Red-headed Stepchild of Climate Change: Effect of Ocean Acidification on Marine Life and Calcification Rates

The other CO2 problem: Ocean acidification affects the calcification and behaviours of marine species.

Climate change and the atmosphere is a hot topic today; however, little focus is directed toward how climate change affects our oceans. Yet, the oceans will and have been directly affected by increasing carbon dioxide (CO2) concentrations related to anthropogenic activities such as fossil fuel use. Often called “the other CO2 problem”, ocean acidification (OA) is the absorption of CO2 by the ocean. This uptake of CO2 leads to reduced pH and calcium carbonate availability (see figure below for details) (3). In fact, the average pH of the oceans has decreased from 8.21 pre-industrial revolution to 8.10 in 2009 (3) and is predicted to fall to about 7.60 by 2100 (1). This may not seem like a drastic decrease – after all, 7.6 is about the same acidity as milk, so harmless right? – but, for many creatures of the ocean, minor changes in acidity levels and especially carbonate levels can affect their growth, reproduction, and behaviour. Negative effects of ocean acidification could lead to major decreases in biodiversity and possible issues for fishery markets. Despite these warning signs, OA continues to be an understudied and mainly ignored problem. Essentially, OA is the red-headed stepchild of atmospheric climate change – neglected and overlooked.

Picture1 Ocean Acidification, by unknown, UK Ocean Acidification Programme
Absorbed CO2 reacts with water to create carbonic acid and hydrogen atoms which eventually form bicarbonate ions (HCO3), reducing the availability of carbonate for CaCO3 (used in shells and exoskeletons) and reducing the pH (4).

Dr. Katharine Hayhoe, a quirky climate scientist at the Texas Tech University, gives a quick summary of OA in her video channel, Global Weirding with Katharine Hayhoe:

In general, the marine organisms most at risk from OA are those that calcify their shells, skeletons, or carapaces. These organisms, including corals, use the compounds (calcium carbonate) in the water to create their outer layers in a process called calcification. The problem is that under OA conditions the amount of those compounds in the water is low, so corals can’t grow their exoskeletons. Lower pH levels can also lead to erosion of those exoskeletons (4). When faced with OA, corals have the following options: (a) decrease skeletal growth, (b) reduce skeletal density, or (c) invest more energy into keeping the same skeletal growth rate and density. Think of it this way: say you have a set amount of money to build a house. You have enough money to either build it tall (skeletal growth) or strong (skeletal density). If you build it tall (high growth), the walls are weak (low density). But if you build it strong with thick walls (high density), it will be short (low growth). A third option may be to build it tall and strong regardless, but then you don’t have any money left to buy furniture (high skeletal growth and density, but not energy left for reproductive/other activities). This is the kind of trade-offs corals face under OA conditions. Studies in the Great Barrier Reef have shown that the first scenario is occurring in Porites, a type of stony coral (4). With this lower growth rates in corals,

Picture2.pngGreat Barrier Reef, by Toby Hudson, 24 July 2010

we see a loss of  habitat for fish and invertebrates which could eventually lead lower biodiversity levels. In other cases, corals continue to grow but with weaker skeletons. In this case, the coral faces harm from predation, such as parrot fish, or storm and wave damage (4). The third option has other problems. If corals allocate more energy toward maintaining their exoskeletons, they have less energy for larval production. But without larvae, corals risk reduced genetic diversity and ability to recolonize or disperse (4). This could be disastrous in maintaining coral populations and if sustained, could lead to extinction. None of the options are quite promising and may lead to decreases in coral numbers especially when coupled with increased temperature and bleaching events.

Picture3.pngOcean Acidification, NOAA Environmental Visualization Laboratory, 2011

Shellfish (mussels, clams, oysters), gastropods (marine snails), and echinoderms (urchins, sea stars, sea cucumbers) are also at risk under OA. Like corals, they have calcium carbonate shells. Many studies have found ocean acidification to have a negative effect on calcification rates and shell growth. One study found a decrease of 25% and 10% in calcification rates in the blue mussel and pacific oyster respectively (3). Another study found six more species (pencil urchins, hard clams, conchs, periwinkles, whelks, soft clams) have similar problems (9). Break-down of shells occurs in low pH environments and results in decreased fertility and larval development rates. For example, urchins fertilize less eggs and their larvae are smaller and develop slower in lower pH (3). Blue mussels and shrimp also had smaller larvae sizes (1). This is most likely due to energy trade-offs like that of corals discussed above. Although it seems to vary between species, the overall effects of OA are negative and can have detrimental effects on their populations. A decrease in biodiversity due to OA would not only disrupt ecological functioning, but also humans; less seafood for us to harvest and eat.

