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.
Figure 1: Optimally cryptic
coloration represented as human-perceived color (viewed under
northern daylight). Predicted reflectance spectra for oceanic
and coastal waters are converted to CIE XYZ coordinates using
standard methods (Wyszecki and Stiles, 1982), then converted
to RGB coordinates using color conversion software (Munsell
Conversion Program, GretagMacbeth Inc.), and finally printed
on a CMYK printer using color management software (ICM 2.0,
Microsoft Inc.). Dorsal, lateral, and ventral coloration are
from Johnsen 2002, and Johnsen and Sosik, 2003. Coloration
at intermediate locations is given by linear interpolation.
The surface of the animal is assumed to be diffusely reflective.
Note that while the white ventral surface in each case is
optimal, it is not ideal, because the required reflectance
for perfect ventral crypsis is several orders of magnitude
higher than one.
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.
Figure 2: Optimally cryptic reflectance
for mirrored fish represented as human-perceived brightness
(viewed under northern daylight). See Figure 1 caption for
more details. The white lateral surfaces when viewed into
the sun and the white ventral surfaces in all cases are optimal
but not ideal, because the required reflectance in both cases
is greater than one.
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).
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and conspicuous coloration in the pelagic environment. Proceedings
of the Royal Society of London: Biological Sciences 269: 243-256.
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