Reef sharks may already be adapted for climate change

The oceans are getting more and more acidic as they slurp up all the carbon dioxide that our cars and factories are churning into the air. What will happen to marine life as their environment becomes more and more diluted with CO2? To try to predict the affects of acidification on ocean biodiversity, some researchers have simply subjected animals to more and more acidified water to see what happens. Small reef fishes act riskier, and some plankton seem able to rapidly adapt. The truth is plenty is known about the effects of acidification on teleost fishes, but not much is known about the elasmobranchs, which include sharks, rays, and skates.

That’s what James Cook University marine biologist Dennis D. U. Heinrich set out to change. Together with colleagues from James Cook University, the University of Tasmania, and the Australian Institute of Marine Science, Heinrich rounded up a group of reef-dwelling sharks, dumped them into tanks with varying levels of dissolved carbon dioxide, and waited to see what would happen.

The researchers focused on the epaulette shark (Hemiscyllium ocellatum), a species that already has high tolerance to the low oxygen environments of the shallow coral reef flats in which it resides. Because it can tolerate low oxygen, they reasoned, perhaps it is also tolerant to the levels of CO2 that will characterize our seas in the near future. However, these sharks only have to contend with brief periods of low oxygenation, on the order of hours, and laboratory-based experiments on sharks’ responses to high levels of CO2 have also relied upon exposure periods lasting only minutes or hours.

To look at the longer-term effects of acidified seawater, Heinrich exposed his sharks to seawater with varying levels of dissolved CO2 for between 60 and 90 days. Some of the sharks were placed into a control seawater solution with just 390 µatm CO2, others were placed into a 600 µatm solution (medium), and others into an 880 µatm solution (high). The researchers measured the sharks’ oxygen consumption rates to see if the elevated CO2 required the sharks to expend more energy just to breathe. They also assessed the sharks’ responses to low oxygen, to see if the elevated CO2 had any negative effects on their ability to continue to thrive in their reefs.

They discovered that the sharks’ oxygen consumption was not affected by living in acidified seawater, nor was their ability to survive in an oxygen-depleted environment. There were detectable changes in the way their blood transported oxygen, but not in a way that incurred any cost to the animals. “Our findings suggest that, for this reef-inhabiting benthic elasmobranch, neither the energetic costs of basic maintenance nor sensitivity to hypoxia may be compromised in the elevated CO2 conditions projected for the end of this century,” they concluded.

On one hand, this is good news for the epaulette shark and for similar species. However, their ability to thrive in relatively acidic seawater may result in a shift in ecosystems in which they live, such as by increased the pressure from predation on the fish they eat.

Importantly, these findings may not apply to elasmobranchs more broadly; pelagic species, for example, may respond in an entirely different way to their changing environment. “Investigating both sensitive and tolerant species from an array of habitat types would help to tease apart the role of the environment from other factors, including evolutionary history and behaviour,” Heinrich writes, which is important when considering how to best conserve sharks in an acidifying ocean. Some species, it seems, may be able to take care of themselves. Indeed, the ability of epaulette sharks to survive is probably linked with their previous adaptations to life in shallow lagoons and reef platforms. In other words, epaulette sharks have already been practicing their climate change survival skills. – Jason G. Goldman | 22 October 2014

Source: Heinrich D.D.U., Rummer J.L., Morash A.J., Watson S.A., Simpendorfer C.A., Heupel M.R. & Munday P.L. (2014). A product of its environment: the epaulette shark (Hemiscyllium ocellatum) exhibits physiological tolerance to elevated environmental CO2, Conservation Physiology, 2 (1) DOI: 10.1093/conphys/cou047

Header image: An epaulette shark swimming in the shallows, copyright M. Heupel.