JOHNSEN, Sonke; Duke University: Troubles in flashlight world: The effects of water turbidity and visual acuity on the success of counterillumination in deep-sea species

Many deep-sea species, particularly crustaceans, cephalopods, and fish, use photophores to illuminate their ventral surface and thus disguise their silhouette from predators viewing them from below. Despite its apparent success and widespread distribution, this strategy has at leas two potential limitations. First, a predator with acute vision may be able to detect the individual photophores on the ventral surface. Second, The intensities of the counterillumination and the background light may not decrease at the same rate as a predator moves father away or closer to the counterilluminator. Using Monte Carlo methods, image transfer theory, and radiative transfer software, these two issues were examined by modeling the appearance of photographed bioluminescent signals as a function of the distance from the signaler and the visual acuity of the viewer. Four different water types were examined: coastal water at 5 m depth, and oceanic water at 5, 210, and 800 m depth. Surprisingly, visual acuity of the viewer influenced the appearance of the counterillumination more than the turbidity of the water, even at large distances. Species with visual acuity better than 0.05 degrees were able to distinguish the individual photophores of the counterilluminating signals at distances up to 2-4 m, thus breaking the camouflage. In addition, the intensity of the bioluminescent signal decreased more rapidly than the intensity of the background. Therefore, even if the signal perfectly matches the background when viewed from a particular distance, it will be brighter than the background at shorter distances and dimmer than the background at longer distances. This issue is particularly important in more turbid waters and at longer wavelengths. In summary, counterillumination may be of limited benefit against predators with high visual acuity and in more turbid waters.