Distinguishing between the abyssal macrourids Coryphaenoides yaquinae and C. armatus from in situ photography

The scavenging fish communities at abyssal depths of the Pacific Ocean are dominated by two species of macrourids; the rough abyssal grenadier Coryphaenoides yaquinae Iwamoto and Stein, 1974 and the abyssal grenadier C. armatus (Hector, 1875). These two species are morphologically very similar, and in the absence of physical specimens are notoriously difficult to distinguish from photographic data. In an era of increasing reliance on imaging technology in the deep sea, we provide an analysis of images of the two species from around the Pacific Rim with supplementary data from the Atlantic and Southern Oceans. Our results show that image-specific morphometric characters are inadequate to distinguish the two species. However, the way in which artificial illumination is reflected from the body is both sufficient, and consistently different to distinguish between the two species. The results are also corroborated by known geographic and bathymetric distributions. This analysis is intended to provide a reliable method of identification from deep-sea imaging systems in the absence of standard fishing techniques.