Cormorant Eyes Rapidly Refocus in Dives Into Murky Water
You’re hang gliding over a lake, and you spot a fish below. From your hovering position, you drop into a rapid, steep dive headfirst into the water. Whoops; your eyes just went out of focus, and you lost your fish in the murky depths. Too bad you’re not a cormorant.
Cormorants (a kind of waterfowl) are able to adjust the lenses of their eyes from air-focus to water-focus in a split second, according to an article in Current Biology May 25.1 Four Israeli scientists bedazzle us:
Cormorants (Aves; Phalacrocoracidae) are active fliers, yet they forage by pursuit diving and capture of fish with the bill. In air, the cormorant’s cornea provides most of the total refractive power of the eye. Underwater, however, corneal power is lost, as the cornea is now bathed in liquids of similar refractive index. The retention of a sharp image, while performing precise visual tasks underwater, requires that the cormorant’s optical system compensates for the loss of refractive power of the cornea. In addition, the underwater photic environment differs markedly from the aerial one, with the image quality undergoing a rapid deterioration through scatter and absorption. Upon submergence, cormorants compensate for the loss of corneal power (>55 dioptres, D) and rapidly (>1000D/sec) attain a state of emmetropia, i.e. they are well focussed, by marked changes in the shape of their very flexible lens.
The scientists somehow acquired accurate measurements of the birds’ optical acuity in air and in murky water. The birds’ vision is as good as that of fish, seals and whales who spend most of their time underwater. Cormorants, however, need outstanding vision in water as well as air. “The requirements to perform precise visuo-motor tasks in two optically different media, and the uniqueness of the lenticular system of these birds,” they note with some admiration, ”make the vision of pursuit-diving birds a model of vertebrate capacities at the extreme.”
1Strod, Arad, Izhaki and Katzir, “Cormorants keep their power: visual resolution in a pursuit-diving bird under amphibious and turbid conditions,” Current Biology, Vol 14, R376-R377, 25 May 2004.
Strod and Arad work at the Hula Valley Nature Preserve in northern Israel; perhaps that is where they made some of the observations. TV nature programs sometimes show these birds in action. Next time you see one, you’ll have reason to appreciate even more the elegance of their fishing expertise.
Once again, this excellent intelligent-design paper was marred by a worthless insertion of the E word, probably because Current Darwin-Worship wouldn’t print it otherwise: they conclude, “Low turbidity levels are commonly encountered in natural water bodies and thus are of crucial importance in our understanding of the evolution, sensory ecology, and micro-habitat selection in aquatic organisms.” This, as usual, means that nobody understands how these optical marvels evolved, but the Darwin Party hopes to some day. I wonder if the authors really buy that promissory note. So how many billion cormorants died of starvation till they got their optics right? Sorry, 990 dioptres per second isn’t good enough; let’s bump it up to 1000 and make sure all the less fit go extinct. For sure. We don’t need such evolutionary whistling in the dark. Cormorants knew the tune from the top, and in the right key, too; see sharp or be flat.