How UV rays threaten ocean life at the tropics

UV radiation, the same rays that cause sunburn and skin cancer, are also harmful to marine animals. Copepods, small crustaceans that are one of the most abundant animals in the ocean, are especially at risk. These little critters keep ocean food webs healthy as an important food source for young fish and other animals, and their numbers can indicate whether their ecosystems are thriving. Scientists already know about UV radiation's harmful effects in higher latitudes, but it is unclear how animals in the tropics that face stronger sunlight year-round protect themselves from the damaging rays. In 2018, Samuel Hylander and colleagues at Linnaeus University, Sweden, investigated how marine copepods protect themselves from UV light at the Inhaca Marine Biology Station, Mozambique.The research team collected copepods at depths down to 10 m from the sea near the station and placed them outside in small plastic containers submerged in small pools flushed with seawater, to keep them cool. First, they covered each container in Plexiglas, which blocked UV light in the natural sunlight from passing through. Then, when they wanted the copepods to experience potentially damaging UV rays, the researchers replaced the UV-blocking Plexiglas with Plexiglas that allowed UV rays through for either 2 or 4 h, tracking how many of the tiny creatures survived. Their results were troubling. The death rate amongst the copepods that experienced even a few hours of UV light was 40–50% higher than that of the protected copepods, so UV radiation is a threat to these essential members of the marine community.Then the team wondered how the damaging rays might affect copepod swimming. They placed some of the animals at the surface of a 99 cm deep tube of water at midday, when the light was most intense, and the copepods swam down, away from the surface to avoid where the UV light was strongest. However, the researchers also noticed that the copepods migrated down even when there were no UV rays, meaning this may not be a reaction to UV radiation.Finally, the researchers measured the amount of mycosporine-like amino acids and astaxanthin produced by the copepods in their bodies. Mycosporine-like amino acids are natural sunscreens produced by other animals to protect themselves from UV damage and astaxanthin is an antioxidant that provides protection from the damaging effects of UV rays. Although the researchers found evidence that the copepods produced natural sunscreen mycosporine-like amino acids, they weren't in large enough quantities to protect them from the UV rays, especially for copepods in tropical areas that may have fewer pigments and compounds compared with those living in freshwater or higher latitudes. And the tiny creatures did not produce any protective astaxanthin and were transparent. All of these observations point to UV radiation posing a serious threat to the survival of copepods.As climate change continues, scientists expect that UV radiation will grow stronger in many regions, including the tropics where sunlight is already intense. If copepods can't survive through these changing conditions, this spells trouble for the rest of the food web, which relies heavily on them for food. This means that we might see fewer of the tiny plankton in the ocean, in addition to the large fish that we eat. This study is an important reminder of the large role even minute creatures in the ocean play in maintaining the ocean's health. It is important for us to protect them to keep the entire marine ecosystem balanced.