by Max Breitbarth
Anthropogenic emissions of carbon dioxide (CO2) have already directly affected corals, algae and other low-trophic level organisms in our oceans through the process of ocean acidification—the absorption of around 25% of atmospheric CO2 by the ocean. The increasing acidity has made forming calcified exoskeletons more difficult for corals, destroying localized ecosystems. The effects of a declining coral population have climbed up the food chain to threaten even predators near the top of the list. But what about the primary predators of the oceans…the feared, fascinating and ferocious sharks that have provided insight on marine feeding patterns, inspired tales like the one shown through the film Jaws, and are recognized by most as the biggest, baddest fish in the sea? Dixson et al. (2016) were interested in observing whether higher levels of ocean acidification sharks and rays, specifically their enhanced olfactory organs.
These olfactory organs are more powerful than those of other marine groups and are vital for hunting and finding food in water, as “Olfaction is considered especially important as a distant sense because chemical signals can become…transported much further in the marine environment than visual, mechanical, or electric signals.” (Altima, 2012) While elevated olfactory sensitivity serves as a competitive advantage for these species, ocean acidification can disrupt their ability to use their olfactory systems to find prey and other sources of energy through their chemical “odor.”
Experiments conducted by Dixson et al. described in their 2015 Global Chance Biology article “Odor tracking in sharks is reduced under future ocean acidification conditions” suggest that seawater with the high-CO2 conditions predicted for the future inhibit the ability of sharks to find food through their olfactory sensors in the water. This “smell” inhibition present in sharks living in a high-CO2 ocean environment links a direct effect of ocean acidification on the ability of sharks to sustain themselves.
The experimental design by Dixson et al. compared sharks in three groups over a 5-day period to observe if a significant effect on “odor tracking ability” existed. The authors used the smooth dogfish shark, Mustelis canis, which they were able to easily and responsibly collect, which also represented sharks and rays with similar olfaction organs. In the control group, sharks were exposed to current ocean acidity conditions, while there were two groups of sharks exposed to elevated dissolved CO2 treatments—an “elevated” treatment with a slightly higher CO2 PPM count and a “high” CO2 PPM count, consistent with the upper-bound acidity levels projected for 2100 (Caldeira & Wickett, 2005; Meehl et al., 2007). By designing specially constructed holding cages and flumes, the authors carried out their test trials, using a uniquely formulated odor attractant dubbed “squid juice.” The experimenters would observe whether or not the attractants would warrant a shark’s preference to a side of the flume where the attractant was present or not. Observing the environments with an increase of controlled CO2 concentration with their control, present-conditions holding tank and flume, the rate at which the sharks were able to effectively trace food sources on certain sides of the flume decreased with statistical significance.
While this experiment was conducted in a closely controlled laboratory and not in real ocean water, it is still alarming to see that as atmospheric CO2 from anthropogenic climate change continues to accumulate in our oceans, a well-known member of many aquatic ecosystems is in danger of losing its feared hunting abilities. While the effects of climate change are often subtle and less visible than other imminent dangers, the risk of losing sharks in our oceans will certainly grab one’s attention. Without any changes in human behavior, a favorite topic of biologists, filmmakers and all members of society will be absent from its habitat.
Danielle L. Dixon, School of Biology, Georgia Institute of Technology, Ashley R. Jennings, Jelle Atema, Philip L. Munday. “Odor tracking in sharks is reduced under future ocean acidification conditions.” Global Change Biology, Volume 21, Issue 4. 2015. 22 January 2016. Web.
Atema J (2012) Aquatic odor dispersal fields: opportunities and limits of detection, communication and navigation. Chemical Ecology in Aquatic Systems, (eds Bronmark C, Hansson L-A), pp. 1–18, Oxford University Press, New York.
Caldeira K, Wickett ME (2005) Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean. Journal of Geophysical Research, 110, C09504.
Meehl GA, Stocker TF, Collins WD et al. (2007) Global climate projections. Climate Change 2007: the physical science basis. Contributions of working group 1 to the fourth assessment report of the intergovernmental panel of climate change. In: IPCC Fourth Assessment Report: Climate Change 2007 (eds Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL), pp. 747–845. Cambridge University Press, Cambridge UK.