The distribution and abundance of organisms can have a profound effect on an ecosystem's structure and function. Nutrient cycling, predator-prey interactions, and community structure may all be influenced by distribution patterns. As such, numerous studies have been conducted to understand factors influencing vertical and horizontal distribution and abundance [2,78]. However, UVR has histori cally received less attention. Implications of behavioral responses to UVR in diel vertical migration and predator-prey interactions are discussed further below.
One of the most interesting behavioral responses to solar radiation is the phenomenon of zooplankton diel vertical migration (DVM). Large zooplankton often exhibit strong migrations during the day to deeper, darker depths in the water column. Smaller zooplankton, in turn, remain in the surface waters during daylight and migrate to the deeper waters at night to avoid predation or interference by larger zooplankton [52,79,80]. Many hypotheses have been proposed to explain these patterns. Some of the earliest works on DVM demonstrated that solar radiation was a potentially important proximate as well as an ultimate factor inducing migrations [81-83]. These experiments, however, were conducted in the laboratory and no field studies were conducted to demonstrate a clear link between damaging solar radiation and zooplankton migration patterns in nature. Consequently, other factors such as temperature, food, and especially predation have typically been more widely studied and identified as the primary factors inducing DVM .
In spite of the importance of predators inducing migrations, predation alone does not explain the variety of DVM patterns observed in nature [84,85]. For example, vertical migrations have been detected in organisms inhabiting Ashless systems . Most of these systems tend to be high alpine or desert lakes in which damaging solar radiation can be intense. Several experiments have shown that ambient levels of UVR can lead to a decrease in survival as well as a decrease in growth and reproduction in both freshwater and marine organisms [6,8,14,86], and negative phototactic behavior has been demonstrated in the laboratory and field [28-38,40-51]. Given these recent findings, UVR may be more important than previously thought in influencing the vertical migration and distribution of organisms [34,84], serving as both a proximate and an ultimate cause of DVM.
Indeed, zooplankton often migrate deeper than the depths to which damaging UV-B radiation penetrates, in both freshwater and marine systems. While damaging UV-B may not be present, UV-A radiation continues to penetrate through the water column. For example, in the open ocean, the 1% level of 375 nm is four times as deep as the 1% level of 310 nm (Jerlov type II oceanic water) , and in freshwater lakes, UV-A penetration can be two times or greater . Given that many fish species use UV-A light to forage, zooplankton may migrate to deeper depths in order to avoid visually feeding predators with UV-A photoreceptors. Many freshwater and marine species, however, continue to migrate to even deeper depths, suggesting that other factors besides UVR, such as temperature and predation, are inducing migrations.
While both UV-B and UV-A can be damaging, UY-B is generally more damaging than UV-A radiation per photon. The UV photoreceptors in many species peak in the UV-A range (see Table 1). If animals are cueing to UV-A wavelengths that penetrate more deeply into the water column, they would be protected from potentially more damaging UV-B found closer to the surface. These alterations in depth to prevent UVR exposure may in turn influence the overlap of predator and prey in both time and space. For example, UV-tolerant zooplankton may find refuge from larval fish predators, which are susceptible to UV damage [46,86], in the surface waters of high UV systems.
Many species of larval fish have retinal cones that perceive UV-A (350-370 nm) and these are thought to help larvae locate and capture their prey [63,64]. However, some prey species also have UV-A photoreceptors. Responses to UV-A wavelengths in these organisms may therefore also be a means of predator avoidance in the surface waters. In this case, predation may be the ultimate cause of DVM but UV-A light would be the proximate cue. Further investigation is needed to test these types of hypotheses.
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