BACKGROUND: Acanthocephalans of the genus Bolbosoma Porta, 1908 are trophically transmitted parasites that infect marine mammals (mostly cetaceans and less frequently pinnipeds) worldwide. There are 12 species currently considered as valid; however, most records lack information on the maturity stage of the specimens. This, coupled with the scarce phylogenetic information available, hinders a correct understanding of their patterns of host specificity, evolutionary history, and taxonomy. A particularly intriguing case is that of Bolbosoma vasculosum (Rudolphi, 1819), which has been frequently reported in odontocetes but rarely as an adult, having been suggested to be synonymous with Bolbosoma capitatum (von Linstow, 1880). METHODS: We used a comprehensive approach to investigate the concept of Bolbosoma. First, we conducted a bibliographic review of records of Bolbosoma spp. to clarify which are the final hosts for each species. We paid particular attention to B. vasculosum, using morphological and molecular analyses to compare it with B. capitatum. Second, we characterized the complete mitochondrial genome of Bolbosoma balaenae (Gmelin, 1790), Bolbosoma turbinella (Diesing, 1851), B. capitatum, and B. vasculosum. Then, we reconstructed the phylogenetic relationships of Bolbosoma spp. and related taxa using full mitochondrial genomes (or only cox1 when full mitogenomes were unavailable) and nuclear ribosomal genes (18S and 28S). RESULTS: Bolbosoma spp. exhibit high specificity for cetaceans, with no confirmed records of adult specimens in other host groups. Within this genus, B. vasculosum appears to be conspecific with B. capitatum based on both morphological and molecular evidence. This species shows high affinity to odontocetes, while the remaining species are specific to mysticetes. Phylogenetic analyses showed strong support for the monophyly of Bolbosoma spp., which appeared as sister taxa to Corynosoma spp. and Andracantha spp.. The resulting topology aligns with the patterns of specificity indicated by host records, revealing two distinct clades for species specific to odontocetes and mysticetes, respectively. CONCLUSIONS: The phylogenetic relationships obtained support the hypothesis that the association of Bolbosoma spp. with cetaceans originated through a host-switching event from aquatic birds.

Centro Para el Estudio de Sistemas Marinos Consejo Nacional de Investigaciones Científicas y Técnicas Puerto Madryn Argentina

Institute of Parasitology Biology Centre Czech Academy of Sciences České Budějovice Czech Republic

Marine Zoology Unit Cavanilles Institute of Biodiversity and Evolutionary Biology University of Valencia Valencia Spain

Natural History Museum London UK

Scottish Marine Animal Stranding Scheme School of Biodiversity One Health and Veterinary Medicine College of Medical Veterinary and Life Sciences University of Glasgow Glasgow G12 8QQ UK

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