| الوصف: |
To examine the effect of wave-induced light fluctuations on the appearance of objects to fish, we recorded the spatial and temporal fluctuations of light reflected from a diffusely reflecting target that served as a simplified proxy for the body of a fish, and of light from the water background that a fish might be viewed against. Measurements were repeated at diverse depths, viewing azimuths, distances to the substrate, and sun conditions. Two conditions that are necessary for wave-induced light fluctuations to make objects more apparent to fish were satisfied. The contrast of light fluctuations reflected from either the object or water background was higher than the minimum contrast value that is detected by fish, or, alternatively, the contrast of light fluctuations reflected from both the object and water background was higher than the minimum contrast value detected by fish, but differed from one another. Furthermore, the frequency range where most of the power of wave-induced radiance fluctuations matched the frequency range of maximum contrast sensitivity in fish. Thus, light stimuli having spatial and temporal characteristics similar to those of wave-induced light fluctuations may make objects more apparent to fish. We suggest that the frequency characteristics of the visual systems of fish were likely shaped by wave-induced light fluctuations in aquatic ecosystems. Variability is one of the most distinctive and often ignored features of the underwater light field. This variability in photon flux, directionality, spectra, and polarization diverges in magnitude and timescale, ranging from seasonal, through diurnal, to short-term fluctuations lasting less than a second. In shallow water, surface waves are the principal cause for short-term variation in the underwater light (Snyder and Dera 1970; Dera and Olszewski 1978; Zaneveld et al. 2001). This variation arises as a result of focusing and defocusing of sunlight rays refracted at the water surface, producing spatial and temporal light fluctuations (Schenck 1957; Snyder and Dera 1970; Stramski and Dera 1988). These fluctuations in underwater light were suggested to influence the visibility of submerged objects (McFarland and Loew 1983; Loew and McFarland 1990). However, this possibility has not yet received adequate attention. Contrast sensitivity (CS) is the relative detection efficiency of contrast between light stimuli as a function of their frequency of modulation. The contrast between light stimuli is commonly estimated as the Michelson contrast (Lmax 2 Lmin)/(Lmax + Lmin), where Lmax and Lmin are the maximum and minimum radiance in the stimulus, respectively (Michelson 1920). Visual systems have a characteristic frequency at which spatially or temporally modulated light stimuli are best detected (Kelly 1972; McFarland and Loew 1983; Douglas and Hawryshyn 1990). This is the frequency at which spatial or temporal CS is highest. The frequency of maximum CS depends on the adaptation state of the eye (i.e., adjustment of the eye sensitivity to the background light), the mean intensity of the modulated light stimulus, and the visual angle |
