The balbo sabretooth (Evermannella balbo) is a sabertooth of the family Evermannellidae, found circumglobally in tropical and subtropical seas, at depths of between 100 and 1,000 m. Its length is up to 17 cm.
Vision
E. balbo, like other evermanellids, have eyes which display radical adaptations to the light conditions of their mesopelagic environment. The tubular shape and dorsally-directed orientation allow for detection of backlit prey during daylight hours, while binocular overlap improves acuity and may enable the fish to determine object distance. The lenses in this species are pigmented yellow-green, functioning as a filter that excludes short-wavelength light, improving detection of bioluminescence (and thus overcoming prey counterillumination). As these eyes have a field of view restricted to the ~50° of visual space directly above the animal, these fish possess "optical folds", lamellated structures on the lateral surface of the eye that direct incident light onto accessory retinae on the opposite walls of the barrel-shaped eyes, analogous in function to the pearl organ found in the scopelarchids and ocular diverticula found in some opisthoproctids. This allows E. balbo to detect bioluminescence occurring laterally and ventrally to the animal which may signal the presence of a potential mate, prey item, or predator.
References
Tony Ayling & Geoffrey Cox, Collins Guide to the Sea Fishes of New Zealand, (William Collins Publishers Ltd, Auckland, New Zealand 1982) Wagner, H.-J., Partridge, J. C., & Douglas, R. H. (2019). Observations on the retina and “optical fold” of a mesopelagic sabretooth fish, Evermanella balbo. Cell and Tissue Research, 378(3), 411–425.