Behavioral responses to light have long been of interest to aquatic scientists, both freshwater and marine. Light-mediated behaviors such as mate recognition, visual foraging, and especially vertical migration are the focus of numerous studies spanning a wide diversity of taxa [1-3]. However, the role of UVR in these and other behaviors has only recently been more closely examined.

Until recent decades, UVR was not thought to be an important factor influencing aquatic ecosystems, as it was believed to rapidly attenuate through the water column. UVR is now known to penetrate deeply into many freshwater and marine systems, with dissolved organic carbon (DOC) as one of the primary factors regulating UV attenuation [4,5]. In addition, aquatic organisms occupying all trophic levels from viruses and phytoplankton to zooplankton and fish are susceptible to damage or mortality from UVR [6-9]. UVR may directly affect organisms via cellular and tissue damage, genetic mutation, or mortality; or it may indirectly affect organisms by constraining them to suboptimal habitats where temperature and food concentrations may be low and predation risk high. Tolerance to UVR differs among species [8-12] and therefore some species are more likely to respond behaviorally to damaging UVR than others. Consequently, UVR can alter species composition and trophodynamics within an ecosystem, possibly shifting communities towards more UV-tolerant species [13].

There are three means by which organisms can respond to potential UVR damage: (1) avoidance, (2) photoprotection, and (3) photorepair [14]. The extent to which organisms use each mechanism differs both within and among taxa. For example, among freshwater organisms, many species of the cladoceran Daphnia are capable of photorepair while copepods such as Diaptomus oregonensis and Acanthodiaptomus denticornis depend more on photoprotective compounds [15,16]. In the southern hemisphere, three species of the freshwater calanoid copepods within the genus Boeckella vary in their use of photoprotection versus photorepair [17]. Differences in photorepair and photoprotection are also seen among marine organisms. Photorepair in two closely related marine fish, the tautog Tautog onitis and the cunner Tautogolabrus adspersus, appears to be related to longevity, with the longer-lived tautog possessing greater photorepair capabilities than the shorter-lived cunner [18]. In Antarctica, where the ozone hole is the greatest, photoprotection by mycosporine-like amino acids is prevalent in several marine organisms from algae and invertebrates to fish [19]. Although our understanding of the photorepair and photoprotection capabilities of aquatic organisms is increasing, less is known about behavioral avoidance of UVR in nature.

While some wavelengths of UVR are damaging, others are potentially beneficial to aquatic organisms. For example, UV photoreceptors have been described in a variety of aquatic organisms from bacteria to fish [20]. The adaptive significance of these UV photoreceptors is not fully understood; however, research suggests that they may enhance navigation, communication, and foraging

[20]. It is also possible that UV photoreceptors may help organisms to avoid depths at which damaging wavelengths are present.

This chapter first describes the underwater UV environment. The different types of phototactic responses, such as positive versus negative phototaxis, are then described and related to UV tolerance as well as UV vision. Finally, implications for behavioral responses to UVR are addressed, including the role of UVR in diel vertical migration (DVM) and predator-prey interactions.

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