Oarfish are rarely seen and seldom studied, which makes their parasite fauna even more enigmatic. Necropsy of 12 oarfish, Regalecus russelii (Regalecidae) (Cuvier, 1816), from Japan yielded 2 species of acanthocephalans. One species was found in 2 oarfish and a total of 76 specimens was collected, but only a single, immature specimen of the second species was found. The former represents an undescribed species from the order Echinorhynchida and is described here. Morphological and phylogenetic analyses of the small subunit (SSU) rDNA place this species in the family Gymnorhadinorhynchidae, and genus Gymnorhadinorhynchus which is characterized by a cylindrical proboscis with longitudinal rows of hooks, basal circle of enlarged hooks, asymmetry of hook shape, 4 cement glands, and a spineless trunk. Diagnostic characters of this species within the genus are the number of longitudinal rows of hooks (14), smaller body size (males: 4.8–6.6 mm and females: 5.3–6.3 mm) and a number of molecular autapomorphies including a number of long insertions in both the SSU and large subunit rDNA (LSU). A single immature female of Bolbosoma sp. (Palaeacanthocephala: Plagiorhynchidae) was also found with its anterior end embedded in the mucosa of the pyloric ceca. The characters of this specimen are not consistent with any other known species of Bolbosoma; however, because only 1 immature specimen with a partially invaginated proboscis was recovered, it was not designated as a new species.

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