Fish Can't Smell in Acidic Seawater

Fish use their olfactory sense of smell, which is highly acute, across many aspects of their lives. Olfaction plays important roles in feeding, migration, reproduction and predator avoidance. Thus, it would not be unimaginable that if there was a reduction in their ability to smell, it could very well compromise their survival. A new study published in Nature Climate Change has just provided evidence that the smelling abilities of some economically important species could be affected by increased CO2.

As if global temperature wasn’t problematic enough, increased carbon in the atmosphere from the combustion of fossil fuels is also the culprit for the rapid acidification of our oceans. The world’s oceans are an enormous carbon sink, absorbing about 25% of the anthropogenic CO2 produced every year. When CO2 is absorbed by seawater carbonic acid is formed, and thus the seawater become more acidic.

University of Exeter researcher Dr Cosima Porteus, who led this study, said: "Our study is the first to examine the impact of rising carbon dioxide in the ocean on the olfactory system of fish”. Researchers from the University of Exeter, in collaboration with scientists from the Centre of Marine Sciences (CCMar, Faro, Portugal) and the Centre for Environment, Fisheries and Aquaculture Science (Cefas) compared the behaviour of commercially important juvenile European sea bass, Dicentrarchus labrax, at CO2 levels typical of today's ocean conditions, and those predicted for the end of the century. By recording the activity in the nervous system while their nose was exposed to water with different levels of CO2 and acidity, they also tested the ability of the sea bass’ nose to detect different smells. Finally, the team also studied how the elevated CO2 and acidity in the water affected the genes being expressed in the nose and brain of sea bass.

Dr. Porteus said: “sea bass in acidic waters swam less and were less likely to respond when they encountered the smell of a predator. These fish were also more likely to "freeze" indicating anxiety".“The sense of smell of sea bass was reduced by up to half in sea water that was acidified with a level of CO2 predicted for the end of the century. Their ability to detect and respond to some odours associated with food and threatening situations was more strongly affected than for other odours. We think this is explained by acidified water affecting how odorant molecules bind to olfactory receptors in the fish’s nose, reducing how well they can distinguish these important stimuli. Therefore, rising atmospheric CO2 levels threaten natural aquatic ecosystems and our food supply.” Finally, the genes expressed in the nose and brain of the juvenile sea bass when subjected to elevated CO2 and acidity, were altered and were genes involved in sensing smells and processing of this information.

Dr Porteus said: “I wanted to examine if fish had any ability to compensate for this reduced sense of smell, but found that instead of increasing the expression of genes for smell receptors in their nose they did the opposite, exacerbating the problem.”Although, European sea bass was the only species used in the study, the process of sensing of smells in aquatic species are common and thus their findings could be broadly applied.

For this interesting paper please see the full text

Porteus CS, Hubbard PC, Uren Webster TM, van Aerle R, Canário AVM, Santos EM, Wilson RW (2018) Near-future CO2 levels impair the olfactory system of a marine fish. Nature Climate Change. https://doi.org/10.1038/s41558-018-0224-8

https://www.nature.com/articles/s41558-018-0224-8