Searching for zooplankton just outside Snell's window'

Abstract

Blueback herring (Alosu aestivalis) feeding in a visual "gulping" mode preferentially swim toward the water surface at a region just outside Snell's window. Outside of Snell's window the water's surface reflects light from the dark water beneath the fish. This dark background provides maximum contrast for zooplankters which reflect and refract light entering through their own Snell's window. The principle is similar to that used in dark field microscopy. © 1981, by the American Society of Limnology and Oceanography, Inc.