Metazoan parasites and pearls in coexisting mussel species: Mytilus californianus, Mytilus galloprovincialis, and Septifer bifurcatus, from an exposed rocky shore in Baja California, Northwestern Mexico

Metazoan parasites, Modiolicola gracilis Wilson (Copepoda), Pseudomyicola spinosus Rafaelle & Monticelli (Copepoda), Urastoma cyprinae Graff (Platyhelminthes), unidentified encysted trematode cercaria, and pearls were found in coexisting mussel species: Mytilus californianus Conrad, Mytilus galloprovincialis Lamarck, and Septifer bifurcatus (Conrad) from the upper intertidal zone of an exposed rocky shore in Baja California, northwestern Mexico. Incidence of parasites and pearls was greater in M. californianus, the largest species of mussels examined, than in the other mytilids. The lowest incidence of parasites and pearls was in S. bifurcatus, the smallest species of mussels examined. The highest parasite prevalence coincided with the autumn and winter months when M. californianus and M. galloprovincialis had a lower condition index and were reproductively active. All parasites produced histological alterations in their hosts; a hemocytic reaction and compression of tissues were commonly observed. In spite of M. californianus being the most parasitized species, it is the dominant component in the upper intertidal zone. These results suggest that factors specific to the infesting metazoan parasites in M. galloprovincialis and S. bifurcatus reduce their competitiveness capacity against M. californianus.