Plunging birds' eyes are keen enough to detect fishy prey, even through the erratic shimmer of the ocean's surface, before the birds dive rapidly into the water. For some of these birds, however, merely seeing prey from the sky is insufficient for catching a meal. They must also have exceptional vision while submerged, which is no easy feat. While flying in air, the eye's ability to focus is provided by the refractive power of the cornea. However, while submerged, the refractive index of the cornea is about the same as that of the surrounding water – so there has to be some other mechanism for focusing an image on the retina.
Gabriel Machovsky-Capuska of Massey University, New Zealand, and colleagues from around the world wanted to know whether Australasian gannets, birds that plunge dive and swim to catch prey, have found a solution to this visual tradeoff. Gannets are unique in being one of the few species of bird that both plunge dive and ‘wing flap’ underwater. This suggested to the researchers that the gannets could be visually tracking fish even while submerged.
To determine whether gannets caught fish during the plunge section of their dive or after a visually guided underwater chase, Machovsky-Capuska and colleagues filmed diving gannets both above and below water in the field, noting when, and how many, successful prey captures resulted from each dive. The vast majority of successes occurred during the wing-flap stage of a dive – when the birds must be tracking the fish underwater.
Encouraged by these findings, the researchers photographed the eyes of several gannets both above and below the water. From the photographs, they used the reflection of light off the gannet's eyes to measure the curvature of the cornea, a proxy for its refractive power. They also used infrared photorefraction to determine how well focused an eye is from the position of light reflected off the retina.
The gannet's eyes were usually hyperopic, or far-sighted, in air and underwater. However, the birds were capable of achieving myopia, or near-sightedness, within about a tenth of a second of submergence. This ability to focus differently represented the birds' capacity to adapt to underwater vision even without the refractive power of their cornea.
Machovsky-Capuska and colleagues suggest that gannets may change the shape of their eye lens, instead of the retina, to retain the ability to focus. Many aquatic animals, including fish, amphibians, whales and penguins have evolved spherical lenses to cope with the challenges of underwater vision. Specialized lens morphology may permit the gannet's remarkable ability to switch from aerial to aquatic vision quickly and compensate for the cornea's ineffectual refractive power in water. More research will undoubtedly reveal the mechanism of the gannet's remarkably flexible visual system. In the interim, New Zealand fish might consider camouflage.