Research Projects

Visibility of Aquatic Animals

Pelagic species are visually exposed to a degree not found in any other ecosystem, due to the simple fact that there are no physical objects to hide within or behind. This has led to the evolution of complex adaptations for camouflage including whole-body transparency, mirrored sides, countershading and counterillumination, morphological adaptations to minimize body profile, and cryptic coloration. Conversely, several of these adaptations are also employed to increase visibility for sexual signaling, luring prey, and advertising chemical defenses. Concurrent with these adaptations, complex visual abilities have evolved to break camouflage. These are generally contrast increasing mechanisms and include ultraviolet vision, polarization vision, coloured ocular filters, and offset visual pigments.

Unlike terrestrial systems, light in aquatic systems is strongly affected by the surrounding medium. Therefore, the success or failure of either camouflage or a conspicuous signal depends not only on the visual capabilities of the viewer, but also on the depth of the viewed organism, the angle from which it is viewed, and the optical properties of the water. We have calculated optimally cryptic and conspicuous coloration -- as a function of viewing angle, depth, and solar elevation -- from the underwater radiance distribution.

This work showed that optimal cryptic coloration, like other camouflage strategies, depended strongly on all the above factors. In fact, while all cryptic strategies are highly successful when the organism is viewed under the conditions for which the strategy is optimized, they are much less successful when viewed under a different set of optical conditions.

A transparent organism accommodates trivially to changing conditions by simply transmitting the background light. Many counterilluminating organisms are known to alter the intensity, angular distribution, and spectrum of their emitted light to remain cryptic over a wide range of optical environments. Certain colored and mirrored pelagic organisms are known to change color in an apparent cryptic response to changing optical conditions, but cryptic color changes and the potential need for them remain poorly understood for pelagic species.

Therefore, we have also examined how robust cryptic coloration and mirroring are under varying optical and viewing conditions. First, the ideally cryptic reflectances for organisms in coastal water were calculated for a variety of optical conditions. Then, using the Atlantic Cod Gadus morhua as the viewer, the sighting distances of organisms optimally cryptic in one condition, but viewed in another, were determined. The overall goal was to determine the relative success of the two cryptic strategies when faced with a varying optical environment, and to determine the potential importance of the ability to change color.

Figure 3: Appearance of a cryptic fish viewed in the azimuth in which it is completely cryptic (left-most image), and at 36°, 72°, 108°, 144°, and 180° from that azimuth (in which crypsis is lost). Fish is viewed in the coastal water studied in Johnsen and Sosik (2003) at a depth of 5 m with a solar elevation of 10° (i.e. near dawn or dusk). In this case, the fish is optimally cryptic when viewed in the solar azimuth (where the horizontal background radiance is greatest). As the viewpoint rotates around the fish, the background radiance decreases, but the irradiance illuminating the fish increases (because viewed side of the fish moving towards the solar azimuth). This results is a large radiance mismatch (and thus high visibility) when the fish is viewed from the opposite azimuth (right-most image).

Publications

Johnsen, S. (2005). The red and the black: Bioluminescence and the color of animals in the deep sea. Integrative and Comparative Biology. (in press)

Johnsen, S. and H. M. Sosik (2004). Shedding light on light in the ocean. Oceanus.

Johnsen, S., Widder, E. A., Mobley, C. D., and P. J. Herring (2004). Propagation and perception of bioluminescence: factors affecting the success of counterillumination as a cryptic strategy. Biological Bulletin: 207: 1-16.

Johnsen, S. (2003). Lifting the cloak of invisibility: the effects of changing optical conditions on pelagic crypsis. Integrative and Comparative Biology 43: 580-590.

Johnsen, S. and H. M. Sosik (2003). Cryptic coloration and mirrored sides as camouflage strategies in near-surface pelagic habitats: implications for foraging and predator avoidance. Limnology and Oceanography 48: 1277-1288.

Johnsen, S. (2002). Cryptic and conspicuous coloration in the pelagic environment. Proceedings of the Royal Society of London: Biological Sciences 269: 243-256.

Johnsen, S., and E. A. Widder (2001). Ultraviolet absorption in transparent zooplankton and its implications for depth distribution and visual predation. Marine Biology 138: 717-730.

Johnsen, S., and E. A. Widder (1998). The transparency and visibility of gelatinous zooplankton from the north west Atlantic and Gulf of Mexico. Biological Bulletin 195: 337-348.