Introduction

There are a variety of reports in the literature concerning UV-induced injuries in aquatic organisms, particularly fish [1,2]. An excellent treatise on the vulnerability of fish and amphibians to UVR was recently published [3]. Laboratory studies with different fish species have shown that the short-wave UV-B portion of the solar spectrum is the most damaging. The most commonly reported harmful effect of solar radiation in fish has been skin damage, which is often referred to as sunburn (Figure 1). Fish skin is sensitive to UV-B because the skin generally lacks a keratinized outer layer, has dividing cells in all layers of the epidermis, and normally does not contain protective melanin-containing cells in the epidermis [1].

A variety of reports have documented the harmful effects of UVR on fish.

Figure 1. UV-exposed rainbow trout (Oncorhychus mykiss) on right showing darkened sunburn on dorsal surface (arrow), an unexposed fish is on left.

When the eggs and alevins of sockeye salmon (Oncorhynchus nerka) were irradiated with UVR, histologic examination of the alevins revealed changes in the epidermis and fibroelastic layers of the irradiated skin [4]. Dunbar [5] observed necrotic areas behind the head and around the base of the dorsal fin in rainbow trout (Oncorhychus mykiss) fingerlings within three days when the fish were exposed to solar UV after being transferred from a hatchery to outdoor ponds. DeLong et al. [6] observed sunburn lesions and subsequent fungal infections at the site of these lesions in chinook salmon (Oncorhynchus tshawytscha). Bullock and Roberts [7] observed sunburn lesions in rainbow trout and Atlantic salmon {Salmo salar) fry exposed to solar radiation. Bullock et al. [8] observed solar radiation induced sunburn in koi carp (Cyprinus carpio) held outdoors in clean water with no shading cover, but the symptoms subsided in water that had algae or contained aquatic plant cover. Bullock and Coutts [9] observed solar radiation induced lesions on the dorsal surface of broodstock rainbow trout held on a high altitude fish farm in Bolivia. In another report Bullock [10] mentions other cases of probable solar radiation induced skin lesions in farmed finfish. Dermatitis was observed in two species of cultured salmonids held in unshaded raceways in British Columbia [11]. Ramos et al. [12] observed sunburn-like dorsal skin lesions in juvenile paddlefish (Polyodon spathula) held in outdoor raceways. The number of mucus secreting goblet cells was significantly reduced in the dorsal epidermis of two cyprinids and two salmonids exposed to both simulated and solar UV-B [13]. Goblet cells on the ventral side of the fish were unaffected. The authors speculated that the reduction in number of goblet cells could result in less mucus production with a subsequent reduction in nonspecific defenses. "Sunburn cells" can be observed histologically in UV-B-injured fish skin and are characterized by dense granules of fragmented nuclear material [10,14,15]. These cells can be classified as either Type A or B and are differentiated by their morphological appearance under light microscopy [10], where the morphology of Type A cells most closely resembles that described for human skin, while the morphology of Type B cells closely resembles cells in eye and brain lesions in larval anchovy and mackerel as described by Hunter et al. [16].

Much of the information on UV-B-affected fish has come from laboratory studies. For example, Lahontan cutthroat trout (Oncorhynchus clarki henshawi)

were much more sensitive to simulated solar UV-B than were razorback suckers (.Xyrauchen texanus) [15]. Cutthroat trout showed darkening of the dorsal skin resulting from UV-B-induced melanosome dispersion with 48 h of exposure. Histological observations included sloughing of mucous cells, epidermal and dermal necrosis and edema, and in some cases secondary fungal infection. Cells that appeared to be Type B sunburn cells were occasionally observed in cutthroat trout. More free melanosomes were observed in the skin of UV-B exposed cutthroat trout. Conversely, the response of razorback suckers was very different. Razorback suckers showed no visible signs of sunburn after 72 h of UV-B exposure. Nevertheless, histologic examination revealed that cellular necrosis had occurred, but not nearly to the extent that it had in cutthroat trout. There was an increase in epidermal thickness as a result of large low-electron-dense cells that appeared to be club cells. There were some focal areas of increased melanocyte accumulation in the dermis. No sunburn cells were observed. Brown trout (Salmo trutta) were also found to be sensitive to simulated UV-B exposure [14]. Loss of cell layers occurred in exposed fish by increased surface cell sloughing or by detachment of the outer and middle cellular layers. Mucus cells disappeared from the epidermis of exposed fish. In addition, sunburn cells were also observed, although no distinction could be made as to whether they were Type A or B. Channel catfish (Ictalurus punctatus) fingerlings were found to be highly sensitive to simulated solar UV-B [17]. After 24 h exposure all the body surfaces were slightly darker than in control fish. Thinning of the most dorsal epidermis was accompanied by edema and occasionally both Type A and B sunburn cells were observed. Focal necrosis and sloughing of the outer epidermal layer was widespread by 48 h exposure (Figure 2). Three fish were dead and all remaining live fish appeared to have fungal infection on the dorsal skin. At 72 h exposure all remaining fish were dead. There appeared to be a time dependent decrease in the number of mucus cells in the epidermis.

UV injuries are not limited to fish. For example, aquatic tiger salamander (Ambystoma tigrinum) larvae were highly sensitive to UV-B, and developed lesions over the dorsal skin areas (Figure 3) within seven days of exposure to simulated UV irradiances as low as 2 p,W cm-2 [Carey personal communication]. Cellular damage was found in the epidermis of surviving alpine newt (Triturus alpestris) larvae exposed to both simulated and solar UVR [18]. Using a combination of ambient solar UV and simulated UV-B, Flamarique et al. [19] observed substantial skin damage and prominent eye lens opacities in tadpoles of two frog species, Hyla regilla and Rana aurora. Darkened dorsal areas were evident among certain pelagic aquatic invertebrates prior to mortality during simulated UV exposure [20].

In addition to skin injuries, increased UV-B exposure of fish in natural waters and shallow water hatchery pools and raceways may cause eye lens damage in the form of cataracts [21]. Cataracts, evident in observations of blinded brown trout in European streams (http://s.o.w.tripod.com/dyingbrowns.htm) have also been observed in eye lenses of hatchery reared lake trout (Salvelinus namaycush) exposed to solar UV [22-24].

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