What do pelagic metazoans have in common, except for sharing an aquatic habitat? Not too much, apparently. Their habitats range from the tropics to polar regions, from deep sea to alpine ponds, and from highly transparent oceans and oligotrophic lakes to brownish, humic lakes or eutrophic localities with very high UV attenuation. They cover numerous species and taxa over several phyla from jellyfish to fish and mammalians, from the minute rotifers or nauplii to the largest creatures on the planet. In addition, there is also a wide variety of benthic or sessile animals with larval stages that for brief periods belong to the pelagic community. Within the pelagic community there is a vast number of spatial niches, life cycle strategies, morphology and color. There are also generation times ranging from few days to many years, implying widely different growth rates, longevity and metabolic turnover. This multitude of evolutionary strategies has several bearings on the role of UVR, and could lead to the conclusion that the search for general phenomena in marine and freshwater ecosystems, as well as among the highly diverse pelagic community is overly optimistic. Hopefully this chapter will provide evidence that this is not the case, that UV is a common challenge to all pelagic animals and that this challenge can be met by a finite number of strategies.

First of all; the role of salinity per se is probably minor, except that UVR mediated membrane damage could cause an osmotic distortion that would be most pronounced at low salinity. Also ecosystem attributes such as longitude or latitude, depth, size or transparency of habitats are all a matter of gradients. These are important gradients, but does not call for distinct categories of aquatic habitats. Thus when treating major taxa, there is made no straight division between ecosystems, although the special challenges in some habitats have been stressed. There are some pronounced differences between major taxa, however, that call for special attention. Although basic cellular processes, and thus the UVR mediated damage, are uniform and most organisms share a set of cellular means of UV-protection, there are still some inherent differences attributed to level of organism organization and life cycle strategies.

It is also important to recall that there may be pronounced variability in UV-responses also within taxa or even species. The individuals of many species undergo ontogenetic shifts that may create bottleneck periods with regard to UV-susceptibility (e.g., hatching, larval stages, periods of rapid growth, moulting) that yet are poorly understood. There may also be strong variations in abiotic or biotic factors such as temperature, oxygen, ionic composition or nutritional status, burden of parasites or predators. In fact these intra-species properties may be more prominent than inter-species differences. In general, small organisms could be more susceptible than larger ones since all body components, including reproductive organs, can be reached by UVR. Also, rapid cell division or growth, implying a very active gene expression, could be a major determinant of susceptibility [1]. Thus is would be expected that, within species, eggs and embryos would be the most vulnerable stages. On the other hand organisms with a short life span and short generation time may be less vulnerable to accumulated damage with age, hence for such organisms it would pay off to invest in protective means for eggs and offspring on the expense of somatic tissue.

There is a scattered, and in general limited, knowledge on the effect of UVR for most aquatic organisms, and for a number of major categories there is no empirical support at all for judgement of UV-effects. Hence a systematic screening of all groups will be impossible, and this chapter will rather focus on general aspects of UVR in major aggregated and functionally related groups and draw on existing knowledge within these groups to generalize predictions. Although care should be taken when extrapolating effects from one taxon to another, some general knowledge may nevertheless be derived since the biological effects of UV at the cellular level are fairly similar across taxa. This means that some of these general effects need not to be reiterated for all systematic groups. The major questions are how the animals are able to cope with this stress, i.e., what kind of protective means and life cycle strategies are evolved - and at which costs? What are the ecological and evolutionary consequences of UVR? How could this affect communities and food webs?

This book covers an extensive list of UV-mediated damage to a wide range of aquatic organisms. Some chapters are devoted to specific types of protection or damage, with emphasis on particular taxa. Thus although reiterating some of these aspects, the main part of this chapter is devoted to truly planktonic metazoans and aspects of their ways of coping with UVR that is not extensively covered in other chapters.

The first and most important question is whether there are reasons to believe that UVR indeed is an ecologically or evolutionary relevant factor for pelagic animals? If so, does this mean that UVR poses a significant stress on these organisms? To really be concerned with present or future UVR, these basic questions demand a yes. For the sake of argument, this chapter will first address the variety of defense mechanisms that are evolved to cope with UVR. The very presence of such mechanisms in pelagic animals is in itself a strong argument for UVR as a major player in an ecological and evolutionary context. The various kinds of UV-mediated cellular damage will be the focus of other chapters; hence the scope here will be the ecological and evolutionary consequences. Later in this chapter we will proceed to case studies that evaluate the role of UVR for particular organisms. One particular challenge is that while UVR may pose both stress and constraints on organism performance, these sub-lethal effects may be hard to trace in nature. Pelagic organisms are evolved under some UV-stress and hence should be able to tolerate this, but at some costs that may be quite subtle. Nevertheless, there are several observations of in situ effects of UVR, and these are of course particularly important for the final judgement of the ecological role of UVR.

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