Ultraviolet radiation (UVR) reaching the surface of earth has been recognized as a major environmental stressor for marine organisms due to the potential of UVB (280 - 315 nm) to induce DNA damage such as cyclobutane pyrimidine dimers (CPDs) that can lead to cell death. Despite inhabiting a UV-rich environment, the levels of DNA damage in reef fish and factors influencing these levels are unknown. Whether reef fish are able to avoid UVR and repair UV-induced DNA damage is unclear despite the importance of these protection mechanisms in other animals. The presence of UV-absorbing Mycosporine-like Amino Acids (MAAs) in the external mucus of reef fish has been confirmed, however the efficiency of these compounds in preventing CPDs has not been studied. The aim of this PhD was to assess the impact of ambient and elevated levels of UVR on reef fish in terms of UV-induced DNA damage, and to evaluate the protective mechanisms available to fish with and without UV vision.
In order to determine the net level of DNA damage in skin samples of 15 species of fish from the reefs surrounding Lizard Island, a CPD-specific antibody was used in an ELISA. Analysis using a boosted regression tree shows that the most important factors governing CPDs were species and size, with higher damage being detected in smaller individuals. Other factors such as family, depth and the presence or absence of UV vision contributed the least to the variation in damage. The length of exposure to natural levels of UVR over the course of a day was found to have no significant influence on net DNA damage levels, which were relatively low in situ, compared to levels that were detected in later experiments using elevated UVR.
The first protection mechanism, behavioural avoidance to UVR, was tested using behavioural experiments in which fish with (Pomacentrus amboinensis) and without UV vision (Thalassoma lunare) were given a choice between UV-protected and UV-exposed compartments. Additionally, foraging behaviour of settlement-stage larvae of P. amboinensis was determined under ambient levels of UVR. Automated analysis of video footage using MatLab shows that neither species showed a specific avoidance response to varying levels of UVB. Although P. amboinensis showed a preference for deeper sections of the experimental tanks, fish spent equal amounts of time in exposed and protected compartments. The foraging activity and distance to shelter of P. amboinensis that were exposed to UVR were significantly reduced compared to fish that were observed under light conditions that lacked UVR.
Next, the efficiency of natural sunscreens, MAAs, in preventing CPDs was tested in P. amboinensis and T. lunare. The levels of MAAs in the mucus of the two species were either reduced or maintained during captivity before exposure to a short pulse of high UVR. Spectrometric measurements of light transmission through mucus samples collected after irradiation were used to quantify the amount of MAAs available for protection. In both species, DNA damage levels in skin samples from UVR exposed individuals was higher than in control groups that were exposed to light lacking the UV component. Spectrometric measurements of external mucus of both species revealed a clear link between higher mucus absorbance, i.e. MAA levels, and lower DNA damage levels. Furthermore, a significant increase in mucus absorption was observed in P. amboinensis after UV exposure.
The last protection mechanism, the ability to revert DNA damage via photoreactivation and dark repair was investigated in four species (P. amboinensis, Pomacentrus moluccensis, Lethrinus variegatus and Siganus corallinus). All species examined showed significant increases of DNA damage after exposure to elevated UVR levels. Interspecific variation in the susceptibility to UVR was observed, with L. variegatus showing the highest damage levels. Significant reductions in DNA damage levels were found in P. moluccensis and L. variegatus that were exposed to photoreactivating light after the initial damage accumulation. Individuals of P. moluccensis that were shielded from any light exposure post UVR exposure also showed less DNA damage at the end of the experiment.
This is the first study to address levels of UV-induced DNA damage in reef fish under natural conditions as well as under elevated doses of UVR that could occur in a changing climate. The relatively low levels of CPDs in a diverse group of reef fish indicate that current levels of UVR pose only low threat and underline the importance of protection mechanisms against UVR. Increases in UVR could have an impact not only on adult fish capable of adjusting their protection mechanisms, but also settlement stage larvae which show some of the highest levels of DNA damage in situ as well as mortality during slight increases of UVR. Whether these and other effects such as decreased foraging activity have broader implications on the recruitment and reproduction of reef fish and the community structure on coral reefs needs to be examined in the future.