The Effect of Ultraviolet Vision on Predation
The issue of ultraviolet transparency is particularly intriguing.
Recent research has shown that UV radiation is more abundant in
near-surface ecosystems than previously supposed. This presents
two problems for zooplankton that have evolved a concealment strategy
based on transparency.
Figure 1: Identical underwater views taken with
green (top) and UV (bottom) filters. Note the enhanced contrast
between fish and the background in the UV-filtered image.
Tom Cronin, UMBC.
The first involves UV vision among potential predators and prey.
UV vision has been demonstrated in many marine species and it has
been conservatively estimated that there is sufficient UV light
for vision down to 200 m in clear ocean water. Visual pigments with
UV sensitivity (though not necessarily peaking in the UV) have been
found in the Atlantic halibut, 22 (out of 41 examined) species of
coral reef fish, juvenile salmonids, and decapod and stomatopod
crustaceans. Among freshwater teleosts UV vision appears to be fairly
widespread. Several researchers have hypothesized that UV vision
is primarily used to improve detection of planktonic prey, and some
have shown that the presence of UV light improves the search behavior
of certain UV-sensitive zooplanktivorous fish. The presence of UV
sensitivity in planktivorous but not in non-planktivorous life stages
of salmonids, the correlation between UV vision and planktivory
in coral reef fish, and the correlation between ocular UV transparency
and planktivory all suggest that UV vision is often used to increase
the contrast of planktonic prey.
The second problem related to UV radiation is potential radiation
damage. Numerous studies have shown that pelagic organisms are damaged
by UV radiation in various ways, including deleterious effects on
DNA, proteins, tissue, activity, growth, reproduction, and chemical
defenses. Differential levels of these effects have been shown to
influence biomass, sex ratios, and species compositions of both
terrestrial and marine ecosystems. These effects are primarily due
to UV-B radiation (280-320 nm), and in clear polar water have been
observed to depths of 20-25 m. At lower latitudes, where surface
UV irradiance is higher, these effects are likely to be observed
Figure 2: Another set of views taken with green
(top) and UV (bottom) filters. Tom Cronin, UMBC.
One of the primary defensive mechanisms against UV radiation damage
is the use of UV absorbing pigments. However, because UV-protective
pigments must attenuate UV light to be effective, their presence
reduces an organism's transparency in the UV, and thus increases
its sighting distance for predators and prey with UV vision. This
presents a potential dilemma for transparent epipelagic zooplankton:
protection or concealment. This conflict is particularly difficult
to resolve in clear, oceanic waters where UV radiation levels are
high, and camouflage is especially challenging. Reports of decreasing
ozone levels at polar, temperate, and tropical latitudes create
an additional complication, because transparent zooplankton may
face concomitant increases in UV radiation. A responsive increase
in UV-protective pigmentation (at either an individual or population
level) increases UV visibility, resulting in potentially increased
predation or decreased feeding success. A responsive increase in
depth may decrease access to prey, phytoplankton, or warm water.
We have investigated this topic in several ways. First, we have
measured the UV and visible transparency of zooplankton and the
waters they inhabit, and then modeled the effect of the measured
UV absorption on radiation protection and visibility to UV-visual
predators. Second, we have measured the effects of UV radiation
on the sighting distance of zooplankton when viewed by the Bluegill
in a laminar flow tank.
and S. Johnsen (2003). Avoidance and UV vision. Pp. 455-484 in UV
Effects in Aquatic Organisms and Ecosystems, (W. Helbling, H. Zagarese
eds.), Royal Society of Chemistry, London.
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.