Pelagic metazoans cover organisms from rotifers to whales that inhabit a variety of localities from alpine pond to oceans. Knowledge on specific responses on UVR is restricted to very few taxa or species, yet the fact that all organisms share a basic set of biochemical and cellular properties and have a limited range of protective mechanisms and responses, allows for some generalizations across taxa. The main discussion in this chapter is devoted to the best examined groups: zooplankton and fish. From the early development of life on this planet without oxygen (and ozone), UVR has probably been a key player in the early evolution of aquatic life. As an agent of mutations, UVR may operate directly at the gene level, but also by shaping phenotypic traits and behavior by adaptation. The fact that a range of energetically costly protective mechanisms has been evolved is a strong argument for the ecological and evolutionary relevance of UVR. This is further supported by in situ studies that confirm a variety of direct and indirect UVR effects under natural conditions. Such studies do, however, also confirm a wide range of responses (or lack of such) between and within organisms, suggesting that a key effect of changes UVR climate could be community shifts. For some organisms that appear rather insensitive to UVR, increased UVR could in fact be beneficial, yet the multitude of effects on food webs within ecosystems and the entangled bank of life are hard to predict.

Based on the present literature is may be concluded that UVR may play a key role for pelagic metazoans, and that it often plays in concert with other ambient factors that may be mutually strengthening.

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