Abstract
Hyperostosis, excessive bone growth along bone that stems from bone, periosteum or articular or epiphyseal cartilage, occurs in at least 22 families of fishes most of which are tropical or subtropical marine species. While the presence of hyperostosis is well documented in fishes, the mechanism driving the development of the excessive bone growth is unclear. This study documented hyperostosis along the dorsal pterygiophores in both sexes of oarfish, Regalecus russellii; however, it was not present in all specimens examined. This is the second lampridiform fish with hyperostoses and the first case documented in a deeper-water, epi-mesopelagic fish. In oarfish, the majority of the dorsal pterygiophores tissues are poorly mineralized, anosteocytic bones with some fish displaying localized stiffened, hyperostotic growths near the distal edge. Oarfish lack a swim bladder so they must continuously beat their bi-directional dorsal fin to maintain position within the water column while engaged in locomotory behavior. These fishes have areas of localized, hyperostotic skeletal elements along the dorsal pterygiophores that, presumably, function as a stiffened lever system to support fin undulation. It was noted that hyperossification was not present in all fish examined and was only documented in fish with total lengths greater than 3 m.