Ocean acidification will interfere with fish eyes

The idyllic coral sands and crystal seas that lap the Great Barrier Reef are probably most people's definition of a tropical paradise: but all is not well in paradise. As global CO₂ levels rise, the pH at the surface of the oceans is gradually falling. Göran Nilsson from the University of Oslo, Norway, explains that dissolved CO₂ levels are predicted to rocket by the end of the century, increasing by approximately 500 μatm from today's level of about 400 μatm. The resulting 0.4 drop in the water's pH will dramatically affect the reef's inhabitants by altering their ion balance and disrupting one of the brain's key neurotransmitters: GABA. ‘GABA performs a function in virtually all neural circuits in the brain’, says Nilsson, who explains that alterations to the system can dramatically disrupt behaviour, making predators attractive and increasing the boldness of usually shy creatures. Nilsson adds that juvenile damselfish also fail to respond correctly to glimpses of a predator after exposure to elevated CO₂ and says, ‘This suggests that the function of the visual system is affected by high CO₂’. Curious to find out how increasing ocean acidification might affect the vision of residents of the Barrier Reef, Nilsson, Wen-Sung Chung, Justin Marshall, Sue-Ann Watson and Philip Munday decided to find out how increased CO₂ alters the visual responses of damselfish retinas by focusing on the speed of the retina's response to flickering light (p. 323).

The team explains that we can see lights flickering until the flicker reaches a specific frequency – the critical flicker fusion (CFF) – at which point the retina no longer responds fast enough and the image appears to stop flickering. The retina's response is often correlated with an animal's lifestyle. Creatures that live in brightly lit environments that are also good at evading predators usually have higher CFFs than sluggish animals that live in dim conditions.

The team recorded the electrical activity of the damselfish's eye as they shone a flickering light into it, increasing the flicker rate until the pattern of the eye's electrical activity no longer matched the light's flicker; this was when they reached the fish's CFF and the animal could no longer distinguish the flicker. The fish kept at today's CO₂ level had a high CFF of around 90 Hz, while it had fallen to about 78 Hz in fish that had experienced 6 days at the CO₂ levels that are predicted by the turn of the next century (944 μatm). ‘Having good temporal resolution is critical to detect fast-moving objects’, says Chung, and Nilsson adds, ‘It is likely that the reduction will translate into a reduced ability to react to fast events, probably by 10–15%.’

Next, the team tested whether the increased CO₂ exposure had affected the fish's GABA signalling system. They activated the GABA_A receptor – which is usually activated by GABA – with an agonist to see whether they could restore the CO₂-exposed fish's impaired flicker response, and the treatment worked, successfully restoring the fish's performance.

It seems that increasing CO₂ levels will impact the vision of reef residents, but it is hard to predict how this will affect the reef's ecology. Nilsson says, ‘We expect that the sensitivity of the CFF to high CO₂ will vary between species… If a particular prey is more sensitive than some of its predators, it could have negative consequences for the prey… but one can speculate that the opposite situation may also occur (that the predators become slower in their visual responses), which would be beneficial for the prey.’