infecting species of Philometra Costa, 1845 (Nematoda: Philometridae) are described from marine fishes of the genus Lutjanus Bloch (Perciformes: Lutjanidae) in the northern Gulf of Mexico: P. longispicula sp. n. from the ovary of the northern red snapper L. campechanus (Poey) (type host) and silk snapper L. vivanus (Cuvier); P. latispicula sp. n. from the ovary and rarely testes of the grey snapper L. griseus (Linnaeus); and P. synagridis sp. n. (only males available) from the ovary of the lane snapper Lutjanus synagris (Linnaeus). These species are mainly characterised by the lengths of spicules (378-690 µm, 135-144 µm and 186-219 µm, respectively) and spicule shapes, structure of the distal portion of the gubernaculum and the structure of the male caudal end. These are the first valid, nominal species of gonad-infecting philometrids reported from fishes of the family Lutjanidae in the western Atlantic region.