It’s a story that many of us know well; a charismatic, touching and often hilarious tale of a father’s relentless pursuit to find his lost son. In Disney-Pixar’s 2003 blockbuster, “Finding Nemo”, a young clownfish by the name of Nemo ventures off into the open ocean, is unfortunately captured by humans, and is sent off to reside in a dentist office fish tank in Sydney, Australia. As a result, Nemo’s father, Marlin, with the company of his newfound pal, Dory, sets off across the big blue ocean to find his son and bring him home, while Nemo frantically plans his escape from the dentist office fish tank. After much hullabaloo, including Marlin and Dory encountering a hungry shark and a humpback whale, Nemo eventually escapes the dentist’s office and catches up with his dad (who unfortunately thinks Nemo is dead) and together with Dory they head for home – a touching story indeed. Though the ending is certainly of the feel-good genre, would the story have turned out the same in an acidified ocean?
Ocean acidification is a process whereby increasing atmospheric CO2 dissolves into the oceans, lowering their pH and making them more acidic. Though calcifying organisms are thought to be among the most negatively impacted, research has also suggested that an increasingly acidifying ocean can impair the behaviour of various marine fishes, with coral reef fish being the most intensively studied. Furthermore, among the coral reef fishes that have been studied, clownfish (Amphiprion percula) have been a fairly popular species of interest.
In 2010, Danielle Dixson and colleagues reported the results of their research comparing the ability of juvenile clownfish to avoid their predators under both ambient (present day) and elevated (2100 projection) CO2 conditions. What Dixson et al. (2010) found was that, in comparison to fish raised in present day CO2, juvenile clownfish reared under elevated CO2 were unable to distinguish between predators and non-predators, spending equal amounts of time hanging around both (though newly-hatched larvae were unaffected; Figure 2A). Likewise, Simpson et al. (2011) reported that clownfish lost their ability to avoid the sounds of their predators when raised under elevated CO2 conditions (Figure 2B). Additionally, negative impacts on the homing ability of clownfish have also been reported. Munday et al. (2009) found that baby clownfish raised under elevated CO2 conditions lost the ability to choose their preferred and optimal habitat and were also unable to distinguish their parents from other clownfish (Figure 2C).
These negative changes in clownfish behaviour are likely induced by a change in their biochemistry when reared under elevated CO2 conditions. Studies have suggested that high levels of CO2 interfere with one particular chemical in the brains of fishes (and other animals as well) – GABA (chief inhibitory neurotransmitter vertebrate central nervous systems; e.g. Nilsson et al. 2012). Since the GABAA receptor has a specific conductance for Chlorine (Cl−) and bicarbonate (HCO3−) – the two ions most likely to be impacted under elevated CO2 conditions – this explanation seems probable.
So how would this have impacted Finding Nemo? Well, if we revisit the plot of the movie, there are two ways in which acidification may have changed the outcome of Finding Nemo. Firstly, in the original movie, Marlin and Dory swim away when they come across a predatory shark (though shark odour tracking may also be hindered in a more acidified ocean; see Dixson et al. 2014). However, in a more acidified ocean, as research has suggested, they may have done exactly the opposite and would likely have become dinner for the shark. Secondly, at the end of the movie, Nemo pops out of a sewer pipe and swims along, eventually finding his dad and Dory. Unfortunately, in a more acidified ocean, research suggests that Nemo likely wouldn’t be able to find his father.
Though I like to think that adult behaviour may not be altered and Marlin would have eventually found the dazed and confused Nemo (though other inaccuracies in the movie, if corrected for, would have led to Marlin and Dory’s demise, let alone a much more incestual tale), it is likely that the feel-good ending that we’ve all come to love would have been anything but feel-good. So we need to find ways of mitigating ocean acidification and saving our precious ecosystems from the myriad of destructive consequences that they will face if we continue our ways – not only for our own sake, but so that the Nemos of the future will have a chance to swim away from their predators or find their way home if they ever decide to venture away from their anemonemone.
Munday, PL et al. 2009. Ocean acidification impairs olfactory discrimination and homing ability of a marine fish. PNAS, 106: 1848-1852.
Dixson, DL et al. 2010. Ocean acidification disrupts the innate ability of fish to detect predator olfactory cues. Ecology Letters, 13: 68-75.
Simpson, SD et al. 2011. Ocean acidification erodes crucial auditory behaviour in a marine fish. Biology Letters, doi:10.1098/rsbl.2011.0293.
Nilsson, GE et al. 2012. Near-future carbon dioxide levels alter fish behaviour by interfering with neurotransmitter function. Nature Climate Change, 2: 201-204.
Dixson, DL et al. 2014. Odor tracking in sharks is reduced under future ocean acidification conditions. Global Change Biology, doi: 10.1111/gcb.12678.