Sunday, April 20, 2014

Real Ducks Don't Wear Swim Goggles

Hooded Merganser pair (c) John Ashley
Hooded Mergansers have special adaptations for sharp vision above and below water 

Ever opened your eyes underwater? Of course you have. Doesn't work so well for us terrestrial types, does it? But what about aquatic animals like fish or even semi-aquatic ducks? How can they see underwater without wearing goggles?

Can I clear up the fuzziness in a few words? Probably not.

Eye diagram (c) National Eye Institute
Land-lubber eye diagram (courtesy NEI)
Modern animal eyes are quite a marvel. The whole complicated assembly evolved to serve one purpose - bending light. A combination of eye parts redirects light waves to converge on the fovea (Latin for "pit"), a small spot on the inner light-sensitive layer, the retina, lining the back wall. Half of all the optic nerves reporting from the eye to the brain originate from this pit, which has the highest density of color-sensitive cones and is responsible for sharp vision.

For the most part, our modern eyes use just two parts to bend light into focus, the cornea and lens. The cornea is that curved outer part that you rub when sleepy. Behind the corneal cover and iris opening (the pupil), sits a clear and flexible lens that gets mushed about by tiny muscles, changing shape to bend light into different magnifications. Changing the eye's magnification power is just another way of saying that you are changing your focus between near and far.

In land animals, our curved cornea accounts for a whopping two-thirds of the eye's ability to focus light (called "accommodation"). It works mostly because light changes speed when moving through different mediums, faster through air in front of the cornea but slower through our liquid eyeball. This bends the light rays in towards the lens.

For us mere humans, our eyes working above water have about 40 diopters of optical power. (One diopter means the lens can focus at 1 meter, or 39 inches. Two diopters focuses at one-half a meter, and so on. More diopters equals more magnification power.) A young person can reshape their lenses to gain an additional 20 diopters. But this drops to 10 extra diopters by age 25 and fades to only about 1 extra diopter by age 50. Thus, the reading glasses I'm currently wearing and the extra diopters I keep adding to my camera's viewfinder.

Fine. But what about underwater?

Our land-lubber eyes are mostly filled with liquid, so when we open our eyes in water, the light is moving from water into a similar liquid (with a similar refraction rate), and this effectively removes our corneas from the equation. Limited to just the lens, our land eyes can only manage to focus incoming light behind our pits. The result is fuzzy fish - and perhaps the inspiration for those fur-bearing trout in Iceberg Lake.

So how do aquatic animals see? Well, it depends. We used to think that animals with clear nictitating membranes (a retractable, lubricating outer layer) used them like goggles whenever underwater. That theory was quickly dispatched, but several different solutions would eventually come to light.

Westslope cutthroat trout (c) John Ashley
Westslope cutthroat trout
Fish have corneas to protect and contain their eyes, but these don't really help with clear vision. However, most fish have convex (bulging outward), almost round, lenses with an internal density gradient. This gradient allows fish eyes to bend light inside the lens itself instead of just at the lens surface.

But some fish also spend parts of their lives out of water. Some of these semi-terrestrial fish have an extra chamber behind the cornea, which allows them to still see well when the focal point is projected beyond the primary cornea, when these fish moves between air and water. The famous four-eyed fish has hourglass-shaped pupils, with the top half of the retinea adapted to seeing in air and the lower half adapted for seeing in water - It can see above and below at the same time! Unfortunately, the nearest of these cool species lives in South America.

Other fish and some birds, like penguins and albatross, have a flattened cornea, so the small amount of optical power lost by the cornea underwater is still within the focusing ability of their lenses. Other diving birds that chase fish underwater arrived at yet another answer. Some of the duck species here at the lake can see clearly in air (flight, landing, predator avoidance, etc.) and also underwater when in pursuit of pursue fish. Goldeneyes and Hooded Mergansers fall into this fish-eater category.

Remember those tiny muscles that reshape the lenses? Well, they're not so tiny in fish-eating bird species. These birds use their comparatively-larger eye muscles to squeeze their lenses into the opening (the pupil) formed by the colorful, constricted iris. By squeezing lens against iris, the lens distorts enough to become close-focusing. With these super-deformable lenses, diving ducks can immediately compensate for corneas - so valuable above water - that become useless underwater.

So diving ducks have sharp vision above and below water? Well, yes and no. According to one research team, these ducks don't switch over to their "underwater eyes" until they get close to something of interest (usually a fish), within about a bill's length away.

Like me, diving ducks appear to have brief flashes of clarity sandwiched between longer periods of fuzziness. Otherwise, I could have cleared this up in half as many words.