Effect of solar radiation on cetaceans

Abstract

Despite the marked deceleration in the amount of ozone lost at the poles each year, high levels of solar ultraviolet radiation (UVR) continue to reach our biosphere, potentially threatening living organisms, which owing to their life-histories and physiological constraints, are unable to avoid exposure to UVR. I aimed to demonstrate that cetaceans are affected by UVR and that they have adaptive mechanisms against exposure. Using histological analyses of skin biopsies and high-quality photographs, I characterized and quantified UVR-induced lesions in 184 blue, fin and sperm whales sampled in the Gulf of California, Mexico, and estimated indices of skin pigmentation for each individual. To examine the molecular pathways by which whales counteract UVR-induced damage, levels of expression of genes involved in genotoxic stress pathways (heat shock protein 70: HSP70, tumour protein 53: P53, and KIN protein genes: KIN) and melanogenesis (tyrosinase gene: TYR) were quantified. I not only detected evidence of sun-induced cellular and molecular damage but also showed that lesions were more prevalent in blue whales, the study species with lightest pigmentation, and sperm whales, the species that spends longest periods at the surface. Furthermore, within species, darker whales exhibited fewer lesions and more apoptotic cells, suggesting that darker pigmentation is advantageous. When accounting for interspecific differences in melanocyte abundance, sperm and blue whales presented similar amounts of melanin, although sperm whales overexpressed HSP70 and KIN. This suggests that sperm whales may have limited melanin production capacity, but have molecular responses to counteract more sustained exposure to UVR. By contrast, increased UVR in the study area led to increases in melanin concentration and melanocyte abundance of blue whales, suggesting tanning capacity in this species. My study provides insights into the mechanisms with which cetaceans respond to UVR and reveals the central role played by pigmentation and DNA-repair mechanisms in cetaceans.