. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 344 S. JOHNSEN AND E. A. WIDDER. 20 30 40 50 60 70 80 90 100 percent transparency Figure 5. Sighting distance of a prey item vs. its percent transpar- ency when viewed from below by predators with visual systems that have different minimum contrast thresholds for object detection. The sighting distance is divided by the sighting distance of an opaque object to control for water clarity, prey shape, size. etc. The ratio gives an estimate of the advantage of transparency for crypsis in a given situation. parent, alter the pola
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 344 S. JOHNSEN AND E. A. WIDDER. 20 30 40 50 60 70 80 90 100 percent transparency Figure 5. Sighting distance of a prey item vs. its percent transpar- ency when viewed from below by predators with visual systems that have different minimum contrast thresholds for object detection. The sighting distance is divided by the sighting distance of an opaque object to control for water clarity, prey shape, size. etc. The ratio gives an estimate of the advantage of transparency for crypsis in a given situation. parent, alter the polarization characteristics of transmitted light and are therefore detectable by animals that can detect these characteristics (Shashar el ai, 1998). Since many crustaceans have polarization vision (Waterman, 1981), this method of detection may be quite common. Relationship of measured transparency to in situ light transmission The great majority of the collected animals either have a mesopelagic daytime distribution in oceanic water or are found at moderate depths in coastal water (see Results). In both cases, the light field has reached an asymptotic state. The asymptotic light field is considerably simpler than its epipelagic counterpart. The intensity and spatial and temporal aspects of the epipelagic light field are strongly influenced by solar elevation and azimuth, waves, and clouds (Jerlov, 1976; Lythgoe, 1979; Loew and McFar- land, 1990). In addition, downwelling light has a broad angular distribution. Therefore, it can be difficult to relate percent transmission of a narrow beam of light to percent transmission of the in situ light field (Chapman, 1976a). In contrast, while the intensity of the asymptotic light field is affected by surface light levels, its spatial charac- teristics are essentially constant (Jerlov, 1976; Denton, 1990). Additionally, the angular distribution of most of the light is narrow and centered around the vertical. As mentioned in the Materials an
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