Elevated levels of dissolved carbon dioxide (CO2), such as those expected by the end of this century, have been shown to harm marine fish by affecting how they sense and interact with their environment, ultimately increasing their mortality. The sensory and behavioural changes caused by CO2 have largely been attributed to chemical changes in fish brains, with little investigation into other mechanisms. However, sensory information is first gathered outside the brain through systems such as the olfactory system, with which fish sense food, find potential mates, detect predators and locate suitable habitats. Cosima Porteus and colleagues from the University of Exeter, UK, in collaboration with scientists from Portugal and other UK institutions, hypothesized that fish olfactory tissue – which is in direct physical contact with water – is also likely affected by elevated CO2 and that changes in this tissue may thus be responsible for some of the effects of high CO2 on fish behaviour.
To test their hypothesis, the team first confirmed that the behaviour of their study animal, the European sea bass, was affected by exposure to water with high dissolved CO2. Indeed, they found that the fish swam less and were more likely to ‘freeze’ in the presence of a predator when exposed to high CO2. Next, the researchers exposed only the odour-sensing tissues of the fish to water with normal CO2 or high CO2 levels that also contained chemicals that the fish should be able to smell. They then measured how strongly the nerves in the scent-sensing tissue responded to the different chemicals. High CO2 levels reduced the sensitivity of the nerves to most of the odour chemicals, and the sensitivity was more impaired for some chemicals than for others. In fact, the scientists estimated that by the end of the century, fish might have to be almost twice as close to the source of an odour to smell it.
Porteus and colleagues also wanted to find out how exposure to elevated CO2 might affect the sea bass on a molecular level, so they measured the expression of genes in both the odour-sensing tissue and the odour-processing area of the brain after they exposed fish to current or end-of-century levels of CO2 for 2 or 7 days. In both the water-contacting tissues and brain, they found that the expression of genes involved in neuron development and function was reduced after high CO2 exposure, suggesting an impairment of smelling ability even at the molecular level.
Overall, Porteus and her fellow scientists showed that the effects of high CO2 levels on fish behaviour are not simply due to changes in brain chemicals; rather, they are likely due to a combination of alterations in the odour-sensing tissue itself and in the odour-processing centre in the brain. Unfortunately, the scientists’ findings suggest that by the end of this century, fish may have a harder time noticing when something smells, well, fishy.