First record of Oschmarinella macrorchis (Brachycladiidae: Digenea) in the Atlantic Ocean parasitizing the liver of Sowerby’s beaked whales (Mesoplodon bidens)

Abstract Background Beaked whales, due to their deep-water habitat and open-ocean lifestyle, are among the most difficult vertebrates to observe, leading to limited knowledge of their biology and the diversity of their associated parasites. Results This study reports the first case of trematode parasitism caused by Oschmarinella macrorchis in the liver of two male Sowerby´s beaked whales ( Mesoplodon bidens) stranded in Galicia, northwestern Spain. Post-mortem examinations of the liver revealed spindle-shaped helminths inhabiting dilated and hypertrophic bile ducts. A total of 62 adult specimens were recovered, from which a subset was subject to morphological and molecular characterization. Main morphological characteristics, such as intestines with anterior and non-diverticulated caeca converging posteriorly on a uroproct, mean body size and shape of testes, were consistent with trematodes belonging to the family Brachycladiidae (Trematoda: Digenea), specifically O. macrorchis . Although, molecular analyses based on partial 28 S rRNA and ITS2 rRNA sequences were inconclusive for species-level identification, partial mitochondrial ND3 sequences supported the assignment, showing the highest nucleotide similarity with reference sequences of O. macrorchis . Conclusions To our knowledge, this is the first report of O. macrorchis infection in M. bidens and would also represent the first report of O. macrorchis in the Atlantic Ocean. These findings contribute to the limited knowledge of the parasitofauna of beaked whales and expand the known host and geographic range of brachycladiid trematodes.