• Sea World Research & Rescue Foundation - Funded Project

How Do Coral Reef Fish Cope With Elevated UV Levels

PROJECT TITLE: How do coral reef fish cope with elevated UV levels
RESEARCHERS: Dr. Ulrike E. Siebeck and Christoph Braun
LOCATION: Lizard Island on the Great Barrier Reef, Australia and The University of Queensland, St Lucia

OVERVIEW

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Despite living in an environment with high ultraviolet radiation (UVR) levels our knowledge about the impact of UVR on reef fish and their protection mechanisms (avoidance behaviour, DNA repair and protection with natural sunscreens) is limited.

During his PhD Chris discovered that:
• the most important factors influencing UV-induced DNA damage in reef fish are species and size
• UV vision did not result in UVR avoidance behaviour
• Some reef fish species can repair DNA damage caused by UVR
• Some reef fish can secrete natural sunscreens that protect against UVR

The results of this study serve as a valuable baseline and open the door for future studies that will investigate possible connections of the impact of UVR on reef fish on their reproduction and community structure within a coral reef.

QUESTION ONE: DNA DAMAGE

What is the net level of DNA damage present in reef fish exposed to natural levels of UVR?

METHOD: DNA was isolated from skin and tissue samples from a total 125 individuals from 15 species of reef fish collected from the reefs surrounding Lizard Island.

RESULTS: The most important factors influencing UV-induced DNA damage in reef fish are species and size. Surprisingly other factors such as the depth, where the fish were caught and the presence or absence of UV vision contribute the least to the differences in DNA damage.

QUESTION TWO: UV PROTECTION

How do reef fish protect themselves from UVR? Are these behavioural avoidance responses to UVR different between fish with UV vision (Ambon Damselfish Pomacentrus amboinensis) and without UV vision (Moon Wrasse Thalassoma lunare).

METHOD: Fish were given a choice between UV-protected and UV-exposed compartments. Additionally, boldness and foraging behaviour of juvenile P. amboinensis were determined.

RESULTS: Although fish with UV vision showed a preference for deeper sections of the experimental tanks, no specific UV avoidance behaviour could be detected in both species. The foraging behaviour and boldness of newly settled fish without UV vision that were exposed to an ambient dose of UVR were significantly reduced compared to fish that were observed under light conditions that lacked UVR.

QUESTION THREE: LIGHT-ACTIVATED REPAIR MECHANISM

Are there differences between two DNA repair mechanisms in settlement stage larvae of different species of reef fish?

METHOD: Settlement stage larvae of four species of reef fish were exposed to elevated levels of UV radiation. Upon exposure, fish were split into two different treatments that allowed for either a light activated repair mechanism to take place, or only allowed for a light-independent repair mechanism (commonly referred to as dark repair).

RESULTS: The tracking of the accumulation and removal of UV-induced DNA damage (Cyclopyrimidine dimers, CPDs) from skin and tissue samples showed significant increases of DNA damage and mortality after UVR exposure, as well as interspecific variation in the susceptibility to UVR. Significant reductions in DNA damage levels due to a light-activated repair mechanism were found in two of the four species up to eight hours after the initial damage accumulation. Evidence for a second, light-independent repair mechanism was found only in one species, again highlighting differences in vulnerability to UVR between species.

QUESTION FOUR: NATURAL SUNSCREENS

Do natural sunscreens, Mycosporine-like Amino Acids (MAAs), protect reef fish against UV-induced DNA damage?

METHOD: Two species of reef fish, the Ambon Damselfish and the Moon Wrasse were caught from the Lizard Island lagoon and held in captivity for up to ten days. During this time, the amount of naturally occurring sunscreens in the external mucus of the fish was reduced by altering the food composition and exposure to natural sunlight. Initially after capture and after ten days, cohorts of both species were exposed to elevated levels of UV radiation. After exposure, the amount of sunscreens in the fish mucus and the level of UV-induced DNA damage were measured. MAA content in fish mucus was measured using light transmission of mucus samples using a spectrometer and a custom-made microscope.

RESULTS: In both species, we found a clear link between higher mucus absorbance, i.e. sunscreen levels, and lower DNA damage levels. This means that the more sunscreens a fish had in its external mucus layer, the lower was the amount of DNA damage caused by UV radiation. Furthermore, a significant increase in mucus absorption after UV exposure was observed in individuals of the Ambon Damselfish, a species that has UV vision. This phenomenon was not observed in the other species used in this experiment, the Moon Wrasse, which does not have the ability to see UV light. This modulation of mucus absorbance is, to our knowledge, the fastest change in sunscreen protection observed in a vertebrate so far and presents an exciting new finding in the ecology of reef fish with UV vision from the Great Barrier Reef.

DISCUSSION

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 a low threat to the species studied here. The presence of up to three protection mechanisms and interspecific variation in their relative contribution to each fishes’ protection highlights the importance of an integrated approach to assess the risks of UVR on reef fish in order to prevent underestimating the influence of UV-induced DNA damage.

The results of this study serve as a valuable baseline and open the door for future studies that will investigate possible connections of the impact of UVR on reef fish on their reproduction and community structure within a coral reef.