We've all tried opening our eyes underwater, but for most of us the world becomes an unfocused blur without the benefit of goggles. However, this problem doesn't seem to affect seals and sea lions; they appear to see equally well whether diving beneath the waves or resting at the surface. Whether, or how, diving animals' eyesight has adjusted to both media has fascinated scientists for more than a century, but little progress had been made in understanding how seal vision works in both media. Intrigued by all aspects of harbour seal life, Guido Dehnhardt from the University of Rostock teamed up with Frederike Hanke from the University of Bochum to take a closer look at harbour seal eyes to find out how well the animals see above and beneath water(p. 3315).
Focusing on the seal's lens, Dehnhardt and Hanke decided to use a modern photorefractive technique to find out more about the animal's eyesight. Training two seals to climb onto a platform and plunge their heads into a tank of water, Hanke was able to collect infrared images produced by the lens on the retina, to see how it focuses light. Analysing the images, the duo were surprised to see that the lenses looked as if they might be multifocal. Hanke explains that Ronald Kröger at Lund University originally discovered multifocal lenses in the late 1990s in cichlid fish. The specialised lenses are usually found in creatures with colour vision, with each layer of the lens focusing a specific region of the optical spectrum to give animals sharp colour vision. However, the concentric rings in the infrared recordings from the seal eyes weren't clear-cut; Hanke needed more evidence to confirm that the lenses were multifocal.
Deciding to work directly with harbour seal lenses, Hanke travelled to Ursula Siebert's lab at the University of Kiel to be close to wild harbour seals. According to Hanke the German Seal Management Program closely monitors the health of newborn harbour seal pups on the Wadden Sea coast and attempts to rescue the weaker pups abandoned by their mothers. Unfortunately the intervention sometimes comes too late, so Hanke was on hand in Siebert's lab to collect the pups' lenses and test their refractive properties. Carefully extracting the lenses, Hanke used two methods to confirm that they were multifocal. In the first she shone a beam of white light through each lens and by filtering the light from the lens with a pinhole was able to produce an image of the lens with the telltale rainbow rings characteristic of a multifocal lens. In the second she scanned the lens with a laser while filming the position of the refracted beam of light as the laser moved across the lens, confirming that it was multifocal.
But why do harbour seals, which lack colour vision, have multifocal lenses?The team suspect that there could be two reasons. Either the lens could focus light in a way that provides the seals with some colour perception at low light levels, or the multifocal properties could improve the depth of focus in the animals' large eyes.