Genetic evidence for the microphallid trematode Maritrema gratiosum in the Northeast Pacific and an overview of its global distribution.

The trematode Maritrema gratiosum Nicoll, 1907 has a heteroxenous life cycle comprising a marine gastropod as its first intermediate host, a barnacle as its second intermediate host, and a bird as its definitive host. Globally, the species is widely distributed from subtropical waters of the South China Sea to arctic waters of the Barents Sea. In European waters (namely, western Europe and the Barents Sea), the complete life cycle of M. gratiosum is known. Elsewhere in its range, however, its life cycle remains incompletely understood. Clarifying its distribution and resolving unknown host associations are essential for advancing a more holistic view of host-parasite dynamics and their ecological role in intertidal ecosystems. In this study, M. gratiosum was detected using DNA-based assays in individuals of the barnacle Balanus glandula Darwin, 1854 that were collected from the rocky intertidal zone along the southern coast of British Columbia, Canada. This finding provides the first molecular evidence of this trematode species in the Pacific, complementing earlier records. The same molecular approach can be extended to identify region-specific hosts, such as the first intermediate (molluscan) host in the Pacific region, which remains unidentified.