Life cycle elucidation and species delimitation hypothesis for the genus Campula (Digenea: Brachycladiidae), trematode parasites of cetaceans

Abstract

The family Brachycladiidae is the most species-rich trematode taxon restricted to marine mammals, with several species being highly pathogenic. The biology of the brachycladiids remains poorly understood due to confused systematics, scarce genetic data, and unknown circulation pathways. All these challenges collide in the case of Campula oblonga Cobbold, 1858, a widespread parasite of porpoises with an unknown life cycle and signs of genetic and morphological heterogeneity across its range. To resolve this putative species complex, we sampled naticid gastropods in the Barents Sea and searched for potential brachycladiid intramolluscan stages. Several Cryptonatica affinis (Gmelin, 1791) moonsnails harboured rediae and ocellate cercariae whose nuclear (18S, 28S rDNA) and mitochondrial ( nad3 mtDNA) sequences confirmed that they belong to C. oblonga. This finding represents the first unambiguous identification of an intermediate host for a trematode parasite restricted to cetaceans. Combining these life-cycle data with available genetic, morphological, and distribution records, following an integrative taxonomic approach, we hypothesised that C. oblonga may comprise three lineages: C. oblonga (to which the newly discovered intramolluscan stages belong), C. folium Ozaki, 1935, and an undescribed species, Campula sp. NWP1.