Losing Nemo - How Acid Oceans Deafen Fish
Chris - It is a proven fact that if you elevate the amount of CO2, carbon dioxide, in the atmosphere (which we also think is linked to climate change) this will have the effect of acidifying the sea, because carbon dioxide when it dissolves forms carbonic acid. This acidification, it turns out, can change the way that fish react to the world around them. Dr Steve Simpson, who's from Bristol University, has been looking at how this affects their ability to sense the sound of danger...
Steve - My research over the last decade has focused on the behaviour that coral reef fish show when they're looking to seek habitat after a period of a few days of developing out at sea in the plankton. So these are very young coral reef fish, seeking the habitat that they'll spend the rest of their lives on. My interest has been particularly on the importance of auditory cues. So this is sounds produced by animals on the coral reef that the small larvae can detect and move towards and use to pick specific habitats. Recent research that's been coming out over the last couple of years has demonstrated that fish that experience ocean acidification lose their natural sense of smell which is the other cue that fish use to detect reef habitat. So the question that I was interested in testing is whether the sense of hearing is unaffected by ocean acidification and so, would be able to compensate for this loss of sense of smell, or whether hearing is also impacted on by ocean acidification.
Chris - So what was the experimental technique? What did you actually do and what fish did you test?
Steve - We worked with clown fish. Clown fish are similar to Nemo, and are readily available through the aquarium trade. They can be bred in captivity. So for scientists, this is great because it means that we can actually work with the embryos and larval stages of these fish. So we took embryonic clown fish and as soon as they hatched, we put them into different treatments of water that were either based on today's CO2 environment, we bubbled air into their tanks, or based on different predictions, for the CO2 environments later in the century. So we had three treatments where we bubbled in what we're expecting the atmosphere to be like in 2050 and then two different scenarios for 2100 and these are based on scenarios from the IPCC, the Intergovernmental Panel on Climate Change. So we reared our fish through the larval stage in these different CO2 environments and then we took the fish and put them into an auditory choice chamber which was in the lab - so it was a long tube facing towards a speaker and we allowed the fish to move around in this choice chamber while we played sounds to them and we monitored their behaviour.
Chris - What, first of all, would a normal fish or young fish do under those circumstances when played the sounds of a reef in the way you did?
Steve - We were running our experiments in the daytime. We used a recording of daytime coral reef noise and we know from both studies following fish out in the open water or from playback experiments that we've ran in the past, fish naturally move away from this sound. A coral reef is a dangerous place to encounter for the first time during the daytime because of the high density of predators. And so, the noise of all these predators cause fish naturally to move away from the sound.
Chris - And what about when you then substituted fish that had been reared in these enhanced CO2 environments? What did they do?
Steve - So the fish that had experienced high levels or elevated levels of CO2 showed no response to the recordings. So were equally spending time towards the speaker as well as away from them.
Chris - Gosh! So that's quite striking, isn't it? Have you any clue as to why they behave like that? How do you know that these fish haven't gone deaf?
Steve - So it's certainly possible that the fish have gone deaf. There are several potential mechanisms as to what's happened here. It may be that hearing has been lost, although we did look at the growth of their ear bone which is a central part of a fish ear and we found there are differences in the shape or the size of the ear bone between the fish from different treatments. But it's still possible that hearing has been impacted on. If it's not hearing that's been impacted, it may then be neural transmission or the processing of information and the impact could be occurring there, or it may be that the fish can hear these sounds but loses its natural avoidance behaviour. Either way, I mean, any of those three scenarios would be bad news for the fish in the natural environment.
Chris - Indeed, but one area where your experiment doesn't really reflect the natural environment is that you took the embryos from a normal situation and then put them into these enhanced CO2 situations for the remainder of their development until you tested them. So that could in itself be quite stressful, so could it just be the fish are responding in a stress related way and that in fact, if you were to study their progeny, if you reared fish from embryo to adult and bred from them and then took the progeny and tested them, you might see they were less stressed and therefore, less affected in this way.
Steve - Yes, so that's a very important question to ask as to whether it is due to an acute response to CO2 environments or whether the fish through generations have the ability to adapt. That will determine how we think, how we can predict the responses of fish will be over the next century. What we're seeing is an unprecedented rate of change of the pH of the sea, so it's occurring 100 times faster than any time in the last 650,000 years. And so, we're expecting that fish, if they can adapt, will be doing this in a period of only a few tens of generations through that century. And that really is a focus of the lab of my collaborator Professor Phillip Munday who is now working with multiple generations to look at the rate of ability to adapt.
Chris - And what do you think the implications are for what you've found?
Steve - Well the implications are that fish losing their sense of hearing or their natural responses to sound are detrimental certainly to fish, in that fish live in a very auditory world, so this sound to communicate, to detect and avoid predators, to find their own shoal mates, and also to detect potential prey items. So there would be fairly detrimental impacts on fish populations. We don't know whether this impact would be seen across the board in terms of different fish species and that's the focus of our research now, to start to look at the variability of response between species. I think also an interesting question is how variable is the response within a species, because it may be that every fish needs to adapt, or that there are some rare fish that already have more tolerance that would then have that ability through selective sweeps in the population to be able to keep pace with the change.