Sunfish Shows the Way Through the Fog

Abstract

Anyone who has driven in heavy fog knows how difficult it is to see things clearly in a medium that scatters light. Yet fish appear to move with ease through a dense aquatic fog of swirling algae, plankton, sand, dirt, and all sorts of plant and animal debris. Now, after studying the retina of the green sunfish ( Lepomis cyanellus ), researchers at the University of Pennsylvania, Philadelphia, think they know how this creature, at least, accomplishes the feat. And they are trying to copy the sunfish's presumed strategy, known as polarization difference imaging, to improve the vision of cameras in murky conditions.

Optical polarization without tools

“…are well over a hundred species of insects able to detect polarization, of which some 36 are known to use that of the blue of the sky as a compass. Many spiders, molluscs, fish, reptiles and birds also use polarization: for example, some fish scales and beetle chitin polarizes [7]. Karl von Frisch was probably the first to attribute the orienting ability of bees to the sky's polarized light [8].…”

Visual discrimination: Seeing the third quality of light

“…This is the reason for flying water beetles mistakenly plunging onto glossy car roofs and for mayflies attempting to lay their eggs on asphalt roads [18]. Polarisation cues have also been implicated for assessing the quality of leaves (A. Kelber, personal communication), haze reduction [19], contrast enhancement [20], intraspecific signalling [14] and breaking camouflage under water [21]. Some of these tasks are optically simple and can be performed by a single class of polarisation-sensitive receptors.…”