Parasitism is an important ecological interaction that may cause dramatic changes in the host population size. As discussed above, solar UVR has the potential to act directly or indirectly in this process, for example by damaging the parasite or by causing damage to the host and increasing its susceptibility to infections. About the latter type of interaction, our knowledge is restricted mainly to studies on fish and amphibians. Solar UV-B radiation is known to cause injury to the skin (sunburn), reduction of goblet cells (mucus secreting cells), and epidermal hyperplasia in fish although sensitivity is species- and developmental stage-specific [96,97]. The damaged skin tissue is usually suscep tible to bacterial and parasite infections. Particularly, Saprolegnia, an oomycete, is a common opportunistic facultative parasite of freshwater fish . Infection by Saprolegnia causes loss of epithelial integrity and tissue destruction due to cellular necrosis or dermal and epidermal damage [98,99]. Infections may result from direct UV-B damage to the skin or from suppression of the immune system. In the case of the parasite Saprolegnia, the decrease in the secretion of mucus appears to be crucial for the infection as it acts as the primary physical barrier . However, UV-B radiation may have a strong immunosuppressive effect on fish, probably weakening their resistance to infectious agents in relation to impairment of the non-specific immune defense . Nevertheless, secondary parasitic infections of fish by Saprolegnia after UV-B exposure appear to have been only documented for laboratory studies . On the other hand, results from field observations and experiments have shown that increased UV-B exposure of western toads embryo, Bufo boreas caused by reduction in water depth at oviposition sites is related to higher infection by S.ferax [103,104]. For example, S. /erax-associated mortality (i.e. the proportion of dead to hatching embryos) was higher than 50% at water depths < 20 cm depth but less than 19% in water deeper than 45 cm .
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