Light scattering by selected zooplankton from the Gulf of Aqaba

Abstract

The open sea is devoid of shelter, making vision an important sense for both predation and predator avoidance (Hamner, 1996;McFall-Ngai, 1990). In this environment, the visibility (or invisibility) of an animal is critical for its survival. Many complex adaptations have developed in response to these unique conditions. Among these are transparency (Chapman, 1976;Johnsen and Widder, 1998;Johnsen and Widder, 1999;Johnsen and Widder, 2001), cryptic coloration (Endler, 1978;Endler, 1990;Endler, 1991;Fuiman and Magurran, 1994;Herring and Roe, 1988;Muntz, 1990), mirrored body surfaces (Denton, 1970), counter shading and counter illumination (Cott, 1940;Denton et al., 1972;Ferguson and Messenger, 1991;Kiltie, 1988;McAllister, 1967), as well as morphological and behavioral adaptations that minimize apparent body or organ size (Seapy and Young, 1986). Further adaptations involve behavior: at least one function of diel vertical migration is to minimize visual predation. It was suggested that animals that have mirrored sides would benefit by orienting their body to or away from the sun to minimize intense light reflections (Johnsen and Sosik, 2003), and cephalopods and heteropods orient their thin intestines into a vertical position regardless of their body's orientation (Seapy and Young, 1986). Counter adaptations for breaking these camouflage strategies include polarization vision, ultraviolet (UV) vision, colored ocular filters and offset visual pigments (Bowmaker and Kunz, 1987;Browman et al., 1994;Loew et al., 1993;Lythgoe, 1984;Muntz, 1990;Waterman, 1981).Many zooplankton use transparency as their form of camouflage. Since many transparent species have refractive indices close to that of water, relatively little light is backscattered (Johnsen and Widder, 1999). However, the amount of light that the tissue scatters to the sides may be a major factor in its visibility (Chapman, 1976;Johnsen and Widder, 1999). Determining how much this scattered light affects an animal's visibility is not trivial and depends on depth, viewing angle, viewing distance and wavelength (Jerlov, 1976;Johnsen and Sosik, 2003;Kirk, 1986;Kitchen and Zaneveld, 1992;Mobley, 1995).Although the pelagic light field varies, there are some general constant attributes. Light intensity decreases exponentially with depth, and the spectrum of light narrows, becoming increasingly blue. At depths greater than a few hundred meters the light field becomes cylindrically symmetrical around the vertical axis, regardless of the position of the sun (Jerlov, 1976;Johnsen and Widder, 1998). In contrast, at shallower depths, the light field is generally asymmetric, especially during sunset and sunrise (Jerlov, 1976;Johnsen, 2002).While much research has examined light scattering from particles and phytoplankton (Ditchburn, 1963), light scattering by zooplanktonic species, and its effect on their visibility, has remained largely unexplored. Some attempts at modeling Light scattering by zooplankton was investigated as a major factor undermining transparency camouflage ...

Fishing methods for sustainable shrimp fisheries in the Canary Islands (North-West Africa)

“…H. grimaldii and H. laevigatus) would tend to move on the seabed, where they would be more susceptible to capture by bottom traps. The biological activity would be affected by a decrease in the organic matter input and lower oxygen levels at increasing depths (Gagnon et al 2007). It is likely that swimming a distance of 2 m to locate the semi-floating trap entrance would require considerable energy and oxygen consumption, so this fishing method is likely inefficient for deep-water shrimps.…”

A detailed investigation of the visual system and visual ecology of the Barrier Reef anemonefish, Amphiprion akindynos

“…The foraging task was to detect zooplankton against the background spacelight. Since zooplankton scatters downwelling light, and has a somewhat flat reflectance into the UV 64,65 , we modelled zooplankton as having a flat reflectance of 10% and assumed it scattered the higher intensity downwelling irradiance. Detecting a predator was modelled as discriminating a dark grey object against the background spacelight as would occur for a looming stimulus.…”

Coping with copepods: do right whales ( Eubalaena glacialis ) forage visually in dark waters?

“…The down-welling radiance is so much brighter than the horizontal radiance (figure 5a) that even weak scattering in the direction of the whale's eye may obliterate the background contrast. Indeed, brightly scattering patches of plankton could themselves become prominent visual targets [37]. At this point, we do not have the measurements or images to determine just what a patch of copepods looks like underwater at any depth.…”