VIDEO PHOTORETINOSCOPY OF THE EYES OF THE SMALL ODONTOCETES TURSIOPS TRUNCATUS, PHOCOENA PHOCOENA, AND KOGIA BREVICEPS

• Published in: Marine mammal science 1998-07, Vol.14 (3), p.584-590
• Main Authors: Cronin, Thomas W., Fasick, Jeffry I., Howland, Howard C.
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Eye and associated structures. Visual pathways and centers. Vision; Fundamental and applied biological sciences. Psychology; Kogia breviceps; Marine; Phocoena phocoena; Tursiops truncatus; Vertebrates: nervous system and sense organs
• ISSN: 0824-0469; 1748-7692
• DOI: 10.1111/j.1748-7692.1998.tb00744.x

While the small toothed whales are best known for their auditory senses and echolocation abilities, they have excellent visual systems as well. Vision is important in signalling and communication and may play a major role in predation. Cetacean vision is certainly acute in water, and in dolphins (at least), it may be equally good under some circumstances in air. This capacity for amphibious vision does not rely on special accommodative mechanisms, as in other vertebrates; in fact, dolphins and other small cetaceans have no apparent ability to accommodate actively. Instead, there seems to be some variation in focal position with direction of view. This may allow the bottlenose dolphin, Tursiops truncatus, to see well throughout most of its visual field underwater, but to see well only ventro-rostrally in air. The optics of harbor porpoise (Phocoena phocoena) eyes, which resemble those of other small whales, have recently been modelled by Kroeger and Kirschfeld. These eyes apparently incorporate a very unusual optical system in which the cornea acts as a diverging element, requiring the spherical lens to refract unusually strongly to form a sharp emmetropic image. However, such an optical system has no obvious advantages for amphibious vision. In general, one would expect an eye like this to be well focussed in water but strongly myopic in air, and this has been confirmed by ophthalmoscopy and retinoscopy of restrained dolphins. In these cases the eye was viewed either directly in the laboratory, or through a small water-containing cup with a flat glass window, while the dolphin held its head in air.