Marine invertebrate crustaceans are a strange group. Unlike the corals and shell-producing organisms, these crustaceans seem to actually increase their calcification under OA conditions. The shrimp, Lysmata californica, increased mineralization, but this led to a fivefold decrease in its transparency (10). This may seem like a good thing – of course, the shrimp is strengthening its shell. But…decreased transparency and stronger exoskeleton might mean that the little shrimp won’t be able to hide from predators or run away from predators as fast (imagine you are running away from a bear, but…while wearing heavy metal armour). Certain species of crabs and lobsters also have increased net calcification (12). Crabs, lobsters, and shrimp are all super important for the fishery industry and more research is necessary to understand the full effect of OA on them.

Picture4.pngAnemonefish (Clownfish Anemonefish) – Amphiprion percula, Jan Messersmith, 28 May 2005

Nemo and his dad, Marlin, are both cute clownfish we know and love. But what would have happened if Marlin never reunited with his son because instead of swimming away from Bruce and the other sharks, he swam straight into their mouths? If he had been in OA affected waters, that is exactly what would have happened. Studies on the clownfish, Amphiprion percula, found that ocean acidification tampered with the fish’s olfactory (smelling) cues. The clownfish were attracted, instead of repelled, to predator smells (2). If fish were to do this, they would surely have lower survival rates. These clownfish also seemed to have a disrupted ability to identify suitable habitats through olfactory cues (6). In this case, Nemo wouldn’t have been able to find his way home even if he tried.

Most likely due to the lack of attention for OA, research is only just beginning to address the effects it has on marine organisms. Data, thus far, show an overall negative effect of  ocean acidification on the calcification of marine species. Although useful, these studies tend to be short term (21 days, etc.) and are overwhelmingly addressing OA in isolation. Not much is known about how combined effects with temperature increase, hypoxia, salinity fluctuations, and other environmental variables may affect these organisms. It is incredibly important that the public and scientific community turn its attention to this “other CO2 problem” and soon before the damage done is beyond repair.


  1. Bechmann, R. K., Taban, I. C., Westerlund, S., Godal, B. F., Arnberg, M., Vingen, S., Baussant, T. (2011). Effects of ocean acidification on early life stages of shrimp (Pandalus borealis) and mussel (Mytilus edulis). Journal of Toxicology and Environmental Health Part A: Current Issues, 74(7-9), 424-438.
  2. Dixson, D. L., Munday, P. L., & Jones, G. P. (2010). Ocean acidification disrupts the innate ability of fish to detect predator olfactory cues. Ecology Letters, 13(1), 68–75.
  3. Doney, S. C., Fabry, V. J., Feely, R. A., & Kleypas, J. A. (2009). Ocean Acidification: The Other CO 2 Problem. Annual Review of Marine Science, 1(1), 169–192.
  4. Hoegh-Guldberg, O., Mumby, P. J., Hooten, A. J., Steneck, R. S., Greenfield, P., Gomez, E., Hatziolos, M. E. (2007). Coral Reefs Under Rapid Climate Change and Ocean Acidification. Science, 318(December 2007), 1737–1742.
  5. 5. Hudson, T, 2010, Great Barrier Reef, digital image, Wikipedia, viewed 28 November 2017, <;
  6. Messersmith, J 2005, Anemonefish (Clown Anemonefish) – Amphiprion percula, digital image, flikr, viewed 28, November 2017, <;
  7. Munday, P. L., Dixson, D. L., Donelson, J. M., Jones, G. P., Pratchett, M. S., Devitsina, G. V., & Døving, K. B. (2009). Ocean acidification impairs olfactory discrimination and homing ability of a marine fish. Proceedings of the National Academy of Sciences, 106(6), 1848–1852.
  8. NOAA Environmental Visualization Laboratory, 2011, Ocean Acidification, digital image, NOAA Coral Reef Conservation Program Ocean Acidification Science Plan, viewed 27 November 2017, < >
  9. Ries, J. B., Cohen, A. L., & McCorkle, D. C. (2009). Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology, 37(12), 1131–1134.
  10. Taylor, J. R. A., Gilleard, J. M., Allen, M. C., & Deheyn, D. D. (2015). Effects of CO2-induced pH reduction on the exoskeleton structure and biophotonic properties of the shrimp Lysmata californica. Scientific Reports, 5(1), 10608.
  11. US Department of Commerce, N. O. and A. A. (n.d.). Coral Reef Conservation Program Ocean Acidification Science Plan Fiscal Years 2012-2016. Retrieved from
  12. Whiteley, N. M. (2011). Physiological and ecological responses of crustaceans to ocean acidification. Marine Ecology Progress Series, 430, 257–271.

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